Sublacustrine Till. — Up ham. 373 
subglacial and thence was pushed out and tirst uncovered 
from the retreating ice under the water, much of it would 
have escaped the lacustrine leveling, and would remain in 
morainic hills and ridges. 
During my exploration of this largest glacial lake of North 
America, I have recognized no indication of icebergs ; and 
thence it seems to me probable that the receding ice-front had 
mostly too steep and high an ascent above the water level to 
permit portions of it to be buoyed up by the lake, which 
attained a depth of 600 to 700 feet in its deepest north central 
part.* 
Lake Minnesota. The area of the glacial lake in the basin 
of the Blue Earth and Minnesota rivers, belonging to a date 
somewhat preceding lake Agassiz, is mainly an exceedingly 
level expanse of till, with obscure stratification near the 
surface. j- 
The Laurentian glacial lakes. Very flat expanses of subla- 
custrine till are seen in many portions of the areas of the gla- 
cial lakes which are now represented by the diminished but 
still great lakes tributary to the St. Lawrence river. The 
cities of West Superior, Chicago, and Buffalo, are built mostly 
on these tracts, and they are very extensive in Canada north 
of lake Erie and along the St. Lawrence between lake Ontario 
and Montreal. 
The till which was brought by the ice-flow from the lake 
Superior basin has been mistaken on parts of its areas in Wis- 
consin for a stratified lacustrine silt. It is characterized by 
comparative scantiness in the supply of granite, gneiss, crys- 
talline schist and gabbro boulders, by the absence of limestone, 
and by the large proportion of fine detritus of dull reddish 
color from the erosion of the Cambrian red sandstones and 
shales, and of the partly sedimentary and partly igneous Ke- 
weenawan series, which form the shores and bed of lake Su- 
perior. This red till, with few boulders, has a typical subla- 
custrine development near Duluth and in West Superior, and 
forms the flat expanse which gradually rises from the west 
*Geol. Survey of Canada, Annual Report, new scries, vol. iv. for 
1888-89. Part E ; U. S. Geol. Survey, Monograph xxv. 
tGeology of Minnesota, vol. i. 1884, pp. 111. 160. 
