124 lievietv of decent Geological Literature. 
ing in conspicuous hills and ridges, forms the boundary along a distance 
of about eighty miles; on the northeastern and northern sides of the 
driftless area its boundary is a drift sheet gradually attenuated and 
terminating in a thin edge instead of a ridged moraine; and on the west 
border there is a tract over which there are only scattered drift pebbles, 
succeeded westward by such an attenuated drift sheet. The morainic- 
border belongs to the later part of the glacial period, when the streams 
of the driftless area had slopes similar to those of the present time, so 
tliat beds of gravel and sand, washed from the melting ice-sheet, were 
deposited along their valleys. The glacial flood-plains have been mostl\- 
removed by post-glacial erosion; and their remnants form terraces at 
hights from 50 to 100 feet above the Mississippi, Wisconsin and Chij)- 
pewa rivers. 
Grand topographic features of the legion north and east of the drift- 
less area, where the Keweenawan range of highland rises between the 
deep basins of lakes Superior and Michigan, are considered the chief 
cause of the exemption of this region from glaciation, as had been 
pointed out by Prof. N. II. Winchell and Prof. Irving. 
From their study of the driftless aiea and the bordering drift, the 
authors deduce the following sequence of events in the glacial period. ^ 
1. The ice-sheet in its maximum extent reached from the north 
around both sides of the driftless area and coalesced south of it. To 
this stage is referred the attenuated drift sheet which forms the border 
on the southeastern and northern sides of the area, and also the pebbly 
border on its west side, which reaches beyond the attenuated drift there 
and seems to be due to floating ice in a marginal lake. 
2. Recession of the ice-sheet permitted the growth of trees and the 
formation of peat-bogs. 
3. By a re-advance of the ice, a mantle of till deeper than the earlier 
one was deposited, burying trees and peat beneath it; and the loess was 
washed down from the melting ice-fields into a lake-like river upon 
their border. 
4. A long period of freedom from glaciation appears to have followed 
the epoch of the deposition of the second till and loess, involving ex- 
tensive erosion and the lifting of the upper portion of the Mississippi 
basin to the extent of 800 to 1,000 feet. The evidence of this is foimd in 
the low altitude and gentle slopes which must hav e pre\ailed when the 
loess was deposited along the great streams from Nebraska to Indiana 
and southward to the Gulf ; and in the higher elevation which made it 
possible for the later glacial streams to flow with rapid, pebble-carrying 
currents at altitudes 700 feet below the summit of the loess. 
5. Following this chief interglacial epoch came the incursion of ice 
which pushed up at its edge the Kettle moraine and sent coursing down 
through the valleys its gravel-bearing streams, filling them and spread- 
ing out broad flood-plains. 
6. There closely followed a succession of stages of retreat of the ice- 
sheet, interrupted by times of halt and partial re-advance. 
