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a
Science
The Transactions
o f the British
Cave Research
ssociation
BeR
I
o lume
15
Number 1
Apr
i
I
1 9 8 8
Sed iment Pa leomagne t i sm
at L langa t twg
Natura l Tunnel
Vi rg in ia
Tufas and
Traver t i nes
f rom Yorksh i re
Pigments in ave
Organisms
Survey
Reduc t i on by
Least
Squares
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aveScience
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ave
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TRANSACTIONS OF THE
BRITISH CAVE
RESEARCH ASSOCIATION
Volume 15
Number
1
Apr i l
1988
Contents
Paleomagnetism
of Cave
Sediments
from Mynydd Llangat twg
Mark
Noel
Natura l Tunnel Virg in ia
Tony Waltham
P o s tg l a c i a l
Tufas
and
Traver t ines from
the
Craven
D i s t r i c t
o f Yorkshi re
Al lan Pen tecos t
and
Tom Lord
Pigmenta ry Development of Cavernicolous Organisms
Mark F Chapman
The
Least
Squares
Method
of Cave Surveying
Data
Reduct ion
based on a Micro
Computer
S Kel ly
and
P B
Warren
Forum
Cover: The
mass ive f o s s i l t runk passage o f
Qian Dong
in the
Bama kar s t of western Guangxi
China.
By J e r r y
Wooldridge.
This
i s
one
of
the
many spec tacu la r
photographs of the Guangxi caves
and
kar s t in the
r e p o r t
o f
the
1987/88 phase of the China Caves P r o j e c t
a v a i l a b l e
t h i s summer fo r £4 inc lud ing
UK
postage from
BCRA
S a l e s .
Edi to r : Dr T . D. Ford Geology
Dept .
Leices te r Univers i ty Leices te r LEI 7RH
3
11
15
21
9
35
Product ion Edi to r : Dr. A.C. Waltham
Civ .
Eng.
Dept .
Tren t
Poly techn ic
Nottingham
NGI
4BU
Cave Science
i s pub l i shed by the Br i t i sh Cave Research Assoc ia t ion and i s i s sued to
a l l
pa id
up members
of
the
Assoc ia t ion
1988
subsc r ip t ion r a t e s a re :
Ind iv idua l £10.00 I n s t i t u t i o n or Club
£12.50
Membership
Secre ta ry :
N.
Br iggs 68 P o i t i e r s Road Cheylesmore
Coventry
CV3
5JX
Ind iv idua l cop ies and
back numbers
of
Cave
Science a re ob ta inab le from:
B.C.R . A. S a l e s 20 Woodland Avenue Westonzoyland Br idgewate r
Somerset
TA7 OLQ.
The
permanent address
fo r
B.C.R.A.
i s : B.C.M.-B.C.R.A. London
WClN
3XX
Copyr i gh t t he B r i t i s h Cave Research
Assoc ia t ion
1988. No pa r t
of
t h i s publ ica t ion may be
reproduced in
any othe r
pub l i c a t i on used
in
a dve r t i s i ng s tored in
an
e l e c t r on i c
r e t r i eva l
sys tem
or o therwise
use d
fo r
commercial
purposes
wi thou t
the pr io r
wr i t t e n
consen t of the
au thors
and
of the Assoc ia t ion .
ISSN
0263-760X
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C VE SCIENCE
Vol. 15, N o . 1 ,
Apri l 1988
Transact ions of the Bri t i sh
Cave
Research Associat ion
Paleomagnetism
of ave Sediments from Mynydd Llangattwg
Mark NOEL
Abstract : The palaeomagnetic
study of
Br i t i sh cave
depos i t s began
with
an
examination of sediments
within Mynydd
L1angattwg,
Powys.
This paper
reviews
the
r esu l t s
obtained from a t o t a l of 77
specimens col lec ted in
the
Agen
Allwedd and Pwll y Gwynt cave systems . The evidence suggests
that
these
s i l t s
and clays
acquired
a s tab le remanent
magnet isat ion
through the
alignment
of
magnetic minerals ,
by
the geomagnetic f ie ld , dur ing
deposi t ion.
Similar i t ies in remanence di rec t ions within
the Cap Mud'
a t
f ive
s i t e s in
Agen Allwedd suggests
t ha t
deposi t ion of th i s
uni t was
simultaneous throughout the cave dur ing a per iod of normal
geomagnetic
polar i ty .
A
sediment
secton
near the top of the
5th
Aven in Pw11 y Gwynt
includes
a
clay
hor izon wi th reversed magnet isat ion
implying
that
these
sediments
record
e i ther
a recen t
geomagnetic
excursion or
f ie lds
dur ing the
reversed Matuyama chron (>720 Ka). Magnetic
fabr ic data provide
evidence
fo r di rec t ions of water currents and mass f low
in
these cave
sediments
INTRODUCTION
Sediments formed
by
slow deposi t ion in water
often contain a weak remanent
magnet isat ion.
This deposi t ional remanent
magnet isat ion
can
therefore
provide a record of
geomagnetic
secular
var ia t ion extending
beyond
his tor ica l observat ions.
This concept
has been
applied to palaeomagnetic
studies
of
glacial varved sediments
from
Scandinavia and North
America
(Noel 1975;; Johnson
e t a l . 1948). Lake and marine sediments (Creer
1974, 1981) and
surface
t e r r e s t r i a l depos i t s
(eg.
Heller Tung-Sheng 1982). These s tudies have
provided important
palaeomagnetic
cons t ra in t s on
models for the geomagnetic dynamo; they have led
to the dat ing and cor re la t ion
of
Quaternary
sediments (eg. Creer
1982)
and
they
have helped to
re f ine and ver i fy
a
reference curve for
archaeomagnetic
dating
(Aitken 1974).
Due to the
glacial
erosion which af fec ted
many lake basins dur ing the Devensian and the
problems
of
re t r i ev ing
longer core samples,
t
has
proved di f f i cu l t to extend exis t ing lake
sediment
magnetostrat igraphies much beyond 13000 yr BP.
Although marine sediments
have
the potent ia l for
providing a much longer t imesca le ,b io turba t ion and
slow sedimentat ion
rates .severely
l imi t the
resolut ion
a t ta inable with t h i s
material ,
Final ly, surface t e r r e s t r i a l deposi ts seldom
provide
the
long
sequence
of
unweathered mater ia l
which
is
a
prerequis i t e for
accurate
palaeomagnetic s tudies . Cave
sediments, on the
other
hand,
are usual ly preserved under favourable
condit ions of low weather ing, wi th l i t t l e
bioturba t ion
or
desiccat ion, par t i cu la r ly
in the
i n t e r io r zone. They therefore
have
a capabi l i ty
f o r deta i l ed palaeomagnetic studies of the
Quaternary geomagnetic f i e ld . Evidence from U/ Th
dating
of
assoc ia ted speleothems
suggests
that
cave
sediments,
a t
some
l oc a l i t i e s ,
may
have
ages
in
excess
of
350 Kyr BP (Atkinson
e t a l . 1978)
.
Cave sediment palaeomagnetic research has
been
slow to
develop largely due
to the
log is t i ca l
problems
of
sampling. The
potent ia l
of th i s
mater ia l
was f i r s t confirmed
in s tudies of
sediments
from
caves in
Spain,
Majorca
and the
Lebanon
(Creer
Kopper 1974, 1976; Kopper
Creer
1973, 1976) .
The ea r l i e s t invest igat ion
in
Bri ta in
concerned
rhythmites from the entrance to
Vic tor ia Cave, Yorkshire (Stober 1978; Homonko
1978) but
Agen Allwedd provided
the
f i r s t
palaeomagnetic
da ta
for sediments
from a
deep cave
environment (Noel e t a l . 1984; Noel 1986; Noel
St .
