Geology of the Region about Beloit 203 



Without dwelling longer upon the stratigraphical relations of 

 our rocks we pass on to note one or two points of interest in their 

 later geological history. 



The two lines of bluffs already mentioned which are the 

 boundaries of the present Eock river bottoms, with their stratified 

 Champlain gravels, were not only the banks of the Champlain 

 lake into which the river expanded as a flood from the melting 

 j glacier, but they were also the banks of a rather remarkable chan- 

 ilnel which the preglacial Rock river cut for itself to the depth of 

 lover four hundred feet through Trenton limestone, St. Peters 

 [i sandstone, Lower Magnesian limestone and into the Potsdam 

 sandstone ; at least this was the depth as shown by the artesian 

 i well at Janesville, and at a point a few miles lower in its course 

 j it could not have been much less. That it is a preglacial valley 

 is evident enough from the fact that the path of the glacier, as 

 shown by stria3 at Hanchett's quarry, was almost exactly west or 

 squarely across the Rock river channel. This fact is in itself in- 

 teresting as being the only case, so far as I know, in which glacial 

 s';rife are found outside the Kettle morain to indicate the direction 

 in which the glacier had moved previous to the retreat and subse- 

 quent advance which formed the Kettle morain. The direction of 

 the glacier in our region had been conjectured from some meagre 

 data to be about southwest ; the discovery of these markings is 

 therefore of interest as showing that the tongue of ice which pro- 

 duced them, apparently a continuation of Ithe Lake Michigan 

 glacier, was, at this point at least, deflected perhaps by a valley to 

 the north of the quarry, so as to move due west ; this bemg the 

 case, the banks of our preglacial valley were doubtless originally 

 higher even than now, so that this ancient channel must have had 

 rather a remarkable depth. Its width at Beloit is three to four 

 miles; a few miles farther south it narrows to one and a half 

 miles. At Rockton the confluence of the Pecatonica with Rock 

 river constituted quite a lake in the Champlain period, but it does 

 not represent so large a preglacial valley since the bottoms be- 

 tween Beloit and Rockton are underlaid by rock as shown m 

 several places, showing that the Champlain floods escaped over 

 the low rim of rock at that point and determined their own limits 



