The direction of glacier movement.  The direction of the flow of vanished glaciers and ice sheets is recorded both in the differences just mentioned in the profiles of overridden hills and also in the minute details of the glacier trail.

Flint nodules or other small prominences in the bed rock are found more worn on the stoss than on the lee side, where indeed they may have a low cone of rock protected by them from abrasion.  Cavities, on the other hand, have their edges worn on the lee side and left sharp upon the stoss.

Surfaces worn and torn in the ways which we have mentioned are said to be glaciated.  But it must not be supposed that a glacier everywhere glaciates its bed.  Although in places it acts as a rasp or as a pick, in others, and especially where its pressure is least, as near the terminus, it moves over its bed in the manner of a sled.  Instances are known where glaciers have advanced over deposits of sand and gravel without disturbing them to any notable degree.  Like a river, a glacier does not everywhere erode.  In places it leaves its bed undisturbed and in places aggrades it by deposits of the ground moraine.

Cirques.  Valley glaciers commonly head as we have seen, in broad amphitheaters deeply filled with snow and ice.  On mountains now destitute of glaciers, but whose glaciation shows that they have supported glaciers in the past, there are found similar crescentic hollows with high, precipitous walls and glaciated floors.  Their floors are often basined and hold lakelets whose deep and quiet waters reflect the sheltering ramparts of rugged rock which tower far above them.  Such mountain hollows are termed cirques.  As a powerful spring wears back a recess in the valley side where it discharges, so the fountain head of a glacier gradually wears back a cirque.  In its slow movement the neve field broadly scours its bed to a flat or basined floor.  Meanwhile the sides of the valley head are steepened and driven back to precipitous walls.  For in winter the crevasse of the bergschrund which surrounds the neve field is filled with snow and the neve is frozen fast to the rocky sides of the valley.  In early summer the neve tears itself free, dislodging and removing any loosened blocks, and the open fissure of the bergschrund allows frost and other agencies of weathering to attack the unprotected rock.  As cirques are thus formed and enlarged the peaks beneath which they lie are sharpened, and the mountain crests are scalloped and cut back from either side to knife-edged ridges.

In the western mountains of the United States many cirques, now empty of neve and glacier ice, and known locally as “basins,” testify to the fact that in recent times the snow line stood beneath the levels of their floors, and thus far below its present altitude.