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Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however, the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Minnesota Geological Survey in St. Paul. In addition, effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification. MINNESOTA GEOLOGICAL SURVEY Harvey Thorleifson, Director DATABASE MAP By Margeurite C. Pettus and V.W. Chandler 2020 LOCATION DIAGRAM COUNTY ATLAS SERIES ATLAS C-47, PART A Rock County Plate 1—Database Map GEOLOGIC ATLAS OF ROCK COUNTY, MINNESOTA Prepared and Published with the Support of THE ROCK COUNTY BOARD OF COMMISSIONERS, THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, AND THE MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND AS RECOMMENDED BY THE LEGISLATIVE-CITIZEN COMMISSION ON MINNESOTA RESOURCES ©2020 by the Regents of the University of Minnesota The University of Minnesota is an equal opportunity educator and employer GIS compilation by R.S. Lively Edited by Lori Robinson Digital base modified from the Minnesota Department of Transportation BaseMap data; digital base annotation by Corey J. Betchwars. Elevation contours were derived from the U.S. Geological Survey 30-meter Digital Elevation Model (DEM) by the Minnesota Geological Survey. Universal Transverse Mercator Projection, grid zone 15 1983 North American Datum 1 0 1 2 3 4 5 MILES 8 KILOMETERS SCALE 1:100 000 1 0 1 2 3 4 5 6 7 CONTOUR INTERVAL 20 METERS Figure 1. Example of a WELL LOG record, showing all the information about the well as reported by the well driller. County Well Name Range Dir Township Section Subsection Well Depth Depth Completed Date Well Completed Update Date Rock DNR OB 67007 47 W 35 AADDAC 102 Yes JOHNSON 489.00 455.00 1995/06/22 ft ft Make Static Water Level Open Hole(ft.) From Last Strat g.p.m. hrs. pumpting Grouting Information Capacity Well disinfected upon completion? Pump Variance Abandoned Wells Screen Nearest Known Source of Contamination Well Contractor Cerfication PLUCKER, D. plastic ü ü ü Quad Depth to Bedrock Pumping Level (below land surface) Model 22C 1385.00 34050 Cretaceous,undiff. Pitless adapter manufacturer ft. Well grouted? feet Direction Type Printed on Model number Manufacture's name REPORT Type Wellhead Completion ü 217.00 MINNESOTA DEPARTMENT OF HEALTH MINNESOTA STATUTES CHAPTER 1031 WELL AND BORING RECORD Description From To (ft.) Color Hardness CLAY, ROCK & SAND GRAY 1 0 DRIFT & SAND BLACK 2 1 SOIL BLACK 5 2 SANDY CLAY BROWN 10 5 SAND 32 10 CLAY SOFT GRAY 121 32 CLAY, SAND LT. GRY 158 121 WOOD, CLAY, LITTLE SAND 161 158 FINE SAND, SHALE BLACK 180 161 CLAY HARD GRAY 217 180 CLAY, LAYER ROCK 220 217 SHALE BLACK 245 220 SHALE DK. GRY 335 245 SAND 340 335 CLAY 341 340 SAND, VERY FINE 349 341 CLAY 354 349 SAND 363 354 CLAY BLACK 365 363 CLAY DK. GRY 376 365 SAND 376 376 CLAY 387 376 SANDY CLAY 389 387 SAND 400 389 SANDY CLAY 408 400 TIGHT CEMENTED SAND 450 408 CLAY DARK 458 450 SAND STREAK 460 458 CLAY 489 460 TOP ROCK 489 489 Remarks DNR OB WELL 67007. HE-01205-07 (Rev. 2/99) Date Lic. or Reg No. Thein Well Co. Was a variance granted from the MDH for this well? Does property have any not in use and not sealed well(s)? 6/1/2018 d e l l a t s n I e t a D d e l l a t s n I t o N Type Length of drop pipe HP Volts g.p.m To 6.13 2.00 in. from to 410.00 0.00 ft. lbs/ft 2.00 45 410 455 10 ft. to ft. to neat cement 400.0 4.00 o T m o r F l a i r e t a M ft. Cubic yards Hole Diameter (in.) Elevation At-grate (Environmental Wells and Borings ONLY) Casing Protection 12 in. above grade Basement offset ü YES NO YES NO ü YES NO YES NO ft. after Date measured Material Diamter Length Set Slot 0.00 2016/05/06 2017/12/23 Entry Date Received Date Unique Well Number County Well Index v.5 Aquifer ft. First Bedrock KSRC PESX Field Located ft. Quad Id MNDNR 565831 Valley Springs License Business Name Name of Driller Diameter 2 Depth 410 Casing Type Plastic Drive Shoe? YES ü NO Use observation well Bentonite Drilling Fluid Well Hydrofractured? YES NO From ft. to Drilling Method Non-specified Rotary Drillhole Angle NOT SPECIFIED Unique Well Number County Rock r i D e g n a R e m a N l l e W Township Section Subsection Depth Drilled Depth Completed Lic/Reg. No. Date Completed Quad Quad Id DNR OB 67007 47 W 35 AADDAC 102 489 455 ft ft 34050 1995/06/22 Elevation 1385.00 ft. Screen 410-455 Method LiDAR 1m DEM (MNDN Aquifer Cretaceous,undiff. Depth to Bedrock 217 ft. Field Located Program Uni No.Verified Location Method Input Source Input Date Universal Transverse Mercator(UTM) - NAD83 - Zone 15 - Meters UTM Northing (Y) UTM Easting (X) 222935 4833051 2016/10/19 Valley Springs 2016/05/06 2017/12/23 Entry Date Update Date Received Date MINNESOTA DEPARTMENT OF HEALTH WELL AND BORING RECORD MINNESOTA STATUTES CHAPTER 1031 Driller Name PLUCKER, D. SWL 565831 Information from owner Mn Department of Natural Resources Minnesota Geological Survey Geological Material Minor Secondary DEPTH Primary ELEVATION LITHOLOGY Hardness Stratigraphy From To Thick From To Color Dale Setterholm MGS Geologic study 1:24k to 1:100k y c n e g A n o i t a t e r p r e t n I c i g o l o e G Interpretation Method 22C CLAY, ROCK & SAND GRAY 1 man-made fill fill 1 1385 1384 0 DRIFT & SAND BLACK 2 man-made fill fill 1 1384 1383 1 SOIL organic deposits BLACK 5 Recent deposit-black soil 3 1383 1380 2 SANDY CLAY sand BROWN 10 clay+sand-brown clay 5 1380 1375 5 SAND 32 sand sand 22 1375 1353 10 CLAY GRAY SOFT 121 clay-gray clay 89 1353 1264 32 CLAY, SAND sand LT. GRY 158 clay+sand-gray clay 37 1264 1227 121 WOOD, CLAY, LITTLE SAND sand clay 161 clay+sand wood 3 1227 1224 158 FINE SAND, SHALE clay BLACK 180 clay+sand sand 19 1224 1205 161 CLAY GRAY HARD 217 clay-gray clay 37 1205 1168 180 CLAY, LAYER ROCK limestone 220 Split Rock Creek shale 3 1168 1165 217 SHALE BLACK 245 Split Rock Creek shale 25 1165 1140 220 SHALE DK. GRY 335 Split Rock Creek shale 90 1140 1050 245 SAND 340 Split Rock Creek sandstone 5 1050 1045 335 CLAY 341 Split Rock Creek shale 1 1045 1044 340 SAND, VERY FINE 349 Split Rock Creek sandstone 8 1044 1036 341 CLAY 354 Split Rock Creek shale 5 1036 1031 349 SAND 363 Split Rock Creek sandstone 9 1031 1022 354 CLAY BLACK 365 Split Rock Creek shale 2 1022 1020 363 CLAY DK. GRY 376 Split Rock Creek shale 11 1020 1009 365 SAND 376 Split Rock Creek sandstone 0 1009 1009 376 CLAY 387 Split Rock Creek shale 11 1009 998 376 SANDY CLAY sandstone 389 Split Rock Creek shale 2 998 996 387 SAND 400 Split Rock Creek sandstone 11 996 985 389 SANDY CLAY sandstone 408 Split Rock Creek shale 8 985 977 400 TIGHT CEMENTED SAND 450 Split Rock Creek sandstone 42 977 935 408 CLAY DARK 458 Split Rock Creek shale 8 935 927 450 SAND STREAK 460 Split Rock Creek sandstone 2 927 925 458 CLAY 489 Split Rock Creek shale 29 925 896 460 TOP ROCK 489 Sioux Quartzite quartzite 0 896 896 489 Figure 2. Example of a WELL STRATIGRAPHY record, which contains a geologist's interpretation of the geologic materials listed by the driller in the WELL LOG record (Fig. 1). Additional downhole information for this well (as noted in the Interpretation Method