THE DRIFT-DEPOSITS OF THE NORTHWEST. 205 



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 rreat 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 be- 

 ing, 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 consid- 

 erable 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 

 Hue of junction between the gravel and sand, and the hai-d-pan of No. 

 3, may be marked by an unusual accumulation of coarse drift materi- 

 als, such as stones and bowlders. These may be mostly surrounded by 