Pierre 1984; Noel
Bull
1982; Schmidt
1982).
There i s now evidence to suggest the
speleothems
may also
contain r e l i ab le
records
of the
geomagnetic
f i e ld (Latham e t
al . 1979).
This paper
summarises
the
palaeomagnetic
r esu l t s
obtained from
samples
of s lt
and
c lay
from f ive s i te s in Agen
Allwedd and
one
s i t e
in
Pwyll
y
Gwynt.
Further de ta i l s are
given in
Noel
3
(1983,
1986). The r esu l t s provide evidence
for
the
relat ive
ages of the deposi ts and for a
palaeocurrent
di rec t ion in Agen
Allwedd.
METHODS
Lightweight, robust sampling equipment was
developed to overcome the
special
di f f icu l t i e s of
sampling underground.
The ent i re
apparatus,
including a camera, can be car r ied in a standard
7.6cm
wide ammunition case
and weighs
approximately
2 kg.
A clean, hor izonta l or
ver t i ca l
sediment
surface
i s
f i r s t
prepared wi th a brass
scraper
blade. 2.54 x
2.54 cm
p las t i c tubes are then
pressed
into
the sediment using an angle
plate
f i t t e d with a bullseye sp i r i t l eve l . The
or ienta t ion
of
each
tube is then recorded in s i t u
and the
di rec t ion
of
down (or magnetic
nor th)
scr ibed on
the
sample tube.
The specimens
are
then
extracted,
trimmed and
placed
end
to
end
in
sealed s torage tubes
for
t ransport
to the
laboratory .
The natural remanent
magnet isat ion of
the
specimens is
measured
in
a
f luxgate spinner
magnetometer (Molyneux
1971). The r esu l t s
are
expressed
in
terms of
the magnitude
and di rec t ion
decl ina t ion ,
incl inat ion)
of the remanence
vector . The s t ab i l i ty of the magnet isat ion in
representa t ive samples i s then examined using
a l te rna t ing f i e ld
demagnet isat ion. For
fur ther
de ta i l s
of
th i s and other
laboratory techniques
the
reader
i s referred
to Coll inson
(1983). On
the bas is on the demagnetisation
t e s t s , each
sample i s next exposed to an a l te rna t ing magnetic
f ie ld
chosen to i so l a t e
the
primary
component
of
magnet isat ion in the
sediment. This
is
f inal ly
measured in the magnetometer.
The
magnetic
suscept ib i l i ty
anisotropy
(magnetic fabr ic ) of the
sediments has
also been
examined.
In
a sediment which contains the
mineral
magnet i te , the
magnetic fabr ic can provide
an
indicat ion of
the
degree
of prefer red
or ienta t ion
of
non-spherical
magnetic gra ins .
This or ienta t ion ar i ses
from the
in te rac t ion
of
magnetic
gravi ta t iona l
and
hydrodynamic forces
on
the se t t l ing
par t i c l es (Hamilton
Rees
1970).
Hence
the
s ty le
of the magnetic fabr ic can
be used
to
in fer
the di rec t ion and the
veloci ty
of flow
through a flooded passage
or
to
indicate
the
nature of gra in
in te rac t ions which occurred
dur ing
a mudflow.
The suscep t ib i l i t y
anisot ropy
i s measured in
modified spinner
magnetometer (Collinson 1983) .
This inst rument measures
the
di rec t iona l var ia t ion
of
suscept ib i l i ty within the specimen and
expresses the
resu l t s
in
terms
of
an
el l ipso id
of
suscept ib i l i ty specif ied
by the
magnitude
and
di rec t ion
of
three pr inc ipa l axes. These
are
general ly displayed on a stereographic projec t ion .
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R SULTS
Ogof
Agen
Allwedd
As par t
of
a prel iminary
study
in
1979;
t en
specimens
of
horizontal ly
bedded sur face
s lt
( Cap
Mud ) were obtained near the
northern
end of
Main
Passage
(Fig.
1) .
This f ine ly laminated s lt
was probably derived from surface
per ig lacial
weather ing
of
Mills tone
Grit
on Mynydd
Llangattwg
which then en te red
the flooded cave
via
ver t i ca l
jo in t s and f i ssures .
These samples
were found
to
contain
a
s tab le
magnetisat ion recording
a
period of Normal
geomagnetic f i e ld
polar i ty
(Noel e t a l . 1979).
The mean remanence inc l ina t ion ,
(40.7°; Fig.
2),
i s
somewhat shal lower than would be
expected for
geomagnetic secular
var ia t ion
a t th i s
l a t i t ude .
However, th i s
can
be
explained
in terms
of an
i n c l i n a t ion e r r o r ,
d,
which
ar ises
from
the
ro l l i ng
of
par t i c l es as
they
come to r es t on the
sediment
sur face
(King 1955).
A Primary Style
of
magnetic fabric
(Hamilton Rees 1970)
was
found
in
these
sediments. This
i s character ised
by
a
near-hor izonta l
magnetic
fo l i a t ion plane
(minimum
axes ver t i ca l ) . The maximum axes
def ine
a
l ineat ion which
i s deviated
away
from
the
remanence di rec t ion ,
implying that
water flow
down
the passage
has
been an important cont ro l on the
magnetic
fabr ic .
The r esu l t s
of
thermomagnetic
measurements on a magnetic
ext ract
from
the
Cap
Mud ,
(Fig.
3) , suggested t ha t
the
main magnetic
mineral
in
the
sediment
was magnet i te .
Encouraged
by these ear ly
r esu l t s , a fur ther
30
samples were obtained from three s i t e s
in
Main
Passage
and
a s i t e in
Keyhole Chamber, (Fig.
1;
Noel e t a l . 1981). Si
es
4 and 5 were
again
located on hor izonta l ly bedded Cap Mud but s i t e s
2 and 3
were
a t two levels on
the
f lanks
of
a
STREAM
N
STREAM
I
PASSAGE
0
25
5
SCALE : metres
Figure
1. The Agen
Allwedd
cave system showing
the
location
of the
sampling
s i tes .
Si te
5 was in the
upper level
of
Keyhole Chamber.
4
SITE 2
200 0 .
SITE 4
200
0 .
- " I
':
.
SITE 1
350 Oe
, ,
i
..
SITE 5
2000.
SITE 3
2 0 0 0 .
Figure
2.
Remanence
directions for the five
Agen
Allwedd
s i tes
after
par t i a l demagnetisation in the alternating
magnetic f ields shown.
steep
sediment connected
to
twin
ver t i ca l cracks
in
the western wal l of Main Passage, (Fig. 4).
The samples
contained
a
s tab le primary
component
of
magnetisat ion. Fig. 2 shows the
di rec t ions
of
remanence
af te r pa r t i a l
demagnet isat ion
in a peak al ternat ing f ie ld
of
200
Oe
(20 mT). The
magnetic fabric r esu l t s
are
shown
in
Fig.
5. The sediments
again contained
a
primary
depos i t ional
s ty le
of magnetic
fabr ic
but
the s i l t s on
the
f lanks
of the
cone
have
a fabric
re la ted to the s t r ike
and dip of the
sloping
surface.
