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. There are even other differences than these, and one who is familiar with a region is often puzzled to explain them; but they are all due to the glacier or to the water furnished by its melting, and a careful study by a student of the subject of Glacial Geology will serve to explain them. Each place has had peculiar conditions and it would be necessary to study each place much more carefully than has been done here in order to explain all the differences. Not only is agriculture influenced greatly by the differences in the soil from place to place, but also by the very fact that they are glacial soils. Being made up of partly ground-up rock fragments the soils are often stony and difficult to till. Unlike the soil of rock decay, the particles of which the glacial soil is made have been derived by mechanical grinding, not by chemical decay and disintegration. There has been less leaching out of the soluble compounds which make plant foods. These are stored up in the rock fragments ready for use when decay causes the proper changes to produce the soluble compounds which plants require. [Illustration: _Fig. 34. Hummocky moraine hills in the background and a level gravel plain--an ancient glacial-stream flood-plain--in foreground._] Slowly the glacial soils are decaying, and, as they do so, are furnishing plant-food to the water which the roots greedily draw in. So the glacial soil is not a mere store house of plant-food, but a manufactory of it as well, and glacial soils are therefore "strong" and last for a long time. That decay is going on, especially near the surface, may often be seen in a cut in the soil, where the natural blue color of the drift is seen below, while near the surface the soil is rusted yellow by the decay of certain minerals which contain iron.