230 The American Geologist. April, i89i 
joining area of its bed, from near Breckenridge, northward beyond 
the mouth of the Sheyenne. Much alluvium was also supplied 
from the erosion of the Sheyenne valley, which, with that of the 
Big Coulee, probably averages three fourths of a mile in width 
and 150 feet in depth along a distance of 200 miles. This chan- 
nel is cut in the drift sheet, mainly till, and in the underMng 
easily eroded Cretaceous shales. The volume of the material sup- 
plied from it would be equal, according to these estimates, to about 
three fourths of the Sheyenne delta, or perhaps to three eighths of 
both the delta and the finer claj-ey sediments that were deposited 
farther out in the lake. But the valley of the Sheyenne was doubt- 
less also both a preglacial and an interglacial valle}'. It was prob- 
ably wholly filled with till in the first glacial epoch, then was 
eroded, chiefly in this drift, to nearly its present size during inter- 
glacial time, and was partially but perhaps not wholly refilled with 
till in the last epoch of glaciation. If it retained in considerable 
degree its trough-like form beneath the last ice-sheet, as was evi- 
dently true of the Minnesota valley, its erosion and its tribute to 
the Sheyenne delta would be less than the proportion estimated. 
When the bed of lake Agassiz was gradually uncovered from 
the water of the receding lake, some parts of its central plain 
through which the Red river flows probably remained as broad, 
shallow basins of water, which that river and its tributaries have 
since filled with their fine cla3'ey alluvium. The similar clayey 
silt brought into lake Agassiz by its delta-forming affluents, the 
Buffalo, Sand Hill, Sheyenne, Pembina, and Assiniboine rivers, 
and others farther north, has been spread over large areas of the 
lake bed, but more extensive portions had a surface of till, with 
no such lacustrine deposit. Over these formations, much alluv- 
ium has been laid down along the avenues of drainage of the old 
lake bed, anil it has filled depressions of the original surface, 
whether of lacustrine sediments or of till, being only distinguish- 
able from the former by its containing in some places shells like 
those now living in the shallow lakes of the country adjoining the 
area of lake Agassiz, remains of rushes and sedges and peaty de- 
posits, as of the present marshes of the Red River valley, and 
occasional branches and logs of wood, such as are floated down by 
streams in their stages of flood. Thus the occurrence of shells, 
rushes and sedges in these alluvial beds at McCauleyville, Minne- 
sota, 32 and 45 feet below the surface, or about 7 and 20 feet be- 