The fo l ia t ion
planes are
t i l t e d in the
downslope di rec t ion , al though a t lower angles than
the
bedding,
thereby causing an imbricat ion in to
the s lope. The
maximum
axes
of
suscep t ib i l i t y are
scat tered within the
fo l ia t ion
plane a t s i t e 2 (8°
slope) but are
more t i gh t ly grouped a t s i t e 3 (29°
slope) . These r esu l t s
conform
c lose ly
with the
6
·c
5
z
0
4
n
i
w
z
( l
3
ct
C
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1( 1
?9
\
200 300
400
500
o
TEMPERATURE
C
600
700
Figure
3. Changes in the
in tensi ty of induced magnetisation
in a
maonetic
extract from
the
Cap Mud
during
a cycle of
heating-and cooling.
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7/40
Figure 4. Plan
vi
ew of the
sediment
cone
near
two
f issures
in the western wall of Ma i n
Passage wi
th
the
posi t ions of
s i t es 2 and 3. The
arrows
are
the
in ferred directions
of
current
flow
based on the
analysi s
of
magnet
i
remanence
and
suscept ibi l i ty anisotropy.
F
igure
S.
Directions
of the
three principal
axes of magnet i c
suscept ibi l i ty in the Cap Mud'
samples. The
dashed
l ines
i ndicate the downward direction
of bed slope .
=
maximum,
.. = intermediate
and
• = mi nimum
axes.
Closed
symbols = posit ive
i nclination ; open symbo ls
=
negative incl inat ion
m tr
' ,
::
.
: ;. :
.
:
+ ~ z
j
; f 0
- 0
0 - 0
Oem
,
: q , '
•
3
,
,
\
\
\
\
\
\
\
I
f indings
of Rees (1966) who
redeposi ted
medium
sands
onto varying
slopes in
the
laboratory.
The sediments a t s i t e s 4 and 5 contain two
or thogonal
l inea t ions
causing
a c lus te r ing of
maximum
and
intermediate di rec t ions
(Fig.
5).
This
unusual
r esu l t was
confirmed
using a low
f ie ld torque
magnetometer
and
is therefo re
not
instrumental
in
or ig in . Hence t
is
probable
that
these sediments
contain
two par t ic le l inea t ions ,
only the
dominant one of which can be resolved by
the
magnetometer . These l inea t ions
are
para l l e l
and
t ransverse
to the
current
flow
di rec t ions
as
defined
by the or ienta t ions
of
the passage
a t
the
two
s i t e s .
A
magnetic fabric
with
l ineat ion transverse
to
the
presumed flow
direct ion
has
been
reported
by Rees (1965) in Eocene sandstones which contain
two grain alignments and
by
Granar (1957) in
Swedish
varved clays. Grain or ienta t ion
perpendicular to the current has also been noted
by Hand
(1961)
.
I t
i s possible t ha t
these Cap
Muds
contain magnetite
grains
whose
shapes
enable them to take
up
one of two
hydrodynamically
s tab le
posi t ions
on a hor izon tal
bed.
The sets of remanence
vectors
for each s i t e
are
well grouped but there is a s igni f icant
disagreement
in
the mean di rec t ions
for
the
two
s i t e s on the cone (
Fig.
2). This
can
be explained
in
terms of the tendency for
s e t t l i ng
magnetic
grains
to ro l l systemat ical ly downslope
before
coming to
r es t thus causing a
ro tat ion
of the
f inal
remanence vector. From the r esu l t s of
laboratory deposit ion experiments in control led
magnetic
f ie lds ,
an
empir ical
correct ion
has been
derived for the '
bedding
er ror ' (Hamilton King
1964). When th i s correct ion i s applied to s i t e s 2
and 3 ,
(Fig.
6) , the mean
vectors
become
5
:-
0.
..... .
..
h
0
, .
, ,
4
3
. , If , ,
5
CURRENT
ROTATIONS
Figure 6.
Correction of
the s i tes 2 and 3 r emanence vectors
f or
bedding
error
fo
and incl ination
error I i
The re
su l t
i ng
vectors
are
then
onnecte
d to
the
estimated
f ie ld
direct ion
by small
ci
r cles (dotted)
which
indic ate the degree of
rotation due to
current
shear
ind is t ingu ishab le
(wi thin
sample
errors) from
the
palaeomagnetic di rec t ions a t other s i t e s in the
cave
(Fig. 7)
. This strongly
suggested
that
deposi t ion of these surface s i l t s has been
simultaneous throughout the cave .
By considering the
hyrdodynamic informat ion
contained
in
the
magnetic fabric
t
i s
possible
to
apply a fur ther
correct ion
to the remanence
vectors for the
ef fec t of water flow. For each
sample,
the
di rec t ions
of
maximum
suscept ib i l i t
y
and remanence are joined by a small c i r c le which
represents the ro ta t ion of grains, about
horizontal axes , due to
f lu id
shear
_ When the
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8/40
small c i r c le
for a su i te of samples are
extrapolated,
they
should converge on the
palaeofield direct ion . Further de ta i l s of th i s
method are
given
by Rees
(1961) and Noel
(1986).
In Fig.
8
th i s technique
i s
applied to s i t e
1. The zone of small c i r c le in te r sec t ions defines
a range of di rec t ions within which
t
i s assumed
the
palaeofield
vector
l ies .
In Fig.
6
th i s is
compared to the
mean
remanence di rec t ions for
s i
es
2 and 3 ( a f t e r correct ion for
incl inat ion
and
bedding
er rors ) . The remaining
angular
discrepancy
may
be
explained
by
current rotat ions
caused by f low over and around the
cone.
The
small c i r c les correspond to the cur rent ro ta t ions
shown,
indicat ing t ha t water flowed up the cone
and
into
the wall
f i ssure .
10
0
20
20
30
30
.0
50
- . 1
Figure 7 . Mean palaeomagnetic directions in sediments from
the five s i t es in Agen
Allwedd.
Corrections
have been
applied for bedding error a t s i t es 2
and 3.
Fi gure 8 . Est i mate of the t rue palaeomagnetic f ie ld
d
i r ec
t io n dotted box) by combining the remanence (circles)
and
maximum
suscept ibi l i t
y
directions (squares).
The solid
l i n es
re
s
ir le
s
SECTION
A S C
I I I
: SITE I I \ I I
I '
6
Ogof Pwll y Gwynt
Pwll y Gwynt i s a
small
cave 213m long)
developed
beneath
Mynydd Llangattwg
near the
upper
l imi t
of the Avonian l imestone sequence and about
8 m above Agen Allwedd (Fig . 9). The
cave
consis ts of a
ser ies
of narrow foss i l phreat ic
passages l inking
large
ver t ica l avens and
probably
represents
the
ea r l i e s t
stage
of
kars t development
in the area.
In common
with
Agen Allwedd (Bull 1976;
1981), Pwll y Gwynt contains an
extensive
sediment
i n f i l l
and
in
places
th i s
has been exposed
in
natu ral stream sect ions to
reveal
a complex
succession
of layered s i l t s and clays.
Fig.
10
shows an approximately
1m
thick sequence
of
sediments
deposi ted
in a
small passage leading
to
the
Fif th
Aven.
These sediments
occur a t
one
of
the highest
levels
in the
cave
and hence almost
cer tain ly predate the deposits in Agen
Allwedd.
The
lower hal f of
the
sect ion comprises a mud
flow, as
shown by the contorted
r e l i c t
laminat ions
and random in t r ac las t s of layered clay .
These
sediments are over la in by nine l i t ho log ica l ly
dis t inc t uni t s of laminated clay , stalagmite
and
homogenous
clay
with
sand. The
presence
of
desiccat ion cracks and the th in
stalagmite
layer
a t a depth of 12cm i s evidence for prolonged
breaks in sedimentation. A prel iminary
in terpretat ion of the sect ion suggests that these
sediments
record
changes
in
surface
cl imat ic
condit ions
which might be equated
with
a
g lac i a l
/ i n t e rg l ac i a l sequence. Unfortunately,
because of de t r i t a l contaminat ion, the speleothem
mater ia l was unsuitable for radiometr ic dating. A
minimum of seven samples
were
obtained from each
of
layers
2,
6
and
9
(Fig.
