Sublacustrine Till. — Upham. 371 
This explanation would evidently greatly diminish the 
work demanded of these streams in cleaning their channels of 
the materials supposed to have been deposited in them by the 
glacier; and it may also have some bearing on the mode oi 
the formation of the river terraces of this border region of 
the Driftless Area. 
March 31, 1896. 
SUBLACUSTRINE TILL. 
By Waeeek Upham, St. Paul, Minn. 
Criteria of Deposits op Till in Glacial Lakes. 
On nearly level expanses enclosed by the beach ridges of 
glacial lakes, surfaces of till are more smooth than on the 
continuation of the same almost level but slightly ascending 
tracts outside the old lake area. The very remarkable flatness 
of the sublacustrine till is succeeded, as soon as the highest 
of the ancient shore lines is crossed, by a moderately undula- 
ting contour, with swells and long wave-like ridges of gentle 
slopes, rising usually 5 to 10 or 15 feet above the intervening 
hollows. In a broad view of such an undulating till expanse, 
when it is prairie, as on extensive portions of the borders of 
the glacial lakes Agassiz and Minnesota, the horizon appears 
as level as that of the ocean, though near at hand the surface 
is uneven with massive low ridges of similar width and highl 
as the heavy undulations into which the sea is raised by great 
storms. The same till deposit, continuously forming the sur- 
face on both sides of the small beach ridge of sand and gravel, 
is commonly very flat within the lacustrine area, often having 
no perceptible undulations and stretching many miles as a 
monotonously uniform plain. From the great extent of this 
smoothed contour of the suhlacustrine till, it appeal's to be 
attributable almost entirely to the action of the lake washing 
the receding margin of the ice-sheet, with only slight later 
modification by the wave erosion which supplied the scanty 
beach deposits. 
Another feature of the sublacustrine till is an imperfecl 
stratification, usually observable in some degree through the 
whole or some pari of any freshly excavated sect ion. as of cellars 
and wells, to depths of In to !•"> feet, or more, below the sur- 
face. Instead of the altogether unstratified condition which 
