Geological Map Interpretation

Outcrops

Where rocks meet the surfaceShape of outcrop depends on the shape of the

surface and shape of structure

Conours

• a curve along which the function has a constant value

• Topographic/ elevation Contours: Lines of equal elevation

• Never crosses each other

Outcrop Patterns

• Horizontal Beds: have outcrop that follow the contours

Outcrop Patterns – Cont’d

• Vertical Beds: have straight outcrop that ignore the contours

Outcrop Patterns – Cont’d

• Dipping Beds: have curved outcrop that cut across the contours

Outcrop Patterns – Cont’d

• Dip Direction: is recognized by the V in Valley Rule: an outcrop of a dipping rock bends round a V shape where it crosses a valley, and the V of the outcrop points (like an arrowhead) in the direction of dip, regardless of the direction of valley slope and drainage

• The rule does not apply in areas of low dip, where outcrops nearly follow contours, so point upstream. On level ground, dipping beds have straight outcrops along the direction of strike

Outcrop Patterns – Cont’d

• Succession is recognized by younger rocks coming to outcrop in direction of dip.

• Conversely, if succession is known, the dip is in the direction of younger outcrops – the easiest way to recognize dip on most maps

Outcrop Patterns – Cont’d

• Width of outcrop is greater at lower dips – and on thicker beds

Recognition of Structures

• Unconformity is recognized where one outcrop (of a younger bed) cuts across the ends of outcrops of older beds

Recognition of Structures

• Faults are usually marked and keyed on maps. They may cut out, offset or repeat outcrops of beds. Fault dip is recognized by V in Valley Rule. is recognized where one outcrop (of a younger bed) cuts across the ends of outcrops of older beds

Recognition of Structures

• Folds are recognized by changes in dip direction, and also by outliers and inliers not due to topography.

• Most important, folds are recognized by bends in outcrop: any outcrop bend must be due to either a fold or a topographic ridge or valley.

Drawing a Cross-Section

• A cross-section is drawn by projecting the data from a single line on the map onto a profile of the same scale

• The topography and each geological boundary are constructed individually from the relevant contours, whose intersections on the section line are projected to their correct altitudes on the profile