10). A
ver t ica l
su i t e
of
th i r teen
samples was
also
obtained from the
slump deposit .
The di rec t ions of remanent magnetisation
before
and af te r par t i a l
demagnet isat ion
in
al ternat ing magnetic f ie lds
are shown
in
Fig. 10.
Detai ls of the pi lo t
sample
behaviour dur ing
demagnetisation are given in Noel
(1986).
The
di rec t ions of maximum
and
minimum magnetic
suscep t ib i l i t y in layers 6, 9
and
10
are presented
in relat ion
to the
passage or ien tat ion in
Fig.
11.
The
in tensi ty
of the natural remanent
magnet isat ion
was
in
the
range
7
.7-4
2 . 2 x 10-
7
Am2 / Kg.
Normal
polar i ty
remanence vectors
were
found
in layers 2
and
6
but
a
reversed
polar i ty
magnet isat ion
was found
in layer
9.
Shallow
values of remanence
incl inat ion
again suggest the
presence of an i nc l ina t ion e r r o r .
The disturbed clay records an
i r regular
prof i le of
remanence
di rec t ion
(Fig.
12). With
the
exception
of the shal lowest sample, the
vectors are of
normal polar i ty , however,
and
within 38° of the present geomagnetic f ie ld.
Three
contrast ing s ty les
of
magnetic
fabric
are present in the Pwll y Gwynt sediment
(Fig.
N
I
10m
Fi
g
ur
e
9.
The
Pw
l l
y G
wyn
t c a
ve
s ystem
sh
owi ng the sampl i ng
lo c a t io n i n
the Fif
t h Aven .
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9/40
Figure 10. Stra
t igraph y
of
the
sediment
section in the
Fifth
Aven with
remanence
direct ion s
in layers 2 . 6
and
9 before
and
a f te r demagnetisation in the
f ie lds shown . Li t hologies a re
as fo l l ows:
1) s talagmite
;
2)
mottled blue-brown
clay
; ( 3)
dr y .
black
an d white
laminate
d
s
la y
; 4) clay and sand; 5)
s talagmite;
6)
br own
laminate
d
c l a
y;
7) dry . orange laminated
c l ay; 8)
clay
with sand
pa r t ings ; 9) laminated
clay;
(
10) slum
p with c la s ts o f
laminated
clay
. Present f i e l d
d ir e
c t i
on i s
ind icate
d by the
s tar .
DEPTH
em.
5
10
15
20
25
30
35
45
5 ~ ~
LAYER
.
o
NRM
.
.
.
:
>II
. .
o 0
A.F
.
7·5mT
-
.
.
:'
15mT
, .0,
+ .
o
o o ·
10mT
i
SEE
FIG.
~
5
100
11) . The suscept ib i l i ty anisotropy of layer 6
contains
a
l i ne a t i
on
para l l e l to the dip di rec t ion
of
the
clay
with
an
imbricat ion
which
i s
upslope
with respec t t o th e
bedding dip-20
0
.
A
similar
s ty le
of fabr ic is
seen
a t Agen
Allwedd
s i t e 3
Fig. 5 ) and in laboratory deposi t ion
experiments
Rees
1966).
A down
-d
ip l inea t ion i s also s e en
in
l ayer
9
Fig
. 11) .
0;'
The
slumped
sediments
contain a well-defined
hor izonta l
magnetic
l inea t ion in a di rec t ion
approximately para l l e l
to
the
passage
axis
Fig.
11).
The
minimum
suscep t ib i l i t y
axes sh ow a
gi rd led
dis t r ibu t ion .
since magnetic reor ient ing
forces
appear
to have
been unimportant in
governing
the
remanent
magnet isat ion
of
the
depos i t it
seems
l ikely that
the magnetic fabr ic
/
/
,/
/
•
. .
,
\
\
LAYER 9
Figure 11 . Direc t ions of
maximum Ie )
and
minimum Ie )
axes
o f ma gnetic s uscept i bi l i t y in
t
hree
of
the
Pwl l y Gwynt clay
la yers . Orienta t io n of t he c ave
passage
i s
shown by the
arrows
.
Sol id symb
o
l s
l ow
er hemisphere,
open symbols upper hemi sphere .
LAYER
6
o
8
o
7
I I I
o
Q)
o
0
o
0 -
0
0
LAYER
1
I I I
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DECLIN TION
180
220
260
300
3.
0
70
E
u
r
80
Q..
w
0
90
- 0
has
been dominated by the
ef fec t s
of
viscous
f lu id
forces
o r
gra in co l l i s ion
dur ing
the slump event .
This caused
the g ra ins to
ro tate about
axes
perpendicular
t o
the flow di rec t ion .
The girdled d i s t r ibu t ion
of minimum
axes
can
then be explained by considering the veloc i ty
dis t r ibu t ion in
the
sediment
as t
flowed through
the
narrow passage
(Fig.
13). The
di rec t ion of
the veloc i ty gradient and hence the shor tes t
axes
of the
moving gra ins
must have been ho
r izontal
near the walls and
approximately ver t ica l
near the
cent re of
the
r i f t .
Since the samples
span a
width
~ 1 0
cm)
which
i s
a
s igni f icant
f rac t ion
of
the wall spacing (15-4) cm) they wi l l , therefore,
contain minimum suscep t ib i l i t y di rec t ions
which
depar t
appreciably
from the
ver t ica l but
which are
conf ined
to
a plane
perpendicular
to the
flow
di rec t ion .
CONCLUSIONS
These
r esu l t s
from Agen
Allwedd
and Pwll y
Gwynt
confirm
the value of deep cave
sediments
for
palaeomagnetic s tudies
of the ear th s
magnetic
f i e ld . Moreover,
since the cave
sediments
were
formed
in
condit ions
analogous
to those crea ted
in
l abora tory flumes, t
has
been poss ib le to compare
equivalent depos i t ional
magnet isat ions
in na tura l
and
a r t i f i c i a l
sediments.
The palaeomagnetic
data suppor t
the
conjec ture ,
based
on
laminat ion matching,
t ha t
depos i t ion of the Cap Mud
was
s imultaneous
throughout
Agen
Allwedd (Bull
1978).
I t would be
in teres t ing
to extend th i s t e s t to
similar
depos i t s
in Darren
Cilau, Eglwys Faen and
other
caves beneath
Mynydd
Llangattwg.
In the
absence
o f rad i ometr ica l ly dateable
material ,
palaeomagnetism i s probably the only technique
which can prove contemporaneity
of
cave
sedimentat ion
on
a regional
scale.
The
discovery
t ha t some
sediments in
Pwll y
Gwynt date from a period
of
reversed geomagnetic
polar i ty ind ica tes t ha t these
clays must have
an
age well
in
excess of the 11000 yr
BP
conjectured
for the Cap Mud
in
Agen
Allwedd (Bull
1980).
The sediments may be contemporary
with
the
Laschamp
Geomagnetic
Event a t -47 Kyr BP
(Hall
&
York
1978). Alternat ively, the
c lays may have
recorded
an
older
Brunhes
geomagnetic
event
or may
da te
from
the
Matuyama
Magnetic Epoch (>720 Kyr).
Cave s i l t s
with a reversed
remanence polar i ty
have
also been found
in Masson Mine, Derbyshire
(Noel,
Shaw and
Ford 1984).
