RELATION OF TOPOGRAPHY TO PLEISTOCENE FORMATIONS. 541 



county. There is also a degradation or lowering of the plain near the 

 margin in the axes of the valleys near Salt river, Cuivre river and the 

 Big Muddy. For example, although the general elevation of the plain 

 on either side is about 850 feet near Troy, in the valley of the Cuivre, 

 the drift overlaid by loamy clay rises to an altitude of less than 750 feet, 

 and more distinctly disturbed masses may be found similarly related 

 even down to the level of the stream. 



Another feature which has a very important bearing upon our problems 

 is the low altitude of the divide between the Missouri and Meramec 

 rivers at Grays summit. This is traversed by a valley or col at an alti- 

 tude of only 665 feet above the level of the sea. This valley, by its relief 

 and its loose material, suggests a former channel, possibly occupied for 

 only a short time. I found no distinctly alluvial deposits. It is bounded 

 on the east by a rather abrupt escarpment rising over 200 feet higher. 

 This tableland is composed of the lower formations of the Silurian 

 rocks, the same which formed the higher knobs bounding the margin of 

 the drift area Avhere it lies north of the Missouri and w^est of the Mis- 

 sissippi. This col rises only a little higher than the loess-topped terrace 

 at Augusta and x\lton, and is 200 feet lower than the edge of the drift, 

 which is only about fifteen miles distant, to the north^vest. 



Closely related to this fact is another, similarl}^ surprising and signifi- 

 cant, namely : While the Pleistocene deposits in eastern Missouri are 750 

 to 850 feet above the sea, similar deposits in Illinois, a few miles east, lie 

 at an altitude of only 500 to 600 feet. In other words, one descends 250 

 feet in going about 50 miles. The formations and topography are alike. 



Hypotheses as to the Origin of the Pleistocene Formations. 



There are but three suppositions which we conceive can be reasonably 

 made concerning the origin of the Pleistocene formations of the region 

 under consideration. Tlie first ascribes them mainly to subgiacial origin, 

 the second to lacustrine conditions assisted by floating ice, and the 

 third attributes them to flooded streams, also assisted by much floating 

 ice. 



DISCUSSIOX OF THE SUBGLACIAL HYPOTHESIS. 



The Hypothesis stated. — The most probable conception of the subgiacial 

 origin is about as follows : At the time of the maximum extent of the 

 o-reat ice-sheet it covered all areas in which are found either striated 

 ledges or accumulations of unstratified bowldery clay. This is probably 

 in harmony with the view generally accepted by glacialists. According 

 to this, the land ice must have extended, at least for a short time, nearly 

 to the boundary of the bowldery drift, or, in other words, to the vicinity 