on the record above) controls the geologist's interpretation, which may not match the driller's description of the geologic material penetrated. D ! E ! l D Record of water-well construction (well driller's log) Scientific investigation hole Diamond drill core sample Rotary-sonic core sample Cutting sample Borehole geophysical log Soil boring Giddings probe hole Field site Textural analysis Gravity data Passive seismic sounding Seismic refraction sounding Bedrock outcrop MAP SYMBOLS Note: More than one symbol can occur at the same location D D D D D D D D D D D D D D D D ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ROSE DELL DENVER BATTLE PLAIN SPRINGWATER MOUND VIENNA BEAVER CREEK LUVERNE MAGNOLIA MARTIN CLINTON KANARANZI NOBLES COUNTY MURRAY COUNTY PIPESTONE COUNTY PIPESTONE COUNTY SOUTH DAKOTA SOUTH DAKOTA NOBLES COUNTY IOWA IOWA 96° 22' 30" W. 96° 15' W. 96° 7' 30" W. 96° 15' W. 96° 22' 30" W. 43° 45' N. 43° 37' 30" N. 43° 45' N. 96° 7' 30" W. 43° 37' 30" N. T. 104 N. T. 103 N. T. 102 N. T. 101 N. R. 47 W. R. 46 W. R. 45 W. R. 44 W. R. 46 W. R. 44 W. R. 45 W. R. 47 W. T. 104 N. T. 103 N. T. 102 N. T. 101 N. § ¦ 90 § ¦ 90 ¤ 75 ¤ 75 ¤ 75 ) 23 ) 23 ) 270 § ¦ 90 Beaver Creek Hardwick Hills Kenneth Luverne Magnolia Steen Kanaranzi Jasper Blood Run Springwater Creek Little Beaver Creek Champepadan Creek Poplar Mound Creek Mud Creek Ash Creek Kanaranzi Creek Split Rock Creek Four Mile Creek Elk Beaver Creek Rock River Rock River Rock River Creek Creek 1 6 1 6 1 6 1 36 31 36 31 36 31 36 1 6 1 6 1 6 1 36 31 36 31 36 31 36 1 6 1 6 1 6 1 36 31 36 31 36 31 36 1 6 1 6 1 6 1 36 31 36 31 36 31 36 480 480 460 480 440 500 520 420 480 460 460 420 460 460 500 440 480 460 460 440 460 440 460 460 440 460 500 460 480 420 500 440 440 440 460 460 500 460 480 520 460 520 480 460 460 440 460 500 480 480 440 460 460 440 460 480 480 500 500 520 480 460 460 460 460 460 460 440 440 440 RKR-1 RKR-2 ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! l l l l l l l l l l l l l l l l l l l l l l l l l l D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E INDEX TO 7.5-MINUTE QUADRANGLES H IL L S NE J A SPE R J A SPE R S E E D G ER TON SOUTH KE N N E T H L U V ER N E M AG NO L IA HIL L S AS H CR E E K EL L S WO R TH VA L L E Y S PR IN G S G A R R E T SON EAST INTRODUCTION The public health and economic development of Rock County are directly dependent on the wise use and management of its land and water resources. Geologic and hydrologic information are essential before decisions are made that affect natural resources. Although the amount of geologic information required for making specific decisions can vary, the information will not be used if it is unavailable when needed, or if it is available only in a highly technical form, or scattered in many different maps and reports. The databases described here address this need. County atlases, prepared jointly by the Minnesota Geological Survey and the Minnesota Department of Natural Resources, present detailed geologic and hydrologic information in an interpretive as well as descriptive form. Maps and texts summarize basic geologic and hydrologic conditions at a county scale, and interpret these conditions in terms of the impacts of possible land- and water-use decisions Several sources commonly provide information about an area or an individual property, but they may use different classification schemes to describe the same geologic materials. As a result, discrepancies in interpreting the data may arise or the different sources may appear to contradict each other. For example, water-well drillers may describe glacial till as "clay," but engineering records will describe it as "clayey sand." Both descriptions are acceptable for their original purpose of describing the physical attributes of the material. "Clay," the term used by well drillers, defines the general inability of the till to yield groundwater to a well. "Clayey sand," the term from the engineering record, defines the physical composition of the till relative to particle size and engineering properties. The geologist must take the analysis one step further and define the material in terms of how it formed rather than how it is to be used. In this example, till consists of an unsorted mixture of rock fragments ranging in size from clay to cobbles and boulders, and it is interpreted by the geologist to have been deposited directly by glacial ice. Understanding the process by which the material formed allows geologists to make predictions about what lies between and beyond data points. All of the types of data described on this plate were interpreted by geologists or hydrogeologists to make them meaningful for mapping purposes. The 1:100,000 scale of the maps in this atlas was chosen because it shows the geologic and topical studies of the county while keeping the physical size of each plate to a manageable level. As a result, some detailed information that was gained by data interpretation and mapping cannot be shown on these maps or discussed in the texts. Some of this information is available in digital files that accompany the atlas. Whether to use the atlas alone, or in combination with the databases, depends on the amount of detail needed. Generally, database information must be used to evaluate site- specific conditions. THE DATABASE MAP The types, locations, and density of information used to prepare the Rock County Atlas are shown on this map. The data are described below to aid the user in assessing what types may be useful for a particular information need. The Database Map serves as a guide to the precision of the other maps in the atlas. It shows where data are sparse or lacking and interpretation and extrapolation were required to prepare the maps. DRILL-HOLE INFORMATION A record of water-well construction (well driller's log) is a water-well contractor's description of the geologic materials penetrated during drilling and the construction materials used to complete the well. Not all wells drilled need to extend to bedrock. In areas of thick, unconsolidated Quaternary deposits such as Rock County, drillers commonly do not need to drill through the entire thickness of overburden to find sufficient groundwater. Hydrologic data, such as the static water level and test-pumping results, are commonly included. Before any driller's log can be used, the location of the well must be verified, and a geologist must interpret the log. Driller's logs are the primary source of subsurface geologic and hydrologic data for Rock County; about 600 logs were used for this atlas. Scientific investigation holes are drilled by various agencies to study the nature of the subsurface material. Geologic logs were generated that describe the material penetrated for each hole. In Rock County, the U.S. Geological Survey, Water Resources Division, drilled a number of soil auger holes, test wells, and observation wells to determine the hydrologic properties of local and regional aquifers. Study results and conclusions may be found in U.S. Geological Survey published reports. Core samples were