INCLIN TION
8
o
70
80
90
0
80
0
m
Figure
12. Prof i les of
remanence
directions in
the
slumped clay
before (0
) and
after
(e)
demagnetisation
in
a peak
alternating
f ield of SmT.
SLUMPE
[
20
em
Figure 13 . Sketch i l lus t ra t ing t he posit ions of samples in
the
s l umped
clay
and
the
form
of the
a l
ong-passage
velocity
dis t r ibut ions , V in the deposit during the flow . The
shor test axes of moving
grains were
aligned paral lel
to
the
velocity
gradient
giving r i se
to
the directions of minimum
suscept ibi l i ty in the
samples
shown
by
the diametric l ines .
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The
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Sarawak,
Trans.
Bri t ish Cave Res. Assn.,
9, 134-141.
Noel,
M., Homonko,
P, Bull,
P.A.,
1979.
The
palaeomagnetism of
sediments from Agen
Allwedd,
Powys,
Trans
. Bri t ish Cave
Res. Assn.,
9,
134-141.
Noel, M , Retall ick, W
G
Bull,
P.A.,
1981. Further
palaeomagnetic
studies
of sediments from Agen
Allwedd,
Trans.
Bri t ish Cave Res. Assn., 8, 178-187.
Noel, M., Shaw, R.
P. Ford,
T.D., 1984. A palaeomagnetic
reversal
in ear ly
Quaternary sediments
in
Masson Hill,
Matlock,
Derbyshire,
Mercian
Geologist,
9, 235-242.
Noel,
M
St Pierre, S. , 1984.
The
palaeomagnetism and
magnetic
fabr ic of cave sediments
from
Gronligrotta and
Jordbrugrotta, Norway, Geophys. J.R.
astr .
Soc., 78,
231-239.
Rees, A.I., 1961. The
effect
of water currents on the
magnetic
remanence
and anisotropy of suscept ibi l i ty of
some sediments, Geophys. J.R. astr . Soc., 5, 235-251.
Rees,
A.I.,
1965.
The
use
of magnetic suscept ibi l i ty in the
estimation
of
sedimentary fabr ic, Sedimentology, 4,
257-271.
Rees,
A. I . ,
1966.
The effect
of depositional slopes on
the
anisotropy
of magnetic
suscept ibi l i ty
of
laboratory
deposited sands, J . Geol. , 74, 856-867.
Schmidt, V.A.,
1982.
Magnetostratigraphy
of sediments in
Mammoth
Cave, Kentucky, Science, 217, 827- 829.
Schmidt, V.A., Jennings, J . Bao
Haosheng,
1984. Dating of
cave sediments
a t
Wee
Jasper,
New
South
Wales, by
magnetostratigraphy,
Australian
J . Earth Sci . , 31,
361-370.
Stober, J.C., 1978.
Palaeomagnetic secular var iat ion studies
on
Holocene
lake sediments,
Ph.D. Thesis, University of
Edinburgh.
Received December 1987
Mark
Noel,
Department
of
Geology,
University of
Sheffield,
Sheffield
S7 3HF
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CAVE
SCIENCE
Vol. 15, No.1 ,
Apri l
1988
Transact ions of
the
Bri t i sh
Cave Research
Associat ion
Natural
Tunnel Virginia
Tony WALTHAM
Abstract :
Natural
Tunnel, in Virgin ia , U.S.A.,
i s a 260m long
cave
t raversed
by
a standard
gauge
rai lway
from
sink
to
resurgence.
I t
is
a
spectacular cave
cur iosi ty ,
yet t remains in
r e l a t ive
obscur i ty .
The cave
i s the rejuvenated remains of a
phreat ic
loop ins t iga ted
by
underground
capture of a surface stream.
Natural
Tunnel
l i e s
in the Allegheny
Mountains about
5 km southeast of
the
small town
of Duff ie ld ,
in
the western
t i p
of Virgin ia ,
U.S.A., where t pinches
out
between
Tennessee and
Kentucky.
I t
i s
therefore in the hear t
of the
Valley
and
Ridge
kars t
region of
the
grea t
Appalachian
fo ld mountains and
should
not be
confused with the
wel l
- known but much
shorter
Natural
Bridge, which
l i e s
300
km to the
eas t .
The Tunnel has dimensions which are
hardly
remarkable, though
t i s a
passage of considerable
cross -sec t ion ,
and t would probably receive few
vis i to r s
t t
did
not
happen
to
have
a
main- l ine
rai lway
l a id throughout i t s length.
History of the cave
I t appears
that
Indians
used
the cave
long
ago
as
a
route between hunt ing grounds, for
a
complete t r averse
i s
j u s t poss ib le
without leaving
a t l eas t
a ves t ige of dayl ight . But t i s
speculateG
t ha t
the f i r s t white man
through
Natural
Tunnel was Daniel
Boone,
soon af te r 1769
when he l ived in the area br i e f ly and was
exploring west in to
Kentucky. Writ ten
descr ipt ions
of
the
cave
date
from as ear ly
as
1832,
but t then seems to have sunk
back
in to
obscur i ty unt i l 1880,
when
an engineer , J .H. McCue
came across
t
while surveying
a route for
the
South Atlant ic and Ohio
Rai l road l a te r
to become
par t of
the Southern Railway .
Figure 1 The area of Purchase Ridge and Natural Tunnel
11
Purchase
Ridge i s a subs tant ia l topographic
feature,
essen t i a l ly composed of sync l ina l
limestone escarpments or iented NE SW in l ine with
the Appalachian
s t ruc tu ra l
t rend. I t separates
the val leys of
the
North
Fork
of the
Clinch River
and the main Clinch
River
f igure 1) , both of
which dra in southwest eventual ly in to
the
Tennessee River. The Ridge r i ses to cres t
elevat ions of
about
600m 200m
above
the
Clinch
Rivers,
and the
modern
highway has
to climb to
500m to cross the r idge. However, the narrow
val ley of Stock Creek
cu ts
r ight through the r idge
a t around the
400m
l eve l ,
and
t h i s was the
route
chosen
by Mr. McCue.
But
par t way
through
the
Ridge, Stock Creek goes underground,
leaving a
dry
saddle cres t ing a t 448m.
This
i s where Mr. McCue
found
that
Natural
Tunnel,
the cave which
Stock
Creek t raverses almost beneath t h i s dry saddle,
was also spacious enough
to take
his
rai lway.
A
l a rge
amount of brushwood
and
debr i s had to
be cleared out of the cave, and a kink, par t way
through, demanded
30m of
blasted
tunnel .
Otherwise,
a
low embankment was bui l t on the f irm
sands
and
gravel
of
the cave sediment to
keep the
rai lway above f lood l eve l s , and, with a gentle
reverse curve
in
i t s
t racks,
the rai lway gained an
underground route 250m
long
for minimal cost .
The
rai lway
through
Natural
Tunnel
i s st ll
in use
today.
This in i t s e l f
i s f a i r ly
remarkable
The caboose of a northbound f reight t ra in
disappears
nto
atural Tunnel
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in America, but the
l ine
t raverses the
great
Appalachian
coal f ie lds and
has
been kept
al ive
by
coal. There have been no regular passenger t r a ins
for many years ,
but
about 10 t r a in s per day
st ll
pass
through
the
Tunnel, mostly diesel-hauled
very
long l ines
of
coal
trucks .
