Geology Reference
In-Depth Information
are smooth-surfaced protuberances in a cave roof.
Scallops are asymmetrical, cuspate, oyster-shell-shaped
hollows with a steep semicircular step on the upstream
side and a gentle rise downstream ending in a point of the
next downstream hollow (Plate 8.16). Scallop size varies
inversely with the flow velocity of the water, and scal-
lops may be used to assess flow conditions. In the main
passage of Joint Hole, Chapel-le-Dale, North Yorkshire,
England, two contrasting-size populations of scallops
were found (Murphy
et al
. 2000). Larger scallops occupy
the walls and ceilings, and smaller scallops occupy the
floor. The floor scallops suggest a higher velocity at the
bottom of the conduit. Presumed solution features in the
phreatic zone include spongework, bedding plane and
joint anastomoses, wall and ceiling pockets, joint wall
and ceiling cavities, ceiling half tubes, continuous rock
spans, and mazes of passages (see Jennings 1971, 156-7).
Depositional forms in caves
Three types of deposit are laid down in caves: (1) cave for-
mations or speleothems; (2) material weathered
in situ
;
and (3) clastic sediments carried mechanically into the
cave and deposited there (White 1976). Cave sediments
are beyond the scope of this introductory text (see
Gillieson 1996, pp. 143-66, for an excellent review), but
the chemical precipitates known as
speleothems
will be
discussed.
Most speleothems are made of carbonate deposits, with
calcite and aragonite accounting for about 95 per cent of
all cave minerals. The carbonates are deposited mainly
by carbon dioxide loss (degassing) or by evaporation.
Formations of carbonate may be arranged into three
groups: dripstone and flowstone forms, eccentric or
erratic forms, and sub-aqueous forms (White 1976).
Dripstone and flowstone
Dripstone
is a deposit, usually composed of calcite,
formed of drips from cave ceilings or walls.
Flow-
stone
is a deposit, again usually composed of calcite,
formed from thin films or trickles of water over floors
or walls. The forms fashioned by dripstone and flow-
stone are stalactites, stalagmites, draperies, and flowstone
sheets.
Stalactites
, which develop downwards, grow from
dripping walls and ceilings. The basic form is a
straw
stalactite
formed by a single drop of water on the ceiling
degassing and producing a ring of calcite about 5 mm
in diameter that grows into a straw (Colour Plate 6,
inserted between pages 208 and 209; Plate 8.17). The
longest known straw stalactite is in Strong's Cave, West-
ern Australia, and is 6.2 m. Leakage and blockage of
a straw leads to the growth of a carrot-shaped stalac-
tite.
Stalagmites
grow from the floor, their exact form
(columnar or conical) depending upon drip rates, water
hardness, and the cave atmosphere. A
column
forms
where an upward-growing stalagmite joins a downward-
growing stalactite. A study of six cave systems in Europe
revealed that, for five sites with a good soil cover, sta-
lagmite growth rate depends chiefly upon mean annual
temperature and the calcium content of the drip-water,
Plate 8.16
Scallops in Joint Hole, Chapel-le-Dale,
North Yorkshire, England, taken underwater.
(
Photograph by Phil Murphy
)
