. A complete geography. Fig. 16. A New England lake formed by a dam of drift left by the glacier. It is very irregularbecause the water behind the dam has risen into many valleys, leaving only the hilltopsabove the surface. rock was swept away and replaced by drift brought by the was made by the grinding of rocks together, much as flour ismade by grinding wheat ; in fact, glacial soil is sometimes calledrock flour. As the glacier scraped along, it ground an enormousquantity of rock to bits, so that when it melted, a layer of drift wasleft, in some places reaching a depth of severa
. A complete geography. Fig. 16. A New England lake formed by a dam of drift left by the glacier. It is very irregularbecause the water behind the dam has risen into many valleys, leaving only the hilltopsabove the surface. rock was swept away and replaced by drift brought by the was made by the grinding of rocks together, much as flour ismade by grinding wheat ; in fact, glacial soil is sometimes calledrock flour. As the glacier scraped along, it ground an enormousquantity of rock to bits, so that when it melted, a layer of drift wasleft, in some places reaching a depth of several hundred feet. Mostof the clays from wdiich bricks are made in the North were alsobrought by the ice sheet. With the melting of the glacier, much water was produced. Thiswashed out and carried off a great deal of clay, in some places leavingextensive sand and gravel plains, where the soil is not very fertile. The bits of ground-up rock left by the glacier have an important effect PHYSIOGRAPHY OF NORTH AMERICA 13. Fig. 17. A field on Cape Ann, Mass., where the glacier leftmany large boulders. upon the soil. Since these fragments were gathered up from many places,and from many different kinds of rock, they sometimes cause a fertile soilin places where the decay ofthe rocks would have naturallycaused a sterile soil. Theconstant rusting, or decaying,of these rock fragments sup-plies the soil with plant food;and for this reason the glacialsoils are usually fertile yearafter year. But, on the otherhand, in some places theglacier failed to grind therock into tiny bits, leavingpebbles and even large boulders to cover the ground and prove a greatnuisance to the farmer (Fig. 17). The Coast Line. — In studying about the Mississippi Valley andthe formation of coal, we have seen that the land and sea bottom arenot fixed, but that they often slowly rise or sink. Such changes in the land level are even now in progress in manyplaces, though so slowly that it requires years, and even centuri
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