Since 1971,
Natura l
Tunnel
has
been a s ta te
park, though i t s
vis i to r
centre and
tour i s t
f ac i l i t i e s
are only
cur rent ly being developed,
and
An empty coal t ra in rolls into the
south
end of Natural
Tunnel, as seen from the top of the limestone c l i f fs
Elevation
on 75
plan scale
5
amphitheatre cliffs
Entrance
Exit
par t
of
i t s
a t t r ac t ion l i e s
in
the campgrounds and
foo t - t r a i l s on the wooded
ridge, This
does make
the s i t e very accessible , with l a id paths to
c r i t i ca l
poin ts
including a spectacular viewpoint
above the c l i f f s around the cave
exi t . A path
descends through
the
woods
and reaches the
cave
af ter
pass ing
through another
short
railway
tunnel .
Perhaps
surpr i s ingly in a society
normally zealous over safe ty cont ro l s ,
vis i to r s
can
wander along the ra i lway
t rack through
Natura l
Tunnel,
a l l of which is a t l eas t gloomily l t by
daylight.
Signs
warn
to
beware
of
t ra ins ,
but
the sound ef fec t
of an
approaching
t r a in
makes
an
accident a
th reat
only to someone completely
devoid
of ears or
brain.
There
i s
l i t t l e
exis t ing
l i t e r a ture
on
Natura l
Tunnel. Together
with Natura l
Bridge
(a lso in
Virginia) t
was described by Woodward
(1936), but
he lacked
cave
surveys and
was
a
l i t t l e
over-enthusiast ic about
collapsed caverns .
Since then
t
has
achieved passing mention in
various
ra i lway
journals , textbooks
of
engineering
geology
and items
of tour i s t l i t e r a ture
(with much
of
the
l a t t e r
containing grossly dis tor ted
s t a t i s t i c s ) . Then in
1977,
a very deta i l ed survey
was prepared by W C. Douty and others for the
Virg in ia
Cave Survey.
The cave today
Stock
Creek i s
entrenched about
100m
in
a
narrow winding ravine
which
cuts through
the
l imestone of
Purchase
Ridge. The stream
enters
Natura l
Tunnel where
t
swings gent ly
l e f t
into
the
c l i f f
forming the eastern ravine
wall .
The entrance i s over 20m wide
and
15m
high,
and the railway
enters on the s tream's l e f t bank,
which has
been modified
with a low concrete wall
to
prevent
erosion beneath the t racks. The cave
is a simple arched tunnel
with
minimal breakdown
and
an undula t ing roof
l e f t
from
i t s phreat ic
o r ~ g ~ n s No rock f loor i s vis ib le , and the stream
i s
s l igh t ly entrenched in beds of sand, gravel
and cobbles ; there are
no s igni f icant
ca lc i te
deposi
t s .
There i s a very gentle overa l l
gradient, but the roof fa l l s to a low poin t on
an
overdeveloped l e f t bend. There the
modern
vadose
stream
swings through
a meander notch cut
in
the
rock
wall .
A
short cut sect ion
of ra i lway
tunnel
avoids both the bend and the low roof.
The downstream hal f
of
the cave i s more
spacious , wi th
the
ra i lway
on
a
low bank almost
down i t s
centre. The
roof r ises in to a shallow
dome, blackened
by smoke from by gone stream
t ra ins , and then r ises towards the exi t . The
resurgence of the cave i s a splendid
arch
25m high
Figure 2 Survey of Natural
Tunnel
NATURAL TUNNEL
Virginia U.S.A.
o
m 1
cliffs
Stock
Creek
~ 4 = ; ; 0 = ; ; 0 : = = = = = = = = = = = = = = = = = ~ r ~ a ~ I I ~ W ~ ~ Y : : : 1 Downst
ream
cr bed
CW
12
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and
wide, which appears to have been
hardly
trimmed
to take the ra i lway agains t the
l e f t
wall .
Above
the
cave
exi t ,
a ver t i ca l l imestone
c l i f f r i ses
about 80m,
wall ing
a
f ine
natural
amphi thea t re . Stock Creek swings round
the
edge
of
t h i s rock bowl,
before
rounding a spur in to a
more open
val ley
beyond . The
railway
uses a
shor t
tunnel
through
the spur , and
then re jo ins
the
creek on
a
dayl ight course .
Total
length
of
the
Natural Tunnel
cave i s
260m,
and it
descends only a few metres. The
survey
(
f igure
2
i s
a
low-grade,
one-man
product ion, but,
because
of the
simplici ty
of the
cave, it
i s
probably
a very
f a i r representa t ion .
Origins of Natura l Tunnel
Purchase
Ridge i s
a
narrow
syncl inal
core
preserved in massive l imestone with outward-facing
escarpments
of which the nor thern
s ide r i ses to
the h igher
cres t
f igure
1) . I t
is par t of
a be l t
of su b - para l l e l s t ruc tu ra l ly control led
r idges,
whose pat te rn
i s
complicated
a
l i t t l e by
some
cross - faul t ing and over
-
th rus t ing . Northeast of
Stock Creek, Purchase Ridge
widens
in to a pla teau
as
the
syncline opens
out
. Both
pla teau and
r idge , and also the other
l imestone
r idges
in
the
area ,
are
r iddled with dol ines , b l ind val leys and
sinkholes, and comprise f ine kars t landscapes.
Both the Clinch River
and
i t s North
Fork
are
t runk streams flowing
para l l e l
to
the
s t ruc tu ra l
controls . The
headwaters of
Stock Creek
or ig inal ly
drained
into
the
North
Fork, and an
obvious wind gap survives
a few
ki lometres
nor theas t of Duffield f igure 1) .
A t r ibu tary
of
the
Clinch River then eroded headward into
Purchase
Ridge and eventua l ly captured the
head
of
the
North Fork -
to form Stock Creek
along
most of
i t s
present
l ine .
There
i s
no c lear remaining
evidence
that
th i s i n i t i a l
capture
involved
any
underground cu t
-
offs , but the poss ib i l i t y cannot
be
ru led
out. And
the
present
drainage
pat te rn
i s
probably
superimposed from
a landscape
of
much
l ess re l i e f .
The or ig inal route
of
Stock Creek took
it
j u s t west
of Natura l
Tunnel, along a
still
well-defined val ley, now followed
by
a
minor road
over a
dry saddle f igure 3).
A
sec t ion of val ley
lS00m
long
was
then
abandoned
when
another capture
took the water through Natural Tunnel;
the wind
gap now
l i e s 4 m
above
the
present stream l eve l .
This
capture
c lear ly
was
underground, as water
leaked through
the i n i t i a l opening
of
Natura l
Tunn
e l and
in to a
shor t
t r ibu tary
val ley enlarging
headwards from the
eas t bank.
of the val ley lS00m
downstream. Development of t h i s underground loop
offered
hardly any shor ten ing of the
surface
route, but
was probably aided
by
fol lowing bedding
The railway track
inside
the Natural
Tunnel
cave
13
m
Figure
3
Surface
features around
Natural
Tunnel
planes
r igh t
across the syncl ina l core.
The
or ig ina l sink in to t h i s phrea t ic loop
was
very c lose to
the
present
northern entrance of
Natura l
Tunnel. The or ig ina l resurgence
was
a
vauclus ian
r i s ing
with
a
phrea t ic li t of c lose
to
40m, as
indica ted by the
level of
the wind gap,
and
appears
to have been
some
shor t way downstrea
m
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14
of the present cave
exi t .
Subsequent regional
lowering of
both Clinch
River
and
Stock Creek
el iminated th i s phrea t ic loop and l e f t the
roof
of Natural Tunnel
above
water
level .
Vadose
modificat ion of the cave since then has
been
minimal
except
for an element
of
collapse
back
from the or ig inal resurgence.
