400 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



but are more likely to be sharp in contour. The large sharp knolls, 75 to 

 100 feet in height, are almost wholly, while those from 75 down to 30 feet 

 are mainly, composed of gravel and sand. This relation between the form 

 and structure is not an exceptional one confined to this particular morainic 

 system, but a common one in nearly all the moraines yet examined between 

 Wisconsin and New York. 



The list of well sections given below serves to bring out a second fact, 

 namely, that the thick deposits of drift in the valleys contain a larger pro- 

 portion of sand and gravel than deposits of similar thickness on the uplands. 

 This is generally true not only of the morainic tracts, but also of plains 

 between the moraines. It is not uncommon to find a surface deposit of till 

 on uplands and valleys alike, while the substrata in the valleys are assorted 

 material, and those on the uplands are till. This relation between the 

 structure of drift in valleys and that on uplands seems to have wide 

 prevalence in hilly portions of the glaciated districts. Among several 

 causes which may have been operative in producing the phenomena are the 

 following: First, streams flowing from the ice sheet as it made its earliest 

 invasion would, under favorable conditions, deposit much assorted material 

 in the valleys in advance of the ice sheet ; second, during oscillatory retreats 

 the ice sheet would be withdrawn somewhat from the valleys, and the 

 assorted material would then, under favorable conditions, be added to the 

 portions outside the ice sheet; third, while the ice was occupying the hilly 

 districts it may have molded itself so imperfectly to the irregularities of 

 the surface as to have left comparatively free passage for waters beneath it 

 along the principal valleys. In the cases cited, where valleys would 

 probably receive contributions of assorted material, the uplands would not 

 be likely to have received such contributions, and it is scarcely probable 

 that an ice sheet under conditions of free drainage, such as the Scioto lobe 

 seems to have had when this morainic system was forming, could do other- 

 wise than fill the valleys with assorted material before the deposition of till 

 had fairl}?- begun. When the valleys became so filled as to greatly impede 

 the flow of water the subglacial drainage would be checked and till would 

 be deposited in the vallej^s as well as on the uplands. To what extent the 

 valleys were filled at the earlier invasions (lowau, Illinoiau, Kansan and pre- 

 Kausan) is not clearly determined, tliough the amount was probably great. 



In the detailed account of well sections and other exposures of the drift 

 given below, the districts are taken in the following order: (1) The inter- 



