156 GEORGE D. HUBBARD 



In the latitude of Bradbury, some three miles south of the 

 outer margin of the moraine, the second bluff attains nearly the 

 same height above the first as the first does above the stream. 

 Farther north the first bluff rises higher with reference to the 

 stream and the outer bluff, until, near the moraine, the second 

 becomes almost imperceptible, while farther south the first gradu- 

 ally decreases in height until it becomes practically 7iil. Fig. 2 

 is a cross-section, somewhat idealized, about opposite Bradbury, 

 at a point where the two eastern bluffs coalesce. Fig. 3 is a 

 bird's-eye view much conventionalized, of the conditions along 

 the west side of the stream, as discussed above. 



What is the relation of all this to glaciation ? Evidently 

 there are two periods of erosion represented. Is one postglacial ? 

 Where, in time, is the other? The nature of the materials in the 

 two sets of bluffs and their relation to surrounding features alone 

 can solve the problem. 



Certainly there must be a preglacial valley somewhere near 

 this present stream, unless the whole plan of the preglacial drain- 

 age has been upset. The Ohio, Mississippi, Illinois, and lower 

 Wabash in this latitude are, in general, about where they were in 

 preglacial time ; hence the general direction of other large streams 

 must be similar to that of their earlier courses, but they are not 

 necessarily in exactly the same beds. The older drift, according 

 to the records of borings and coal shafts, and the exposed sections 

 in stream and railroad cuts, is from twenty to fifty feet thick, 

 rarely more, often less. Between the outer or second bluffs 

 which continue down the stream is a valley, increasing from a 

 mile to three miles in width, and in the deeper portion of its 

 cross-section about fifty feet deep, rarely revealing bed-rock in its 

 floor or sides. Both are practically of continuous drift, correspond- 

 ingin composition, physical characteristics, and state of weathering 

 with the general drift-sheet on either side. The valley may be 

 followed for fifty to sixty miles with no material change of char- 

 acter, so far as these features are concerned. 



There are three time-periods to be considered in the solution 

 of the origin of the valley in question. First, is it a preglacial 

 valley? Second, was it formed since the retreat of the last ice- 



