500 THE ILLINOIS GLACIAL LOBE. 



mouth of the stream is in all probability due to the hardness of the forma- 

 tion excavated. It is not so certain that the construction at Peoria is due 

 to the same cause. Indeed, there is a possibility that the stream here has 

 been deflected across a projecting point of the west bluff. The existence of 

 a broader channel to the east, however, has not been demonstrated. 



Several boriugs have been made along this preglacial valley, and they 

 present variations in the altitude of the rock bottom which are somewhat 

 perplexing. The borings in the vicinity of the bend of the Illinois at Peru, 

 Princeton, and Bureau Junction show a rock bottom only 325 to 340 feet 

 above tide. 1 Below this bend the rock bottom is usually encountered at a 

 slightly higher elevation. The majority of the wells are located within a 

 mile of the bluff of the preglacial valley, which might perhaps signify that 

 the deepest part of the channel had not been struck. But wells at Beards- 

 town, in the midst of the valley, enter rock at a level as high as at the bend 

 of the Illinois, 125 miles up. the valley. Bridge foundations near the mouth 

 of the Missouri and at St. Louis, 103 and 128 miles, respectively, below 

 Beardstown, show the rock bottom to descend to a level slightly less than 

 300 feet above tide, or about 50 feet lower than at Beardstown and at the 

 bend of the Illinois. It is barely possible that these bridge foundations 

 have not extended out to the deepest part of the channel, but it seems 

 scarcely probable that the floor has a much lower level in the middle of the 

 channel. The amount of descent below Beardstown is not remarkably 

 low, but in the 134 miles from Princeton to Beardstown there appears to be 

 no descent. 



This suggests the query whether there may not have been a differen- 

 tial northward depression in the portion of the Illinois Valley north from 

 Beardstown. To fully establish this depression, it will be necessary to 

 make certain that there is no deeper portion of the valley leading past 

 Beardstown and other points in the lower course of the stream. In view of 

 the possibility of northward depression, it seems pertinent to refer to a 

 possible cause for such a depression. To the general cause for northward 



1 In a paper published in the Journal of Geology (Vol. Ill, 1895) the writer called attention 

 to a boring at Princeton, Illinois, which was reported to have penetrated 440 feet of drift and to have 

 entered rock at 270 feet above tide. A subsequent boring, only a few feet distant, entered rock at 370 

 feet, but passed through a soft shale between 370 and 440 feet. It is now thought that the well first 

 made may also have entered this shale at a depth of about 370 feet, or an elevation 340 feet above 

 tide. 



