Fig. 32. Hummocky surface of the boulder-strewn moraine of Greenland.
Going nearer to the ice we find the lower part loaded with pebbles, boulders and bits of clay very like those on the rocks near by. [Fig. 30] shows one of these, scratched and grooved, which I once dug from the ice of this very glacier. The bottom of the ice is like a huge sandpaper, being dragged over the bed rock with tremendous force. It carries a load of rock fragments, and as it moves secures more by grinding or prying them from the rocks beneath. These all travel on toward the edge of the ice, being constantly ground finer and finer as wheat is ground when it goes through the mill. Indeed the resemblance is so close that the clay produced by this grinding action is often called rock flour.
Dragged to the front of the ice, the rock bits, great and small, roll out as the ice melts, some, especially the finest, being carried away in the water, which is always muddy with the rock flour it carries; but much remains near the edge of the ice, forming a moraine ([Figs. 31] and [32]). This moraine, dumped at the edge of the glacier, very closely resembles the hummocky hills of New York (Figs. [33] and [34]), mentioned above, which are really moraines formed at the ice-edge during the glacial period. While their form is quite alike, the New York moraines are generally less pebbly than the Greenland moraines, because the Greenland glacier carries less rock flour than did the glacier which covered New York.
Fig. 33. A view over the hummocky surface of a part of the moraine of the great American ice sheet in Central New York.
In the Greenland glacier, as you can see in [Fig. 31], there is much dirt and rock; in the glacier of the glacial period there was even more. When it melted away the ice disappeared as water, but the rock fragments of course fell down upon the rock beneath and formed soil. If over a certain region, as for instance over your home, the ice carried a great load of drift, when this gradually settled down, as the ice melted, it formed a deep layer of soil; but if the glacier had only a small load a shallow soil was left. Again, if the ice front remained for a long time near a certain place, as near your home, it kept bringing and dumping rock fragments to form moraines, which, of course, would continue to grow higher so long as the ice dumped the rock fragments, much as a sand pile will continue to grow higher so long as fresh loads are brought and dumped.
There are other causes for differences in the glacial soils, but most of them cannot be considered here. One of them is so important, however, that it must be mentioned. With the melting of so much ice, vast floods of water were caused, and these came from the ice, perhaps in places where there are now no streams, or at best only small ones. These rapid currents carried off much of the rock flour and left the coarser and heavier sand, gravel, or pebbles, the latter often well rounded, with the scratches removed by the long-continued rolling about in the glacial stream bed.
One often finds such beds of sand or gravel in different parts of the State, telling not only of ice where it is now absent, but of water currents where is now dry land. The rock flour was in some cases carried to the sea, elsewhere to lakes, or in still other places deposited in the flood-plains of the glacier-fed rivers. Now some of this rock flour is dug out to make into bricks.
Enough has been said to show that the soils of New York were brought by a glacier, and to point out that there are many differences in thickness as well as in kind and condition of the soil. The agriculture of the State is greatly influenced by these differences. In some cases one part of a farm has a deep, rich soil, another part a barren, sandy, pebbly or boulder-covered soil ([Fig. 21]), while in still another part the bed rock may be so near the surface that it does not pay to clear the forest from it. Moreover, some farms are in hummocky moraines, while others, near by, are on level plains ([Fig. 34]), where a broad glacial stream built up a flood-plain in a place where now the stream is so small that it never rises high enough to overflow the plain.