Leverett — Weathering and Erosion as Time Measures. 361 



covers the Illinoian drift, and seems to have been deposited 

 after the Kansan drift had been very greatly eroded, for it 

 covers the valley terraces and valley slopes as well as the 

 uplands, and appears deep down in the valleys in positions that 

 can scarcely have been reached by a secondary deposition or 

 by a creep of the deposit. Beneath the loess on the remnants 

 of the old drift plain, preserved between the valleys, one finds 

 a black soil and muck similar to that at the junction of the 

 loess and the Illinoian drift. The region seems, therefore, to 

 have had alternations of wet and dry climates which will compli- 

 cate to some extent the estimates of erosion. So far as the 

 time element is concerned the periods of aridity will render 

 the length of the Post-Kansan time greater than that required 

 for the mere cutting of the valleys under present conditions of 

 precipitation. The presence of two sheets of loess would seem 

 to call for two periods of aridity. 



The topographic maps of northeastern Missouri, from which 

 figs. 5 and 6 are taken, represent a region where the erosion 

 is almost entirely in glacial deposits, the rock surface being 

 deeply buried. It will be observed that a large part of the 

 original Kansan drift plain has been destroyed, the plain being 

 preserved only in narrow tabular divides, such as that on 

 which the towns of Atlanta and Memphis stand. The valley 

 slopes as well as valley bottoms are very broad. The slopes 

 are so toned down as to be generally below an angle of 5° and 

 not uncommonly are reduced to 3° or even less. By reference 

 to figs. 5 and 6 it will be seen that the 20-foot contours are 

 generally broadly spaced, so that four contours, or 80 feet 

 elevation, occupy a breadth of one-fourth to one-half mile on 

 a valley slope. On streams of comparatively small drainage 

 area the valley bottoms frequently exceed a mile in breadth. 

 In the region covered by these maps and in much of northern 

 Missouri and southern Iowa narrow tabular remnants of the 

 original drift surface are preserved. But as one passes from 

 the tributaries of the Mississippi westward to the tributaries 

 of the Missouri these tabular remnants gradually dwindle and 

 in the vicinity of the Missouri valley entirely disappear. The 

 tributaries of the Missouri have some advantage in directness 

 of drainage, which probably partially accounts for the more 

 advanced state of drainage erosion. There may also have been 

 a flatter surface in the vicinity of the Mississippi, since the 

 underlying limestone there was left with broad tabular surfaces 

 at the oncoming of glaciation, while the sandstone areas to the 

 west were broken up into ridges and hills. The tabular divides 

 in the drift surface, however, do not coincide with preglacial 

 divides in the limestone surface, nor are they restricted to the 

 limestone area, but extend westward some distance into the 



