'Review of decent Geological Literature. 123 
its different portions adjoining the driftless area, conditions attending 
the deposition of the loess, the influence of gravitation toward the ice- 
sheet to change the levels of lakes and the rate of descent of streams, 
and the elevation of the district during the chief interglacial epoch. No 
brief review can even inention the many directions in which important 
researches have been made bj the authors in this work. 
The driftless area, lying chiefly in southwestern Wisconsin, but in- 
cluding also the northwest corner of Illinois, and reaching 10 to 40 miles 
west of the Mississippi into northeastern Iowa and southwestern Minne- 
sota, has an extent of 200 miles from north to south and a width of 
about 100 miles. Thence east to the Atlantic, north to the Arctic ocean, 
and northwest to the Pacific, the continent is overspread by the glacial 
drift, which also covers a width of 340 miles on the west and about 225 
miles in its narrowest portion on the south. Cambrian and Silurian 
formations of sandstone, limestone and shale extend from the driftless 
to the adjoining drift-covered country, which also are nearly alike in 
their average hight. The driftless area therefore, with no till, boulders, 
nor effects of glaciation, such as are found on all sides of it, must be 
nearly the same in its contour and superficial deposits as the surrounding 
region was before the glacial period. 
The most noteworthy element in the contour of the driftless area is its 
isolated cliffs of the nearly horizontal limestone and sandstone strata, 
spared in the process of subaerial erosion, and standing forth like castles, 
towers, and pillars. In many places the valleys of streams arc bordered 
by similarly precipitous walls of rock. The Mississippi has cut a valley 
300 to 600 feet deep and from one to seven miles wide; and the valley of 
the Wisconsin river averages 375 feet in depth and three miles in width. 
Concerning the time when this sculpturing of the surface was effected, 
the authors conclude that the driftless area was a low lying tract until 
the Tertiary age, and hence was subject to but slow and slight erosion, 
and that much of it was accomplished so late as the closing stages of the 
Tertiary age and the transition period to the glacial epoch. 
On the greater part of the driftless area east of the Mississippi the sur- 
face deposit is residuary earth, mostly very fine clay, left from the 
erosion of the overlying strata. Its average depth is about seven feet, 
being far less than the average thickness of the drift upon the surround- 
ing region. On the part of the driftless area west of the Mississipjii and 
that closely bordering this river on its east side, the residuary earth is 
overspread by loess, a fine silt of glacial origin, deposited in a lake or 
broad river with very slow current, which was continuous southward, as 
shown by the extension of the loess, to the lower part of the Mississipjii 
valley. The authors find no reason to believe that a lake, such as might 
be attributable to the barrier of the ice-sheet confluent on the south, 
covered the whole of the driftless area during any considerable time. 
The drift immediately adjoining the driftless area presents three 
phases, similar to those which characterize the southern border of the 
great drift sheet of the continent. On the east the Keltic moraine, ris- 
