be less than an eighth. The aggregate thickness of these alternating layers of clay and sand is sometimes a hundred feet or more. Let it be noticed that these areas of clay subsoil are those in which there is a gentle descent, and drainage to the north or northeast into some one of the great interior lakes of fresh water. The relation this fact bears to the origin of this clay subsoil will be considered farther on.
The gravel or sand subsoil is that which is found in some tracts of rolling land where the drift is heavy, and at points more remote from the valleys of northward drainage, or in the upper portions of those valleys. As a general rule, when present, it will be found on a higher level than that in which the subsoil is clay. It pertains to the interior country like the central part of the southern peninsula of Michigan, the central and southern portions of Ohio, Indiana, and Illinois, and some parts of Central and Northern Minnesota. The area and location of this kind of subsoil are more irregular and more uncertain than the areas of clay subsoil. Such gravel and sand deposits often lie in belts traceable for a great many miles, especially where the general surface is smooth, and the underlying rock of uniform hardness, the country adjoining being, on either side of the belt, one of a clay subsoil, or one formed by No. 3. Such belts are sometimes three or four rods wide, or they may be much wider, and are rolling and slightly raised above the adjoining clay land. Sometimes, instead of lying in belts, such rolling, gravelly land is spread out over areas of no definite shape or limit. The sand or gravel constituting the subsoil in these rolling tracts is, like the clay of the clay subsoil, stratified and assorted. But the layers here are rarely horizontal. They show the most various alternation and change of dip. No two sections could be taken that would give the same succession of parts. The sand sometimes lies in heavy deposits fifteen or twenty feet thick, with lines of deposition running in curving and vanishing layers in all directions. Sudden transitions occur from sand to gravel, or from gravel to bowlders. Sometimes, also, bowlders are found embedded in the gravel; again, nests of bowlders are seen isolated from the rest, and packed closely by themselves. There is also very often a mingling of gravel and sand, with no clay, without stratification, as if the two had been dumped together, after having been first thoroughly washed and assorted. Occasionally, also, in this stratified gravel and sand, may be seen irregular masses of gravelly clay or hard-pan, comparable to those mentioned by Mr. Lewis at Brooklyn. Such gravelly clay sometimes embraces stones of considerable size. Near the bottom of this stratified gravel and sand there are also, often, upward protrusions of the underlying member of the drift (No. 3), somewhat wedge-shaped or oblique, so as to embrace on the lower side a portion of the stratified gravel and sand. Again, the line of junction between the gravel and sand, and the hard-pan of No. 3, may be marked by an unusual accumulation of coarse drift materials, such as stones and bowlders. These may be mostly surrounded by