collected by the Minnesota Geological Survey at three sites in Rock County as a means to establish the nature of the subsurface material. One bedrock core sample was collected by using a diamond bit rotating at the end of a drill rod. During this drilling method, a column of rock moves up the drill pipe and is recovered at the surface for study. Rotary-sonic cores were collected for this project from two sites in the county to aid the interpretation of the Quaternary deposits (labelled RKR-1 and RKR-2). The coring technique enables recovery of a continuous core, 3.5 inches (8.9 centimeters) in diameter, from glacial deposits and bedrock (where intersected). It provides excellent subsurface samples for detailed study and comparison with cuttings, geophysical logs, and driller's logs from surrounding sites. Detailed geologist's logs for both of the rotary-sonic cores are shown on Plate 4, Quaternary Stratigraphy. When studies are complete, the core are stored at the Drill Core Library in Hibbing (the state repository for bedrock and earthen material samples collected during exploration, engineering, and geoscience research programs across Minnesota, maintained by the Minnesota Department of Natural Resources). Cutting samples collected during drilling provide physical evidence of subsurface geologic materials. Cuttings are the samples generated as the drill bit cuts through the subsurface material and are used to interpret and verify driller's logs. They are logged and stored at the Minnesota Geological Survey. Cuttings from the Wildcat Oil exploration hole in Kanaranzi Township (T. 101 N., R. 44 W., sec. 15) are stored at the Drill Core Library in Hibbing. Borehole geophysical logs are collected by Minnesota Geological Survey staff, other government agencies, and private entities by lowering instruments down a well or drill hole and measuring the physical and chemical properties of the geologic materials through which the hole passes. Different logging techniques measure naturally occurring gamma radiation, spontaneous potential, and resistivity. Gamma logs characterize in graphic form the geologic formations penetrated. Spontaneous potential and resistivity are mainly used to locate water levels in wells and the depth of the well casing. An interpretive log is prepared by a geologist from the geophysical log and correlated with drilling samples from the same hole, information obtained from nearby outcrops, or a geophysical log from a nearby drill hole. Geophysical logs can provide high-quality subsurface geologic and hydrologic information for wells that have little or no other information available. Logs can be viewed at the Minnesota Geological Survey or accessed online (http://z.umn.edu/borehole_geo). The geologic interpretations are available through the County Well Index database described below. Soil borings are test holes drilled to obtain information about the physical properties of subsurface materials for engineering, mapping, or exploration purposes. They are logged by an engineer or a geologist using a variety of classification schemes based on particle sizes, penetration rate, moisture content, and color. The Minnesota Department of Transportation soil-boring data used for this project are concentrated mainly along Interstate Highway 90 and Minnesota Highway 23. These data are most useful in determining the composition of unconsolidated deposits. Descriptions of the geologic materials penetrated are interpreted by Minnesota Geological Survey geologists for mapping purposes. The data are available from the Minnesota Department of Transportation, Foundations Unit web page (https://www. dot.state.mn.us/materials/foundations.html). Giddings probe holes are borings of glacial materials, 2 inches (5.1 centimeters) in diameter, collected by Minnesota Geological Survey staff using a truck-mounted hydraulic auger. A description was generated at every site and samples were taken for textural analyses at most locations. Samples were generally taken about every 5 feet (1.5 meters), at unit contacts, or where the geologist believed it was important. OTHER INFORMATION Field sites are natural and artificial exposures of unconsolidated Quaternary deposits that were described in detail; samples from many sites were texturally analyzed. Field sites include stream and river cuts, gravel pits, excavations, and road cuts. Textural analyses express the proportion of sand-, silt-, and clay-size particles that make up a sample. The samples analyzed were taken from natural and artificial exposures, Giddings holes, and the rotary-sonic cores. They are helpful in determining the origin, correlation, and hydrologic properties of unconsolidated sediments. Statewide gravity and aeromagnetic (airborne-magnetic) data that are available at the Minnesota Geological Survey are used to assist mapping Precambrian geology. The utility of these two datasets relies on the strong contrasts in density and magnetization that commonly exist between various Precambrian rocks. In Rock County, Precambrian rocks either form the bedrock surface or directly underlie a variably thick cover of Cretaceous strata. The aeromagnetic data in Rock County were acquired 660 feet (201 meters) above ground along flight lines spaced 0.3 mile (0.5 kilometer) apart, and are compiled into a 328-foot (100-meter) grid (line data are not indicated on the map due to high density). The gravity data are based on ground stations that are spaced 0.25 to 1.00 mile (0.4 to 1.6 kilometers) apart along roads and drivable trails, and these data are compiled into a 3,281-foot (1,000-meter) grid. Derivative-enhanced grids of the gravity and aeromagnetic data are used in conjunction with information from bedrock wells. Passive seismic soundings provide information based on measurement and analysis of ambient shear- and surface-wave energy in the ground that allow a calculation of the depth to bedrock (thickness of unconsolidated deposits) beneath that point. The measurements employ a recording seismograph system that is implanted into the ground surface and records ambient ground vibrations in three orthogonal directions (2 horizontal and 1 vertical) during a 16-minute interval. The method is called passive because no energy is directly input into the ground at the time of measurement, such as is the case for conventional seismic sounding. Instead, the unit measures background vibrations from a variety of natural and artificial sources that include machinery, traffic, and wind. The averaged horizontal spectra of the seismic noise is divided by the corresponding vertical spectrum, and the resulting Horizontal to Vertical Spectral Ratio (HVSR) should display a prominent peak that closely approximates the resonant frequency (shear wave) of the unconsolidated sequence overlying the bedrock surface (Chandler and Lively, 2014). In general, depth to bedrock calculated by this method can have errors up to 25 percent of the depth; however, in areas with little to no other data, this is still useful information and many soundings can be inexpensively