132 THE GLACIAL LAKE AGASSIZ. 



DRIFT DEPOSITS OX THE LACUSTRIIfE AREA AND THE ADJOIN- 

 ING REGION. 



The accompanying map (PI. XVII) exhibits the diverse formations of 

 drift, lacnsti-ine, and alluvial deposits, described in this and following chap- 

 ters, occurring within the somewhat thoroughly examined prairie portion 

 of Lake Agassiz, with considerable tracts of the adjoining country. 



PI. Ill, in Chapter I, drawn on a smaller scale, shows a greater extent 

 of the terminal moraines, and the courses of glacial strise (as noted in 

 Appendix A), upon almost the entire hydrographic basin of Lake Agassiz, 

 with a large area eastward to Hudson Bay and the upper Laurentian lakes. 



Derivation of the drift from preglacial residuary detritus and from glacial 

 erosion. — The loose superficial material provided by preglacial weathering 

 and stream erosion was generally plowed iip and removed by the ice-sheet, 

 being carried forward in the direction of its motion and mingled with other 

 material similarly gathered along the path of the glacial current. Besides 

 the gravel and finer alluvial detritus of valleys and a mantle of residuaiy 

 clay, more or less enveloping all the country, occasional bowlders and rock 

 masses were supplied on the higher lands by the irregular action of the 

 preglacial denudation, ready to be borne along and deposited in the glacial 

 drift. But the ice-sheet commonly did more than to remove the loose 

 material before existing, as is shown by rock surfaces embossed, planed, 

 and sti'iated by glacial erosion. In general, far the greater part of the 

 drift was thus worn off, and most of its bowlders were torn and plucked 

 away, from the rock floor over which the ice-sheet moved, grinding- it with 

 the drift material contained in its basal portion l^nder the pressure of the 

 enormous weight of thousands of feet of ice. The large proportion of 

 limestone present in the sand and finely powdered rock of the drift in 

 regions of limestone foi'mations demonstrates, as Professor Ghamberlin 

 has shown, that the drift was chiefly derived from glacial wearing of the 

 bed-rocks.^ 



' U. S. Geol. Survey, Third Annual Report, p. 312, and Sixth Annual Report, memoir by T. C. 

 Chamberlin and R. D. Salisbury, "The driftless area of the Upper Mississippi," pp. 241, 247, 255; 

 and Am. Jour. Sci. (3), Vol. XXVII, p. 388, May, 1884. Compare "Composition of the till or bowlder- 

 clay," by W. O. Crosby, Proceedings of Boston Society of Natural History, Vol. XXV, 1890, pp. 115-140. 



