SUPERFICIAL DEPOSITS ALONG THE 

 MISSISSIPPI 



Gerard Fowke. 



For the most ]:)art oeologists. and otliers, who have studied 

 the loess formation in tlie states bordering on the Missouri and 

 Mississippi rivers, concur in attributing the deposit to glacial 

 floods which attained their maximum when the ice Avas melting 

 along the front more rapidly than it could advance from the north. 

 The material is clay and sand, in varying proportions, modified 

 more or less In' local detritus. Consiclering the ease with which 

 it is excavated, its power to withstand pressure or erosion is some- 

 thing remarkable. This quality is especially noticeable along the 

 ^Missouri blnffs; below that river it becomes less resistant. On the 

 upper portions of the two great rivers, the loess is hea^^^, forming 

 high blutfs and spreading far inland; southward, it progressively 

 diminishes in extent and thickness. This fact, reinforced by 

 similar conditions observable along tributary streams, have enabled 

 students to determine that coincident with the greatest extension 

 of the glacier, and lasting until the ])resent time, there was a 

 marked subsidence of land, relative to sea-level, in the Mississippi 

 A'alley ; the subsidence being more pronounced toward the north. 

 The current of southward flowing streams was retarded, and the 

 sediment-laden Avaters began to free themselves from silt, by 

 precipitation, almost at once upon their emergence from the ice. 

 There was still sufficient movement, however, to carry the finer 

 sus]>ended matter until sea-level was reached. 



The limit of the ice-sheet, in southern Illinois, was along the 

 hills bordering P)ig Muddy on the north, almost to the mouth of 

 that stream, as it passed into the Mississippi at Grand Tower; 

 tl-ience northward, closely following the line of the larger stream, 

 nearlv to Alton; thence, crossing into Missouri, it skirted the north 

 side of the Missouri river nearly to the middle of the state. On 

 the bluffs at the mouth of the Missouri, on the south side,, is con- 

 siderable glacial drift; until very recently it has been uncertain 

 whether it niarked an extension of the glacier, or whether it is due 

 to floating ice. Within the past year, the excessive rainfall has 

 enabled two little streams to carry away enough overlying gravel 

 to reveal two small areas of typical till ; so it is now certain that 

 tlie main l)ody of ice shut ofl the :\Iissouri and consequently acted 

 as a temporary dam. Further, Brodhead records the occurrence 

 of gravel, which he supposed to be of glacial origin, on the highest 

 point in St. Louis county, about 350 feet above the Mississippi. It 

 seemed possible, from these facts, that the ice had attained sufficient 

 height to back the water up the Missouri a long distance and 