collected over a short period of time with one or two operators. Eighty-eight passive seismic stations were acquired in Rock County as part of this Geologic Atlas. Seismic refraction soundings measure the time required for sound or pressure waves to travel from a source through the subsurface and be refracted back to a receiver. Travel time can be correlated with the density and rigidity of the geologic material. Precambrian and Paleozoic rocks commonly exhibit pressure-wave velocities that are 1.5 to 2.5 times those of unconsolidated Quaternary sediments below the water table. The spacing of the receivers (geophones) and the arrival times (measured in milliseconds) are used to calculate the depth to bedrock. Seismic refraction soundings are more labor intensive than passive seismic soundings, but can provide velocity information and higher-quality depth estimates. Fourteen seismic refraction soundings were previously acquired in Rock County as part of earlier investigations, and two additional soundings were acquired as part of this Geologic Atlas. The seismic refraction data were acquired by the Minnesota Department of Natural Resources. Bedrock outcrops are exposures of solid rock at the land surface. There are numerous naturally occurring outcrops in the northern half of Rock County. They serve as reference points for mapping and for checking the accuracy of subsurface data. Bedrock at or near the surface must be considered in land-use planning decisions such as pipeline routing, sewage- system design, and excavation. DATABASE MANAGEMENT All of the data shown on the map were plotted on maps at 1:24,000 or finer resolution and assigned inventory numbers. Included with the Rock County Atlas are regularly updated databases containing digital GIS files, developed to provide easy access and rapid retrieval of these site-specific data. Computer storage and retrieval systems are ideal for manipulating large amounts of data because automated geologic databases can be designed to interact with other computer files, such as land-use data. Such interaction permits more efficient assessment of cause-and-effect relationships concerning natural resources than is commonly possible with manual files. DIGITAL DATABASES Computerized files were developed for point-source data such as wells and borings in Rock County. They use Public Land Survey descriptions, Universal Transverse Mercator (UTM), and latitude-longitude coordinates as location criteria; thus, they are compatible with the natural-resource databases housed at the Minnesota Land Management Information Center (LMIC). One computerized database developed for Rock County by the Minnesota Geological Survey is the County Well Index (CWI). County Well Index (CWI)—Information from water-well records and other subsurface drill-hole data such as exploration holes is entered into this statewide database. Each well log is assigned a six-digit unique number and each exploration drill hole is assigned either a five- or six-digit unique number. These reference numbers are also used by state agencies and the Water Resources Division of the U.S. Geological Survey. Elevations are expressed in feet above sea level and were determined from either topographic maps (see the index to 7.5-minute quadrangles) or Minnesota's lidar high-resolution elevation dataset. Elevations from the topographic maps are generally accurate to plus or minus 5 feet (1.5 meters), and the lidar elevations are generally accurate to within 3 feet (1 meter). The street address of each well is also included wherever possible to provide data users with a well-location system that is compatible with local regulatory programs. Software at the Minnesota Geological Survey is used to display and tabulate many of the data elements contained on the original well log. The program also denotes if there are additional data such as borehole geophysical logs, cutting samples, or core for an individual record. The County Well Index is stored in a database that consists of ten related tables. These tables contain information such as well depths, well construction, addresses, aquifers, dates drilled, static water levels, and pumping-test data. They also contain alternate well identifiers such as permit numbers or emergency-service numbers, and the well stratigraphy (the geologic materials encountered during drilling). CWI application software developed by the Minnesota Department of Health provides two types of reports: WELL LOG contains all the information about the well as it was reported by the contractor (Fig. 1). There may also be additional location information, land-surface elevation, aquifer designation, and remarks about the drill holes. WELL STRATIGRAPHY contains the geologic log described by a well driller (Fig. 2). A geologist makes stratigraphic interpretations based on this description, and on her or his knowledge and understanding of the geology of Rock County. The interpretations are augmented in some cases by additional data sources, such as cuttings, borehole geophysical logs, or core. Only those drill holes with verified locations have stratigraphy assigned to them. Other digital data—Details about other types of data shown on this plate are available in attribute tables for individual GIS shapefiles included in the atlas products. These include information that is part of the Quaternary Data Index (QDI), an internal working database. The Quaternary Data Index contains information from field sites, rotary-sonic cores, soil borings, Giddings probe holes, soil auger holes, and textural analyses. Some detailed information associated with these data sources is not included in digital files that are part of this atlas. This includes descriptions of cutting samples, borehole geophysical logs, and additional information on passive seismic data. It is stored in various formats and can be obtained by request from the Minnesota Geological Survey. FUTURE DATA COLLECTION Additional geologic information is generated continuously as new water wells are drilled, construction activities expose more bedrock, or additional wells are tested for water quality. To address this, the library of information prepared for Rock County is flexible so that old data can be reevaluated in light of new information, and new forms of data can be added if required. The need to manage groundwater and other natural resources wisely will never become outdated. Future demands on these resources will require current data to assess the impacts. ACKNOWLEDGEMENTS The staff from the Rock County Soil and Water Conservation District Land Management Office contributed greatly to the development of the County Well Index database. We thank local water-well contractors and landowners for their valuable assistance. REFERENCE Chandler, V.W., and Lively, R.S., 2014, Evaluation of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for estimating the thickness of Quaternary deposits in Minnesota and adjacent parts of Wisconsin: Minnesota Geological Survey Open-File Report 14-01. ENVIRONMENT AND NATURAL RESOURCES TRUST FUND

