THE PREGLACIAL CONTOUR. 107 



Ball River, the Grrand and ^loreaii rivers, then united, the Cheyenne, and 

 the White River, tlowed east to the James Valley; and he is inclined to 

 believe that from that valley the great stream formed by these affluents 

 passed northeast to the Red River of the North and Hudson Bay.^ That 

 the greater part of the excavation of the trough of Lake Agassiz could be 

 accomplished by a river of such size during the Lafayette period of conti- 

 nental elevation, following the Pliocene period and inaugurating the Ice 

 age, may be readily believed when we compare it with the Lafayette 

 erosion of the Mississippi, which from Cairo southward, along an extent of 

 about 500 miles, fonned a channel 200 to 300 feet deep and avei-aging 60 

 miles wide.^ 



Tertiary and early Quaternary erosion had sculptured the grand features 

 of the basin of Lake Agassiz, and its whole extent probably had approx- 

 imately the same contour immediately before the accumulation of the ice- 

 sheet as at the present time. The surface of the feldspathic Archeau rocks 

 was doubtless in many places decomposed and kaolinized as it is now seen 

 where they are uncovered in the Minnesota Valley, and as such rocks are 

 frequently changed to a considerable depth in regions tliat have not been 

 glaciated. On these and all the other rock formations the ordinary disinte- 

 grating and eroding- agencies of rain and frost had been acting through long 

 ages. Much of the loose material thus supplied had been carried by streams 

 to the sea, but certainly much also remained and was spread in general 

 with considerable evenness over the sm-face, collecting to the greatest depth 

 in valleys, while on ridges or hilltops it would be thin or entirely washed 

 away. Except where it had been transported by streams and consequently 

 formed stratified deposits, the only fi-agments of rock held in this mass 

 w^ould be from underlying or adjoining rocks. The surface then probably 

 had more small inequalities than now, due to the irregular action of the 

 processes of weathering and denudation, which are apt to spare here and 

 there isolated cliffs, ridges, and hillocks; but most of these minor features 

 of the topography have been oljliterated by glacial erosion or buried under 

 the thick mantle of the drift. 



' Proc, A. A. A. S., Vol. XXXIII, 1884, pp. 381-393, with map. 



-Am. Naturalist, Vol. XXVIII, pp. 979-988. Dec, 1894. Bulletin, Geol. Soc. of America, Vol. V. 1894, 

 pp. 87-100. 



