Page 230 - Fundamentals of Geomorphology

KARST LANDSCAPES 213

are smooth-surfaced protuberances in a cave roof. and ceiling cavities, ceiling half tubes, continuous rock
Scallops are asymmetrical, cuspate, oyster-shell-shaped spans, and mazes of passages (see Jennings 1971, 156–7).
hollows with a steep semicircular step on the upstream
side and a gentle rise downstream ending in a point of the Depositional forms in caves
next downstream hollow (Plate 8.16). Scallop size varies
inversely with the flow velocity of the water, and scal- Three types of deposit are laid down in caves: (1) cave for-
lops may be used to assess flow conditions. In the main mations or speleothems; (2) material weathered in situ;
passage of Joint Hole, Chapel-le-Dale, North Yorkshire, and (3) clastic sediments carried mechanically into the
England, two contrasting-size populations of scallops cave and deposited there (White 1976). Cave sediments
were found (Murphy et al. 2000). Larger scallops occupy are beyond the scope of this introductory text (see
the walls and ceilings, and smaller scallops occupy the Gillieson 1996, pp. 143–66, for an excellent review), but
floor. The floor scallops suggest a higher velocity at the the chemical precipitates known as speleothems will be
bottom of the conduit. Presumed solution features in the discussed.
phreatic zone include spongework, bedding plane and Mostspeleothemsaremadeofcarbonatedeposits,with
joint anastomoses, wall and ceiling pockets, joint wall 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,whichdevelopdownwards,growfrom
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
Plate 8.16 Scallops in Joint Hole, Chapel-le-Dale, revealed that, for five sites with a good soil cover, sta-
North Yorkshire, England, taken underwater. lagmite growth rate depends chiefly upon mean annual
(Photograph by Phil Murphy) temperature and the calcium content of the drip-water,