. REPORT OF THE SECRETARY. 33 



would leave the whole in a condition identical with that in which the 

 drift formation is found. This formation, according to the author, 

 may be divided into three members, in the order of superposition, 

 -, from the surface downward, composed as follows: The first of coarse 

 sand, gravel, loam, and hard-pan, with large boulders from northern 

 rocks, exhibiting little stratification. The second of sand and gravel, 

 less coarse than the preceding, with irregular bands of clay some- 

 what laminated, and smaller boulders. The third of fine laminated 

 sand, of marly clay of great thickness, of various colors, with few 

 boulders and little gravel. Wherever a sufficient thickness of drift 

 occurs these divisions can be readily traced, and always in the same 

 order. The first occupies the highest points of land, and frequently 

 lies upon the rock formation without the intervention of the other 

 beds, and in it occur moraine hillocks and depressions intermingled 

 with the underlying strata at different points, indicating the move- 

 ment of ice, which, while it brought materials from a distance, 

 ploughed up and pulverized the previously existing surface. 



This upper member of the drift contains the coarsest material and 

 the largest and most numerous boulders, a fact inexplicable on any 

 other theory excepting that of their deposit by the melting of glaciers 

 carrying on their surface the heavier masses. The surface is pitted 

 with cavities extending below the general level ten, fifteen, and even 

 a hundred feet, their outline being rudely circular, and their sides as 

 steep as is compatible with the stability of the soil. In travelling 

 through this region the explorer frequently finds these hollows so 

 near together that he no sooner rises out of one than he is obliged to 

 descend into another. They seldom contain water, but boulders are 

 found at the bottom, on the sides, and on the surface around. In the 

 prairie regions of southern Wisconsin, timber grows within the cavi- 

 ties, as well as on the adjacent surface, in clumps, known as oak 

 orchards. The formation of such a system of depressions of so uni- 

 form a character, and over so large an extent of country, while the 

 rocks beneath, wherever uncovered, are found polished and grooved, 

 also indicates the movements and subsequent melting of large masses 

 of ice. In various places there occur patches of boulders from among 

 which the finer materials have been washed away. Near Twin Falls, 

 " on the Menomonee river, in Michigan, on the northern slope of a 

 mountain, such a collection occurs nearly a mile across, covering the 

 surface like a pavement with large masses smoothed and polished by 

 attrition. Along the heights of land collections of boulders also fre- 

 quently occur. After passing northward, above Lake Winnebago-} 

 3 s 