There i s no
evidence nor any apparent s tructural
control
to
suggest that the i n i t i a l phreatic cave rose
sharply
to i t s
resurgence a t
the s i t e
of the
present cave
exi t .
The rock
amphitheatre
a t
the
exi t
has
ver t ical
walls
and
almost
cer ta inly
formed by
collapse
of the
cave
where
t
wrapped
round an
exceedingly
sharp l e f t
bend
perhaps
leaving a cave pi l la r of
inadequate
strength.
Fai lure
of the
roof i t se l f thinning as the
surface
slope
descended to
the south occurred a
long
time ago.
Stock Creek has since removed the
breakdown and graded i t s course to leave a
spectacular
exi t to
Natural Tunnel.
ACKNOWLEDGEMENTS
The author thanks Tammie
Heazlit
for
chasing various
reference items in
America
nd
acknowledges source
data from
the NSS f i les .
REFERENCE
Woodward H.P.
1936.
Natural Bridge and Natural
Tunnel
Virginia. Journal of
Geology
v.
44
pp
604-616
.
Received January
1988
A C Waltham
Trent
Polytechnic
Nottingham
NGl
BU
Looking
out
of
the
south end of
Natural
Tunnel
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CAVE
SCIENCE
Vol
.
15, No.1 ,
Apri l
1988
Transact ions
of
the
Bri t i sh Cave Research
Associat ion
Postglacial
Tufas
and
Travertines
from
the raven
District
of Yorkshire
Allan
PENTECOST
and
Tom LO
RD
Abstract : Pos tg lac ia l tufas , t r aver t ines and
cemented
gravels
are
described
from 29 s i te s .
The
depos i t s are c la s s i f ied
according
to
t he i r
assoc ia ted f lora ,
where t h i s i s present . Most depos i t s exposed to l i gh t were assoc ia ted with
bryophytes ,
cyanobacter ia
and
higher
plant s . Archaeological evidence was
obtained
a t
some s i te s suggest ing a
phase
of
rapid
or
more extensive deposi t ion
pr ior to
or dur ing
the Atlant ic Per iod 6000-8000
years BP)
and t h i s was
suppor ted by the l a rge number of
pos tg lac ia l
s i t e s where there is
l i t t l e
act ive
depos i t ion today. Nineteen s i t e s are described for the f i r s t time.
INTRODUCTION
Tufa
deposi t ion can
occur
in any region
where
l imestones are
over la in
by so i l .
The
so i l
atmosphere , which contains 10-50 times more
carbon
than the
a i r
i s pr imar i ly responsible for the
disso lu t ion
of l imestones
and
subsequent
de-gassing
and
re -
precip i ta t ion
of much of
the
carbonate as
speleothems
and tu fa . The chemistry
of
the process
is
f a i r ly well understood although
the
mechanism of
the r ec rys t a l l i za t ion and
the
s igni f icance
of bac ter ia
and algae requ i res
fur ther study.
Tufa
deposi t ion can
r esu l t
from
several unre la ted changes
occurr ing
in the
percola t ing
water .
Previous stud ie
s
in
the Craven
d i s t r i c t
have
shown t ha t the major
change i s
the
di f fus iona l
loss of
carbon
dioxide to the
atmosphere Pentecost ,
1981).
However, there i s
some
evidence to
suggest
t ha t the uptake
of
carbon
dioxide
by
photosynthesizing
plant s , temperature
ch
an g
es and
water loss by
evaporat ion
also play
some par t
in tufa
deposi t ion
Barnes, 1965),
though genera l ly on a
smal ler
sca le .
Tufa,
when f i r s t
formed, may
be e i ther so f t
and
with
a
mealy
or crumb-like
t ex ture ,
or
massive
and
s tone- l ike .
Several at tempts have
been
made
to
c lass i fy the
var ious
types, though
none
appears
to
be sa t i s f ac to ry when
applied to the range of
depos i t s
found
in
Craven. Gruninger 1965) and
Frey
Probst
1974) have followed
a
system
descr ibed by
Klahn 1923) for tufas
of the
Schwabische
Alb. This
c lass i f i ca t ion i s
based
upon the shape of the depos i t which i s
inf luenced
by
the angle of slope . Although useful , these
def in i t ions do not consider the small
-
sca le
s t ruc ture
of
the
depos i t s .
I r ion Muller 1968),
attempted a biogenic
c lass i f i ca t ion into
s in t e r or
t raver t ine,
which i s predominantly inorganic , and
var ious
c lasses
of
organic a lga l and moss
tufas
.
In
th is paper we
place
emphasis
on
the
inact ive
s i te s of
the
d i s t r i c t because
of
t he i r
potent ia l value as sources of palaeoenvironmental
data
Kerney e t
aI ,
1980)
and
as
a
medium
for
the
preserva t ion of archaeological
material .
Figure 1 .
I l lus t ra t ions ot
moss-tufa
fabr
ics found in
Craven.
a) cascade
tufa
Rhynchoste g ium
r ipar ioides ),
arrow
shows direct ion o f
water
flow
. b) hummock tufa plumose
ratoneuron commut tum nd
Pell ia) .
c)
stromatoid
tufa
TGYrnnOstomum) Bar 1 cm.
15
CLASSIFICATION
The
scheme
adopted
i s
based
upon the
predominant vegetat ion,
where
present
Table
1) .
As plants become
encrusted by the depos i t , they
eventual ly decay but leave t he i r or ig ina l
s t ruc ture
impr inted
on
the
mater ia l
.
Consequently, the tufa fabr ic
i s l a rge ly
determined
by them. Plants are general ly absent
only where l igh t in tens i t i e s are
low
or
nonexistent .
Such
condit ions occur
in
caves
and
in deep t a lu s .
We
have
reserved two
terms
for
abio t ic
mater ia l ;
t raver t ine
for porous
i r regular ly
laminated depos i t s
formed
within
caves, and clas t - suppor ted
brecc ia
for cave
deposi ts
containing >50
brecc ia by
volume with
the i n t e r s t i ces
f i l l ed
with
t raver t ine.
Some doubt remains concerning the def in i t ion
of
t raver t ine.
The term
was
or ig inal ly
used to
describe
depos i t s occurr ing
near Tivol i
in
I ta ly
and
there i s some
evidence indicat ing that
much
of
the mater ia l i s of
bac te r i a l or ig in Chafetz
Folk,
1984).
I f t h i s proves to be cor rec t , the
term calcareous s in te r , which i s also
used
to
denote
abio t ic
depos i t s ,
may
be
more
acceptable .
Bryophytes are
the
most conspicuous plants
associated with
the
i l luminated
and
act ive Craven
tufas
and there are a
number
of
charac te r i s t i
c
species .
The
plumose var ie ty of
Cratoneuron
commutatum
f or
ms yellow-brown
mats
in seepages
or
slow-flowing water and
i s
the charac te r i s t i c moss
of hummock tu fa .
This tu fa
i s found around small
springs
as
i r regular
unlaminated
mounds, sometimes
dissec ted by
narrow
channels Fig.
lb ) .
In par t s
of Belgium such
depos i t s
are
known
as crons
Symoens,
1951).
In
shaded seepages, tufa
- encrusted mosses
with a
radiat ing
cush ion- l ike stromatoid)
s t ruc ture
occur
Fig. l c ) . Two important
genera
forming t h i s type
of
tu fa are
Eucladium
and
Gymnostomum.
Final ly
there are bryophytes which
grow in turbulent condit ions
where
the moss stems
become
al igned
with
the
di rec t ion
of water
f low.