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Page 1: DATABASE MAP - University of Minnesota

Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however, the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Minnesota Geological Survey in St. Paul. In addition, effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification.

MINNESOTA GEOLOGICAL SURVEYHarvey Thorleifson, Director

DATABASE MAP

By

Margeurite C. Pettus and V.W. Chandler

2020

LOCATION DIAGRAM

COUNTY ATLAS SERIESATLAS C-47, PART A

Rock County Plate 1—Database Map

GEOLOGIC ATLAS OF ROCK COUNTY, MINNESOTA

Prepared and Published with the Support of

THE ROCK COUNTY BOARD OF COMMISSIONERS,THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, AND

THE MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUNDAS RECOMMENDED BY THE LEGISLATIVE-CITIZEN COMMISSION ON MINNESOTA RESOURCES

©2020 by the Regents of the University of MinnesotaThe University of Minnesota is an equal opportunity educator and employer

GIS compilation by R.S. LivelyEdited by Lori RobinsonDigital base modified from the Minnesota Department of

Transportation BaseMap data; digital base annotation by Corey J. Betchwars.Elevation contours were derived from the U.S. Geological Survey 30-meter Digital Elevation Model (DEM) by the Minnesota Geological Survey.Universal Transverse Mercator Projection, grid zone 151983 North American Datum

1 0 1 2 3 4 5 MILES

8 KILOMETERS

SCALE 1:100 000

1 0 1 2 3 4 5 6 7

CONTOUR INTERVAL 20 METERS

Figure 1. Example of a WELL LOG record, showing all the information about the well as reported by the well driller.

County

Well NameRange DirTownship Section Subsection

Well Depth Depth Completed Date Well Completed

Update DateRock

DNR OB 67007

47 W 35 AADDAC102

YesJOHNSON

489.00 455.00 1995/06/22ft ft

Make

Static Water Level

Open Hole(ft.) From

Last Strat

g.p.m.hrs. pumpting

Grouting Information

Capacity

Well disinfected upon completion?

Pump

Variance

Abandoned Wells

Screen

Nearest Known Source of Contamination

Well Contractor Cerfication

PLUCKER, D.

plastic

ü

ü

ü

Quad

Depth to Bedrock

Pumping Level (below land surface)

Model

22C

1385.00

34050

Cretaceous,undiff.

Pitless adapter manufacturer

ft.

Well grouted?

feet Direction Type

Printed on

Model number

Manufacture's name

REPORT

Type

Wellhead Completion

ü

217.00

MINNESOTA DEPARTMENT OF HEALTH

MINNESOTA STATUTES CHAPTER 1031WELL AND BORING RECORD

Description From To (ft.)Color HardnessCLAY, ROCK & SAND GRAY 10

DRIFT & SAND BLACK 21

SOIL BLACK 52

SANDY CLAY BROWN 105

SAND 3210

CLAY SOFT GRAY 12132

CLAY, SAND LT. GRY 158121

WOOD, CLAY, LITTLE SAND 161158

FINE SAND, SHALE BLACK 180161

CLAY HARDGRAY 217180

CLAY, LAYER ROCK 220217

SHALE BLACK 245220

SHALE DK. GRY 335245

SAND 340335

CLAY 341340

SAND, VERY FINE 349341

CLAY 354349

SAND 363354

CLAY BLACK 365363

CLAY DK. GRY 376365

SAND 376376

CLAY 387376

SANDY CLAY 389387

SAND 400389

SANDY CLAY 408400

TIGHT CEMENTED SAND 450408

CLAY DARK 458450

SAND STREAK 460458

CLAY 489460

TOP ROCK 489489

Remarks DNR OB WELL 67007.

HE-01205-07 (Rev. 2/99)Date

Lic. or Reg No.

Thein Well Co.

Was a variance granted from the MDH for this well?

Does property have any not in use and not sealed well(s)?

6/1/2018

dellatsnI etaDdellatsnI toN

Type

Length of drop pipe

HP Volts

g.p.m

To6.132.00 in. from to 410.000.00 ft. lbs/ft

2.00 45 410 45510 ft. to ft.

to

neat cement 400.0 4.00oTmorFlairetaM ft. Cubic yards

Hole Diameter (in.)

Elevation

At-grate (Environmental Wells and Borings ONLY)

Casing Protection 12 in. above grade

Basement offset

üYES NO

YES NOü

YES NO

YES NO

ft. after

Date measured

Material

Diamter Length SetSlot

0.00

2016/05/062017/12/23

Entry Date

Received Date

Unique Well Number

County Well Index v.5

Aquiferft.

First Bedrock KSRCPESX

Field Locatedft.

Quad Id

MNDNR

565831 Valley Springs

License Business Name

Name of Driller

Diameter 2 Depth 410Casing Type Plastic Drive Shoe? YES ü NO

Use observation well

BentoniteDrilling Fluid Well Hydrofractured? YES NO

From ft. to

Drilling Method Non-specified Rotary

DrillholeAngle

NOT SPECIFIED

Unique Well Number County Rock

riDegnaRemaN lleW Township Section Subsection Depth Drilled Depth Completed Lic/Reg. No.Date Completed

QuadQuad Id

DNR OB 67007 47 W 35 AADDAC102 489 455ft ft 340501995/06/22

Elevation 1385.00 ft. Screen 410-455Method LiDAR 1m DEM (MNDN Aquifer Cretaceous,undiff. Depth to Bedrock 217 ft.

