484 ELEMENTARY GEOLOGY 
Changes Caused. Since the glacier transported much 
rock material in the form of a ground moraine, as it 
retreated from place to place, and finally disappeared 
by melting, the greater part of this rock load was left 
wherever it happened at the time to be. So the sur- 
face of the country once covered by ice, is now strewn 
over with a sheet of ground moraine, called él or 
bowlder clay (Fig. 114), which forms the chief soil of 
glaciated regions. In some places this soil is so thin 
that the rocks are barely covered, and upon some moun- 
tains not even the thinnest film was left. On the other 
extreme, there are some parts of the country in which 
the glacial soil reaches a depth of 200 or 300 feet. 
By this and other glacial accumulations, many 
changes in-the land surface have been introduced. 
Rivers have been extensively dammed to form lakes, 
and others turned from their ancient valleys, either 
entirely out of their course, or to one side of their 
old channel. As a result of this, in the process of 
cutting new valleys, gorges have been carved and 
waterfalls produced (Figs. 81 and 82, and Plate 7). 
There have been great changes in detail of surface 
outline, though the prominent features of pre-glacial 
times have not been changed. 
The Glacial Theory. No mention can be made of the various 
peculiar forms assumed by the till, and the deposits formed in 
water furnished by ice melting, though there are many interesting 
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