As
t h i s
tu fa
forms, a
ser ies
of l ayers is
b
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low
regl me
assoc
iate
d
vegetat1 n
Table 1 Tufa classificat ion
absent
cyanobacted a
bryophytes hlghe r plants
c sc de
tufa
turbulent
l aml na
ted tu f
a
Rh:z:nchosteslum
non plumose
Cratoneuron
Ho o
thr1
x
travert1
ne
Sch1z o
th r 1x
lam:1nar
l s
supported
R:1vular1a
hummock tufa
breccia
pl u ose
C
raton
eur
on
unconsolida t ed
tufa
SCItonema
stramatoid tufa Carex
u
ncus
seep ge
s
Eu c1adlum
produced.
The
layers are approximately 2-10 m
thick and cons i s t s of
s lan t ing
moss
stems
similar
in appearance
to
a
sect ion
through a thatched
roof.
This
is
defined
here
as cascade
tu fa .
Three
moss species
are associa ted with th i s tufa:
Cratoneuron commutatum non-plumose
var iety) ,
Rhynchostegium
r ipar io ides
and Eucladium
ver t ic i l la tum Fig. 1a) .
I t
should be
noted
t ha t
the flow
regime l a rge ly
determines the
type of
vegetation
presen t
Table
1) .
Cyanobacterium tu fas also
occur. These
consis t
of ser ies of
th in
encrusting
layers.
The
cyanobacterium colonies formerly known as
blue-green algae)
are usually
nodular
and rarely
exceed
10
in
diameter
resu l t ing
in
a
rugose
surface. These tufas are
less common
than
bryophyte tu fas but occur
in
f as t - and
slow-flowing
water
Table 1) . In some s i t e s ,
cyanobacter ia overgrow bryophytes and here tu fa
formation
must be
inf luenced
by
both
groups of
organisms
Pentecost 1978
,
1981,
1987).
Unconsolidated
tu fa or spring chalk
forms
in
mires
where
higher plant s are of ten
dominant.
16
The
deposits often
form
around the
bases
of Carex
and other
wetland
species , o r produce a marl
between
the tussocks.
Animal remains may be a
conspicuous featu re and
some species
of gastropods
of narrow ecological amplitude can be preserved in
this tu fa . These have proved
to
be usefu l
environmental indicators Kerney
e t
a l . , 1980).
I t
i s
important
to
real ise that in any
s i tuat ion , intergradat ions
occur between
these
types of
tu fa .
With
act ively forming deposi ts ,
t
is
easy
to
ident i fy
the dominant form of
vegetation but
with
foss i l ised mater ia l
th i s
i s
of ten
not possib le .
Most
modern
tufas
in
the
Craven d i s t r i c t
cons i s t of micri te . The calc i te crys ta ls are
of ten l
l -defined
,
anhedral
and 5
um
or
less
in
diameter. Bryophyte
stems
and
leaves
are
sometimes
encrusted
wi th
ca lc i e pal isades
consist ing
of
crystals 50-100
um
in
length with
their long axes perpendicular
to
the moss surface .
Microspars
are
also
found, with euhedral ,
drusy
crystals
occurr ing
in
small
cavi t ies ,
probably as
a resul t
of
recrys ta l l izat ion or l a t e r in f i l l i ng .
U
ncon
s o
l idated tufa
on
the
east
bank of Clapham Beck s i te 7 .
The beck has
expcsed
a 2 m
section through the
depcs
i
t , top
marked by arrows .
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75
Figure 2. Map of s i t e s
.
Numbered s i t e s with large
circles are described in
the
t ex t .
Small
c i r c l e s
without
numbers
depict o ther act ive
s i tes .
Circle s ize is
not
related to deposit s ize .
Faul ts : NCF North Cr av
en:
MCF
Mid Craven: SCF South Craven.
65
76
72
68
t
-
7 J
'
~ 9 1 1
'
-
80
Microspars
with
crys t a l s up to 60 urn in
diameter
are
often found cementing scree . Older tufas
generally
have a
similar
petrology
to act ive s i t e s
although organic mater ia l i s usual ly absent . The
tufas of
Gordale
are
exceptional
for t he i r
pronounced laminations and
apparent
r ec rys t a l l i za t ion .
Here
,
microspar
i s
found
al ternat ing
with large ca lc i te
pal isades up
to
3mm
in length. Ser ies of these
laminae
are found
sandwiched between micri
tes bui l
up around
Eucladiurn.
The petrology of some Craven s i t e s i s
described
in
more de ta i l elsewhere
(Pentecost,
1978, 1981, 1985) .
DESCRIPTION
OF
SITES
For
s i t e
posit ions re fe r to Figure 2. All
gr id references
are in square
SD(34)
.
1.
695754
a
l t .
230
m Cascade tufa
2 m
high, jus t eas t of
Thornton Force.
2. 698735 a l t . 130
m
Bench
of cemented r ive r gravel 1 m
above the
R.
Doe,
20 m in l ength wi th
act ive
rimstone
t r ave r t ine nearby. Meal Bank Quarry, adjacent to th i s
Ingle ton s i t e may have s t a r t ed as a tu fa quarry .
3.
757714 a l t . 290 m.
Rel ic t
hummock t u fa ,
extending 15
m
down
slope of eas t
s ide
of dry
valley
above
Ingleborough
Cave.
4. 757713 a l t .
285
m
Rel ic t hummock t ufa extending 10 m
down s lope
o f
eas t
s id e o f
dry va l l ey with protuberant
r e l ie f
.
5. 756713 a l t .
280 m Rel ic t hummock tu f a
jus t below s i t e
4.
An active
Cratoneuron
tu fa
occurs
100m lower down
the
valley near
Ingleborough
Cave. ,
6.
754711 a l t .
265 m Hummock tufa grading
i n to
t rave r t ine
a t the entrance
of Ingleborough Cave.
The
t r ave r t ine bar r i e r
j us t within the cave was removed
by
bl as t i ng in c .
1837
(Dawkins,
1874). Impressions of l iverworts
and mosses
cf .
Conocephalum,
Cratoneuron
occur .
7. 752707 a l t . 250 m Layers of
unconso
l i d a ted and hummock
tu fa exposed in
west
bank of
Clapham
Beck . Exposures up to
50 cm
th ick
extend l a te ra l ly
for
30 m
8 .
751706
alto
indurated blocks,
Beck.
235 m Small bank
of
hummock t u fa with
in
woodland
by the footpa th along
Clapham
9. 788663 a l t
. 245
m
Cave
Ha 1. Large
l imestone rock
s he l t e r
with
f loor deposits
of t ravert ine and
clas t -suppo
r t ed
breccia McKenny
Hughes, 1874)
. Indura ted
cushion
-
l ike
(s t romatoid) tu f a bosses up to 20 cm wide underneath western
extremi ty
overhang.
Some of the t r ave r t ine i s reported to
have a high phosphate content
resu lt ing
from
in f i l t ra t ion
of
water
through
a layer of
guano
(Marr ,
1876).
17
MalhamJ;:?
- ._Tarn
'---.
'-
- t i F ~
- .- .
t K E ~ _
5km
85
90
95
10.
788663
a l t . 245 m Cave Ha 3. Limestone rock she l t e r
with f loor deposits of t ravert ine and clas t-suppor ted brecc ia
over lain by uncemented ta l us. A few s t romatoid tu fa bosses
on the wall. The sit has yielded archaeologica l mater ial
sealed within
travertine
and finds
include
numerous human and
animal bones, a
few
chipped stone too ls
and charcoal . The
bulk of
the t r ave r t ine formation had preceeded the depos i t ion
of
the occupat ion horizons From the typology of the
stone
to o ls and the composit ion
of
the fauna, the occupation
horizons are
most
l i ke ly Atlantic / Su