Field LocatedProgramUni No.Verified

Location MethodInput SourceInput Date

Universal Transverse Mercator(UTM) - NAD83 - Zone 15 - MetersUTM Northing (Y)UTM Easting (X)

222935 48330512016/10/19

Valley Springs

2016/05/062017/12/23

Entry DateUpdate DateReceived Date

MINNESOTA DEPARTMENT OF HEALTHWELL AND BORING RECORD

MINNESOTA STATUTES CHAPTER 1031

Driller NamePLUCKER, D.

SWL

565831

Information from owner

Mn Department of Natural ResourcesMinnesota Geological Survey

Geological Material MinorSecondaryDEPTH

PrimaryELEVATION LITHOLOGY

Hardness StratigraphyFrom To Thick From ToColor

Dale Setterholm MGS Geologic study 1:24k to 1:100kycnegAnoitaterpretnI cigoloeG Interpretation Method

22C

CLAY, ROCK & SAND GRAY 1 man-made fill fill1 1385 13840

DRIFT & SAND BLACK 2 man-made fill fill1 1384 13831

SOIL organic depositsBLACK 5 Recent deposit-black soil3 1383 13802

SANDY CLAY sandBROWN 10 clay+sand-brown clay5 1380 13755

SAND 32 sand sand22 1375 135310

CLAY GRAY SOFT 121 clay-gray clay89 1353 126432

CLAY, SAND sandLT. GRY 158 clay+sand-gray clay37 1264 1227121

WOOD, CLAY, LITTLE SAND sandclay161 clay+sand wood3 1227 1224158

FINE SAND, SHALE clayBLACK 180 clay+sand sand19 1224 1205161

CLAY GRAY HARD 217 clay-gray clay37 1205 1168180

CLAY, LAYER ROCK limestone220 Split Rock Creek shale3 1168 1165217

SHALE BLACK 245 Split Rock Creek shale25 1165 1140220

SHALE DK. GRY 335 Split Rock Creek shale90 1140 1050245

SAND 340 Split Rock Creek sandstone5 1050 1045335

CLAY 341 Split Rock Creek shale1 1045 1044340

SAND, VERY FINE 349 Split Rock Creek sandstone8 1044 1036341

CLAY 354 Split Rock Creek shale5 1036 1031349

SAND 363 Split Rock Creek sandstone9 1031 1022354

CLAY BLACK 365 Split Rock Creek shale2 1022 1020363

CLAY DK. GRY 376 Split Rock Creek shale11 1020 1009365

SAND 376 Split Rock Creek sandstone0 1009 1009376

CLAY 387 Split Rock Creek shale11 1009 998376

SANDY CLAY sandstone389 Split Rock Creek shale2 998 996387

SAND 400 Split Rock Creek sandstone11 996 985389

SANDY CLAY sandstone408 Split Rock Creek shale8 985 977400

TIGHT CEMENTED SAND 450 Split Rock Creek sandstone42 977 935408

CLAY DARK 458 Split Rock Creek shale8 935 927450

SAND STREAK 460 Split Rock Creek sandstone2 927 925458

CLAY 489 Split Rock Creek shale29 925 896460

TOP ROCK 489 Sioux Quartzite quartzite0 896 896489

Figure 2. Example of a WELL STRATIGRAPHY record, which contains a geologist's interpretation of the geologic materials listed by the driller in the WELL LOG record (Fig. 1). Additional downhole information for this well (as noted in the Interpretation Method on the record above) controls the geologist's interpretation, which may not match the driller's description of the geologic material penetrated.

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Diamond drill core sample

Rotary-sonic core sample

Cutting sample

Borehole geophysical log

Soil boring

Giddings probe hole

Field site

Textural analysis

Gravity data

Passive seismic sounding

Seismic refraction sounding

Bedrock outcrop

MAP SYMBOLS

Note: More than one symbol can occur at the same location

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T. 104 N.

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INTRODUCTION

The public health and economic development of Rock County are directly dependent on the wise use and management of its land and water resources. Geologic and hydrologic information are essential before decisions are made that affect natural resources. Although the amount of geologic information required for making specific decisions can vary, the information will not be used if it is unavailable when needed, or if it is available only in a highly technical form, or scattered in many different maps and reports. The databases described here address this need.

County atlases, prepared jointly by the Minnesota Geological Survey and the Minnesota Department of Natural Resources, present detailed geologic and hydrologic information in an interpretive as well as descriptive form. Maps and texts summarize basic geologic and hydrologic conditions at a county scale, and interpret these conditions in terms of the impacts of possible land- and water-use decisions

Several sources commonly provide information about an area or an individual property, but they may use different classification schemes to describe the same geologic materials. As a result, discrepancies in interpreting the data may arise or the different sources may appear to contradict each other. For example, water-well drillers may describe glacial till as "clay," but engineering records will describe it as "clayey sand." Both descriptions are acceptable for their original purpose of describing the physical attributes of the material. "Clay," the term used by well drillers, defines the general inability of the till to yield groundwater to a well. "Clayey sand," the term from the engineering record, defines the physical composition of the till relative to particle size and engineering properties. The geologist must take the analysis one step further and define the material in terms of how it formed rather than how it is to be used. In this example, till consists of an unsorted mixture of rock fragments ranging in size from clay to cobbles and boulders, and it is interpreted by the geologist to have been deposited directly by glacial ice. Understanding the process by which the material formed allows geologists to make predictions about what lies between and beyond data points.

All of the types of data described on this plate were interpreted by geologists or hydrogeologists to make them meaningful for mapping purposes. The 1:100,000 scale of the maps in this atlas was chosen because it shows the geologic and topical studies of the county while keeping the physical size of each plate to a manageable level. As a result, some detailed information that was gained by data interpretation and mapping cannot be shown on these maps or discussed in the texts. Some of this information is available in digital files that accompany the atlas.

Whether to use the atlas alone, or in combination with the databases, depends on the amount of detail needed. Generally, database information must be used to evaluate site-specific conditions.

