Geology Reference
In-Depth Information
Ford 1978). Limestone in the Mackenzie Mountains
is massive and very thick with widely spaced joints.
Karst evolution in the area appears to have begun with
the opening of deep dolines at 'weak' points along
joints. Later, long and narrow gorges called karst streets
formed, to be followed by a rectilinear network of
deep gorges with other cross-cutting lines of erosion -
labyrinth karst
. In the final stage, the rock wall of the
gorges suffered lateral planation, so fashioning towers.
is a
half-blind valley
. A half-blind valley is found
on the Cooleman Plain, New South Wales, Australia
(Figure 8.11a). A small creek flowing off a granodiorite
hill flows for 150 m over Silurian limestone before sink-
ing through an earth hole. Beyond the hole is a 3-m-high
grassy threshold separating the depression from a gravel
stream bed that only rarely holds overflow. If a stream
cuts down its bed far enough and enlarges its under-
ground course so that even flood discharges sink through
it, a
blind valley
is created that is closed abruptly at
its lower end by a cliff or slope facing up the valley.
Blind valleys carry perennial or intermittent streams, with
sinks at their lower ends, or they may be dry valleys.
Many blind valleys occur at Yarrangobilly, New South
Wales, Australia. The stream here sinks into the Bath
House Cave, underneath crags in a steep, 15-m-high
counter-slope (Figure 8.11b).
Fluvial karst
Although a lack of surface drainage is a characteristic
feature of karst landscapes, several surface landforms owe
their existence to fluvial action. Rivers do traverse and
rise within karst areas, eroding various types of valley
and building peculiar carbonate deposits.
Gorges
Steepheads
In karst terrain, rivers tend to erode
gorges
more fre-
quently than they do in other rock types. In France,
the Grands Causses of the Massif Centrale is divided
into four separate plateaux by the 300-500-m-deep Lot,
Tarn, Jonte, and Dourbie gorges. The gorges are com-
monplace in karst landscape because river incision acts
more effectively than slope processes, which fail to flare
back the valley-sides to a V-shaped cross-section. Some
gorges form by cavern collapse, but others are 'through
valleys' eroded by rivers that manage to cross karst terrain
without disappearing underground.
Steepheads
or
pocket valleys
are steep-sided valleys
in karst, generally short and ending abruptly upstream
where a stream issues forth in a spring, or did so in the
past. These cul-de-sac valleys are particularly common
around plateau margins or mountain flanks. In Provence,
France, the Fountain of Vaucluse emerges beneath a
200-m-high cliff at the head of a steephead. Similarly,
if less spectacularly, the Punch Bowl at Burton Salmon,
formed on Upper Magnesian Limestone, Yorkshire,
England, is a steephead with a permanent spring issuing
from the base of its headwall (Murphy 2000). Malham
Cove, England, is also a steephead (Colour Plate 4,
inserted between pages 208 and 209). Steepheads may
form by headward recession, as spring sapping eats back
into the rock mass, or by cave-roof collapse.
Blind and half-blind valleys
Rivers flowing through karst terrain may, in places,
sink through the channel bed. The process lowers the
bedrock and traps some of the sediment load. The sink-
ing of the channel bed saps the power of the stream
below the point of leakage. An upward step or thresh-
old develops in the long profile of the stream, and the
underground course becomes larger, diverting increas-
ingly more flow. When large enough, the underground
conduit takes all the flow at normal stages but cannot
accommodate flood discharge, which ponds behind the
step and eventually overspills it. The resulting landform
Dry valleys
Dry valleys
are much like regular river valleys save that
they lack surface stream channels on their floors. They
occur on many types of rock but are noticeably common
in karst landscapes. Eye-catching dry valleys occur where
rivers flowing over impermeable rock sink on entering
karst terrain, but their former courses are traceable above
ground. In the Craven district, England, the Watlowes is
