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formed not so very long ago. But the gravel is evidently of glacial origin, being of angular and subangular pebbles, of great variety of material. The conclusion seems inevitable, therefore, that these deltas were formed during the retreat of the ice-sheet.
4. Stagnant, melting ice.—In the retreat of the ice-sheet there were parts at least which became too thin to move. As Professor Davis has said:
During this time it must have melted irregularly, presenting a very uneven, ragged front, from which residual blocks may have been frequently isolated; and it must have endured longest in the valleys, where it was thickest, not only by reason of its greater depth, but also because its surface there, where motion had been fastest and longest maintained, must have been higher than on the hills—this being homologous with the variation in the thickness of a Swiss valley glacier from middle to sides."[1]
It seems to me that we must consider the change to have been gradual from a moving glacier to a stagnant one, and that there may have been times of renewed activity with a forward motion, even in the period of decline. Such forward motion may have had some influence in shifting the course of esker rivers and so have determined where the next sand-plain was to be built. So far as I know, this point has not been worked out in the field.
Crevasses are formed as the ice moves, and change their position according to the tensions in the mass of the glacier. When the tension from motion has ceased, and the ice has become a diminishing, drift-covered mass, the condition represented in Fig. 2, we should not expect to find any crevasses remaining. They would either have been closed by the forward motion of the ice, or would have lost their distinctive character by the excessive melting of their sides, while the water would have washed detritus into them covering the underlying ice, and preventing it from melting as fast as that on either side. Such protection of the ice by detritus must have had great influence in determining the surface forms of the stagnant ice-sheet, as is shown in Professor Russell's account of the sand cones and the deposits in glacial lakelets.
- ↑ Bull. Geol. Soc. of Am., Vol. I., p. 196.