Geoscience Reference
In-Depth Information
Figure 13.2. Glacially eroded rock-basin lake with moraine dam at outlet, Blue Lake,
Snowy Mountains, Australia. (Photo: Frances Williams.)
at the surface, these subglacial channels are often seen to have flowed upslope under
extreme hydraulic pressure.
Provided the overall slope of the ice is down valley, basal portions of the glacier
can flow upslope across obstacles, scouring and deepening the rock surface behind
them to form rock basins. These are especially common at the head of the glacier,
where they are termed glacial
cirques
,
cwms
or
corries
. Once the ice has melted,
these hollows often become the sites of rock basin lakes (
Figure 13.2
). The advancing
ice acts somewhat as a bulldozer, eroding any projecting valley spurs and giving rise
to the truncated spurs so characteristic of recently glaciated mountain valleys. The
typical glacial U-shaped valley cross-section has a dual origin, with postglacial rock
avalanches and alluvial fans at the transition between vertical hill slope and relatively
flat valley floor. Tongues of diffluent ice can override the valley side at low points on
the divide and form glacial breaches, leading to the reversal of local drainage after the
ice has gone.
Ice can collect and transport rock fragments that fall from the adjacent mountain
cliffs and steep hill slopes. An analogy for what then occurs is a bar of soft soap falling
onto sand. The sand becomes embedded in the soap, and washing with it becomes
abrasive. Armed with rock fragments, the glacier or ice cap is capable of considerable
erosion or glacial abrasion. The result is often a striated or grooved bedrock surface