THE DATABASE MAP

The types, locations, and density of information used to prepare the Rock County Atlas are shown on this map. The data are described below to aid the user in assessing what types may be useful for a particular information need. The Database Map serves as a guide to the precision of the other maps in the atlas. It shows where data are sparse or lacking and interpretation and extrapolation were required to prepare the maps.

DRILL-HOLE INFORMATION

A record of water-well construction (well driller's log) is a water-well contractor's description of the geologic materials penetrated during drilling and the construction materials used to complete the well. Not all wells drilled need to extend to bedrock. In areas of thick, unconsolidated Quaternary deposits such as Rock County, drillers commonly do not need to drill through the entire thickness of overburden to find sufficient groundwater. Hydrologic data, such as the static water level and test-pumping results, are commonly included. Before any driller's log can be used, the location of the well must be verified, and a geologist must interpret the log. Driller's logs are the primary source of subsurface geologic and hydrologic data for Rock County; about 600 logs were used for this atlas.

Scientific investigation holes are drilled by various agencies to study the nature of the subsurface material. Geologic logs were generated that describe the material penetrated for each hole. In Rock County, the U.S. Geological Survey, Water Resources Division, drilled a number of soil auger holes, test wells, and observation wells to determine the hydrologic properties of local and regional aquifers. Study results and conclusions may be found in U.S. Geological Survey published reports.

Core samples were collected by the Minnesota Geological Survey at three sites in Rock County as a means to establish the nature of the subsurface material. One bedrock core sample was collected by using a diamond bit rotating at the end of a drill rod. During this drilling method, a column of rock moves up the drill pipe and is recovered at the surface for study. Rotary-sonic cores were collected for this project from two sites in the county to aid the interpretation of the Quaternary deposits (labelled RKR-1 and RKR-2). The coring technique enables recovery of a continuous core, 3.5 inches (8.9 centimeters) in diameter, from glacial deposits and bedrock (where intersected). It provides excellent subsurface samples for detailed study and comparison with cuttings, geophysical logs, and driller's logs from surrounding sites. Detailed geologist's logs for both of the rotary-sonic cores are shown on Plate 4, Quaternary Stratigraphy. When studies are complete, the core are stored at the Drill Core Library in Hibbing (the state repository for bedrock and earthen material samples collected during exploration, engineering, and geoscience research programs across Minnesota, maintained by the Minnesota Department of Natural Resources).

Cutting samples collected during drilling provide physical evidence of subsurface geologic materials. Cuttings are the samples generated as the drill bit cuts through the subsurface material and are used to interpret and verify driller's logs. They are logged and stored at the Minnesota Geological Survey. Cuttings from the Wildcat Oil exploration hole in Kanaranzi Township (T. 101 N., R. 44 W., sec. 15) are stored at the Drill Core Library in Hibbing.

Borehole geophysical logs are collected by Minnesota Geological Survey staff, other government agencies, and private entities by lowering instruments down a well or drill hole and measuring the physical and chemical properties of the geologic materials through which the hole passes. Different logging techniques measure naturally occurring gamma radiation, spontaneous potential, and resistivity. Gamma logs characterize in graphic form the geologic formations penetrated. Spontaneous potential and resistivity are mainly used to locate water levels in wells and the depth of the well casing. An interpretive log is prepared by a geologist from the geophysical log and correlated with drilling samples from the same hole, information obtained from nearby outcrops, or a geophysical log from a nearby drill hole. Geophysical logs can provide high-quality subsurface geologic and hydrologic information for wells that have little or no other information available. Logs can be viewed at the Minnesota Geological Survey or accessed online (http://z.umn.edu/borehole_geo). The geologic interpretations are available through the County Well Index database described below.

Soil borings are test holes drilled to obtain information about the physical properties of subsurface materials for engineering, mapping, or exploration purposes. They are logged by an engineer or a geologist using a variety of classification schemes based on particle sizes, penetration rate, moisture content, and color. The Minnesota Department of Transportation soil-boring data used for this project are concentrated mainly along Interstate Highway 90 and Minnesota Highway 23. These data are most useful in determining the composition of unconsolidated deposits. Descriptions of the geologic materials penetrated are interpreted by Minnesota Geological Survey geologists for mapping purposes. The data are available from the Minnesota Department of Transportation, Foundations Unit web page (https://www.dot.state.mn.us/materials/foundations.html).

Giddings probe holes are borings of glacial materials, 2 inches (5.1 centimeters) in diameter, collected by Minnesota Geological Survey staff using a truck-mounted hydraulic auger. A description was generated at every site and samples were taken for textural analyses at most locations. Samples were generally taken about every 5 feet (1.5 meters), at unit contacts, or where the geologist believed it was important.

OTHER INFORMATION

Field sites are natural and artificial exposures of unconsolidated Quaternary deposits that were described in detail; samples from many sites were texturally analyzed. Field sites include stream and river cuts, gravel pits, excavations, and road cuts.

Textural analyses express the proportion of sand-, silt-, and clay-size particles that make up a sample. The samples analyzed were taken from natural and artificial exposures, Giddings holes, and the rotary-sonic cores. They are helpful in determining the origin, correlation, and hydrologic properties of unconsolidated sediments.

Statewide gravity and aeromagnetic (airborne-magnetic) data that are available at the Minnesota Geological Survey are used to assist mapping Precambrian geology. The utility of these two datasets relies on the strong contrasts in density and magnetization that commonly exist between various Precambrian rocks. In Rock County, Precambrian rocks either form the bedrock surface or directly underlie a variably thick cover of Cretaceous strata. The aeromagnetic data in Rock County were acquired 660 feet (201 meters) above ground along flight lines spaced 0.3 mile (0.5 kilometer) apart, and are compiled into a 328-foot (100-meter) grid (line data are not indicated on the map due to high density). The gravity data are based on ground stations that are spaced 0.25 to 1.00 mile (0.4 to 1.6 kilometers) apart along roads and drivable trails, and these data are compiled into a 3,281-foot (1,000-meter) grid. Derivative-enhanced grids of the gravity and aeromagnetic data are used in conjunction with information from bedrock wells.

