While a valley glacier carries much of its load on top, an ice sheet, such as that of Greenland, is free from surface debris, except where moraines trail away from some nunatak.  If at its edge it breaks into separate glaciers which drain down mountain valleys, these tongues of ice will carry the selvages of waste common to valley glaciers.  Both ice sheets and valley glaciers drag on large quantities of rock waste in their ground moraines.

Stones transported by glaciers are sometimes called erratics.  Such are the bowlders of the drift of our northern states.  Erratics may be set down in an insecure position on the melting of the ice.

Deposit.  Little need be added here to what has already been said of ground and terminal moraines.  All strictly glacial deposits are unstratified.  The load laid down at the end of a glacier in the terminal moraine is loose in texture, while the drift lodged beneath the glacier as ground moraine is often an extremely dense, stony clay, having been compacted under the pressure of the overriding ice.

Erosion.  A glacier erodes its bed and banks in two ways,—­by abrasion and by plucking.

The rock bed over which a glacier has moved is seen in places to have been abraded, or ground away, to smooth surfaces which are marked by long, straight, parallel scorings aligned with the line of movement of the ice and varying in size from hair lines and coarse scratches to exceptional furrows several feet deep.  Clearly this work has been accomplished by means of the sharp sand, the pebbles, and the larger stones with which the base of the glacier is inset, and which it holds in a firm grasp as running water cannot.  Hard and fine-grained rocks, such as granite and quartzite, are often not only ground down to a smooth surface but are also highly polished by means of fine rock flour worn from the glacier bed.

In other places the bed of the glacier is rough and torn.  The rocks have been disrupted and their fragments have been carried away,—­a process known as plucking.  Moving under immense pressure the ice shatters the rock, breaks off projections, presses into crevices and wedges the rocks apart, dislodges the blocks into which the rock is divided by joints and bedding planes, and freezing fast to the fragments drags them on.  In this work the freezing and thawing of subglacial waters in any cracks and crevices of the rock no doubt play an important part.  Plucking occurs especially where the bed rock is weak because of close jointing.  The product of plucking is bowlders, while the product of abrasion is fine rock flour and sand.

Is the ground moraine of Figure 87 due chiefly to abrasion or to plucking?

Roches moutonnees and rounded hills.  The prominences left between the hollows due to plucking are commonly ground down and rounded on the stoss side,—­the side from which the ice advances,—­and sometimes on the opposite, the lee side, as well.  In this way the bed rock often comes to have a billowy surface known as roches moutonnees (sheep rocks).  Hills overridden by an ice sheet often have similarly rounded contours on the stoss side, while on the lee side they may be craggy, either because of plucking or because here they have been less worn from their initial profile.