Page 219 - Fundamentals of Geomorphology

202 PROCESS AND FORM

Ford 1978). Limestone in the Mackenzie Mountains is a half-blind valley. A half-blind valley is found
is massive and very thick with widely spaced joints. on the Cooleman Plain, New South Wales, Australia
Karst evolution in the area appears to have begun with (Figure 8.11a). A small creek flowing off a granodiorite
the opening of deep dolines at ‘weak’ points along hill flows for 150 m over Silurian limestone before sink-
joints. Later, long and narrow gorges called karst streets ing through an earth hole. Beyond the hole is a 3-m-high
formed, to be followed by a rectilinear network of grassy threshold separating the depression from a gravel
deep gorges with other cross-cutting lines of erosion – stream bed that only rarely holds overflow. If a stream
labyrinth karst. In the final stage, the rock wall of the cuts down its bed far enough and enlarges its under-
gorges suffered lateral planation, so fashioning towers. ground course so that even flood discharges sink through
it, a blind valley is created that is closed abruptly at
Fluvial karst its lower end by a cliff or slope facing up the valley.
Blindvalleyscarryperennialorintermittentstreams,with
Although a lack of surface drainage is a characteristic sinks at their lower ends, or they may be dry valleys.
feature of karst landscapes, several surface landforms owe Many blind valleys occur at Yarrangobilly, New South
their existence to fluvial action. Rivers do traverse and Wales, Australia. The stream here sinks into the Bath
rise within karst areas, eroding various types of valley House Cave, underneath crags in a steep, 15-m-high
and building peculiar carbonate deposits. counter-slope (Figure 8.11b).

Gorges Steepheads
In karst terrain, rivers tend to erode gorges more fre- Steepheads or pocket valleys are steep-sided valleys
quently than they do in other rock types. In France, in karst, generally short and ending abruptly upstream
the Grands Causses of the Massif Centrale is divided where a stream issues forth in a spring, or did so in the
into four separate plateaux by the 300–500-m-deep Lot, past. These cul-de-sac valleys are particularly common
Tarn, Jonte, and Dourbie gorges. The gorges are com- around plateau margins or mountain flanks. In Provence,
monplace in karst landscape because river incision acts France, the Fountain of Vaucluse emerges beneath a
more effectively than slope processes, which fail to flare 200-m-high cliff at the head of a steephead. Similarly,
back the valley-sides to a V-shaped cross-section. Some if less spectacularly, the Punch Bowl at Burton Salmon,
gorges form by cavern collapse, but others are ‘through formed on Upper Magnesian Limestone, Yorkshire,
valleys’ eroded by rivers that manage to cross karst terrain England, is a steephead with a permanent spring issuing
without disappearing underground. from the base of its headwall (Murphy 2000). Malham
Cove, England, is also a steephead (Colour Plate 4,
Blind and half-blind valleys inserted between pages 208 and 209). Steepheads may
form by headward recession, as spring sapping eats back
Rivers flowing through karst terrain may, in places, into the rock mass, or by cave-roof collapse.
sink through the channel bed. The process lowers the
bedrock and traps some of the sediment load. The sink- Dry valleys
ing of the channel bed saps the power of the stream
below the point of leakage. An upward step or thresh- Dry valleys are much like regular river valleys save that
old develops in the long profile of the stream, and the they lack surface stream channels on their floors. They
underground course becomes larger, diverting increas- occur on many types of rock but are noticeably common
ingly more flow. When large enough, the underground in karst landscapes. Eye-catching dry valleys occur where
conduit takes all the flow at normal stages but cannot rivers flowing over impermeable rock sink on entering
accommodate flood discharge, which ponds behind the karst terrain, but their former courses are traceable above
step and eventually overspills it. The resulting landform ground. In the Craven district, England, the Watlowes is