Passive seismic soundings provide information based on measurement and analysis of ambient shear- and surface-wave energy in the ground that allow a calculation of the depth to bedrock (thickness of unconsolidated deposits) beneath that point. The measurements employ a recording seismograph system that is implanted into the ground surface and records ambient ground vibrations in three orthogonal directions (2 horizontal and 1 vertical) during a 16-minute interval. The method is called passive because no energy is directly input into the ground at the time of measurement, such as is the case for conventional seismic sounding. Instead, the unit measures background vibrations from a variety of natural and artificial sources that include machinery, traffic, and wind. The averaged horizontal spectra of the seismic noise is divided by the corresponding vertical spectrum, and the resulting Horizontal to Vertical Spectral Ratio (HVSR) should display a prominent peak that closely approximates the resonant frequency (shear wave) of the unconsolidated sequence overlying the bedrock surface (Chandler and Lively, 2014). In general, depth to bedrock calculated by this method can have errors up to 25 percent of the depth; however, in areas with little to no other data, this is still useful information and many soundings can be inexpensively collected over a short period of time with one or two operators. Eighty-eight passive seismic stations were acquired in Rock County as part of this Geologic Atlas.

Seismic refraction soundings measure the time required for sound or pressure waves to travel from a source through the subsurface and be refracted back to a receiver. Travel time can be correlated with the density and rigidity of the geologic material. Precambrian and Paleozoic rocks commonly exhibit pressure-wave velocities that are 1.5 to 2.5 times those of unconsolidated Quaternary sediments below the water table. The spacing of the receivers (geophones) and the arrival times (measured in milliseconds) are used to calculate the depth to bedrock. Seismic refraction soundings are more labor intensive than passive seismic soundings, but can provide velocity information and higher-quality depth estimates. Fourteen seismic refraction soundings were previously acquired in Rock County as part of earlier investigations, and two additional soundings were acquired as part of this Geologic Atlas. The seismic refraction data were acquired by the Minnesota Department of Natural Resources.

Bedrock outcrops are exposures of solid rock at the land surface. There are numerous naturally occurring outcrops in the northern half of Rock County. They serve as reference points for mapping and for checking the accuracy of subsurface data. Bedrock at or near the surface must be considered in land-use planning decisions such as pipeline routing, sewage-system design, and excavation.

DATABASE MANAGEMENT

All of the data shown on the map were plotted on maps at 1:24,000 or finer resolution and assigned inventory numbers. Included with the Rock County Atlas are regularly updated databases containing digital GIS files, developed to provide easy access and rapid retrieval of these site-specific data.

Computer storage and retrieval systems are ideal for manipulating large amounts of data because automated geologic databases can be designed to interact with other computer files, such as land-use data. Such interaction permits more efficient assessment of cause-and-effect relationships concerning natural resources than is commonly possible with manual files.

DIGITAL DATABASES

Computerized files were developed for point-source data such as wells and borings in Rock County. They use Public Land Survey descriptions, Universal Transverse Mercator (UTM), and latitude-longitude coordinates as location criteria; thus, they are compatible with the natural-resource databases housed at the Minnesota Land Management Information Center (LMIC). One computerized database developed for Rock County by the Minnesota Geological Survey is the County Well Index (CWI).

County Well Index (CWI)—Information from water-well records and other subsurface drill-hole data such as exploration holes is entered into this statewide database. Each well log is assigned a six-digit unique number and each exploration drill hole is assigned either a five- or six-digit unique number. These reference numbers are also used by state agencies and the Water Resources Division of the U.S. Geological Survey. Elevations are expressed in feet above sea level and were determined from either topographic maps (see the index to 7.5-minute quadrangles) or Minnesota's lidar high-resolution elevation dataset. Elevations from the topographic maps are generally accurate to plus or minus 5 feet (1.5 meters), and the lidar elevations are generally accurate to within 3 feet (1 meter). The street address of each well is also included wherever possible to provide data users with a well-location system that is compatible with local regulatory programs. Software at the Minnesota Geological Survey is used to display and tabulate many of the data elements contained on the original well log. The program also denotes if there are additional data such as borehole geophysical logs, cutting samples, or core for an individual record.

The County Well Index is stored in a database that consists of ten related tables. These tables contain information such as well depths, well construction, addresses, aquifers, dates drilled, static water levels, and pumping-test data. They also contain alternate well identifiers such as permit numbers or emergency-service numbers, and the well stratigraphy (the geologic materials encountered during drilling).

CWI application software developed by the Minnesota Department of Health provides two types of reports:

WELL LOG contains all the information about the well as it was reported by the contractor (Fig. 1). There may also be additional location information, land-surface elevation, aquifer designation, and remarks about the drill holes.

WELL STRATIGRAPHY contains the geologic log described by a well driller (Fig. 2). A geologist makes stratigraphic interpretations based on this description, and on her or his knowledge and understanding of the geology of Rock County. The interpretations are augmented in some cases by additional data sources, such as cuttings, borehole geophysical logs, or core. Only those drill holes with verified locations have stratigraphy assigned to them.

Other digital data—Details about other types of data shown on this plate are available in attribute tables for individual GIS shapefiles included in the atlas products. These include information that is part of the Quaternary Data Index (QDI), an internal working database. The Quaternary Data Index contains information from field sites, rotary-sonic cores, soil borings, Giddings probe holes, soil auger holes, and textural analyses. Some detailed information associated with these data sources is not included in digital files that are part of this atlas. This includes descriptions of cutting samples, borehole geophysical logs, and additional information on passive seismic data. It is stored in various formats and can be obtained by request from the Minnesota Geological Survey.

FUTURE DATA COLLECTION

Additional geologic information is generated continuously as new water wells are drilled, construction activities expose more bedrock, or additional wells are tested for water quality. To address this, the library of information prepared for Rock County is flexible so that old data can be reevaluated in light of new information, and new forms of data can be added if required. The need to manage groundwater and other natural resources wisely will never become outdated. Future demands on these resources will require current data to assess the impacts.

ACKNOWLEDGEMENTS

The staff from the Rock County Soil and Water Conservation District Land Management Office contributed greatly to the development of the County Well Index database. We thank local water-well contractors and landowners for their valuable assistance.

REFERENCE

Chandler, V.W., and Lively, R.S., 2014, Evaluation of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for estimating the thickness of Quaternary deposits in Minnesota and adjacent parts of Wisconsin: Minnesota Geological Survey Open-File Report 14-01.

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