112 S. CALVIN low AN DRIFT 



the surface ; and the more massive ones have been utilized as granite 

 quarries from which contractors have taken large amounts of valuable 

 building material, as fresh as if it had been taken directly from the native 

 ledges. 



Sii.perjiclal j^osition. — At the risk of repetition, I would again emphasize 

 the great size of some of the Towan boulders and the prominence of many 

 of them above the surface of the ground. The common com[)arison with 

 houses and haystacks, suggested to every observer,is b}'- no means strained ; 

 for some of the ponderous blocks rise 12, 15, or 20 feet above the prairie 

 sod. While a large number of the boulders are completel}^ embedded 

 in the till, and while others are buried to a greater or less extent, there 

 are yet man}" that seem to rest practically on the surface Avith their 

 whole mass exposed above ground. At all events, the}'' are no more 

 embedded than would naturally be the case if they had been carefully 

 laid on the surface a century or two ago. All their surroundings preclude 

 the theory that they owe their present prominence to erosion and removal 

 of the finer constituents of the drift ; and the further fact that nearly all 

 those which have been quarried were found planed and worn to a true, 

 flat face on the lower side equally precludes the theory — even if there 

 were not other insuperable objections to it — that they were superglacial 

 during the period of their transportation. These superficial granites were 

 probably completely embedded and incorporated in the ice, with their 

 lower surfaces in position to be abraded and worn, yet coinciding with 

 the bottom of the glacier, while the detrital matter constituting the ground 

 moraine occupied the true subglacial position. There are reasons for 

 l)elieving that the amount of clay and other fine material carried beneath 

 the lowan ice was, locally at least, very small, for lowan boulders not 

 infrequently rest directly on residual clays, on Kansan drift, or on l)ald 

 rocky surfaces, the proper lowan till being absent. 



THICK NESS OF THE 10 WAN DRIFT 



The Towan sheet of till is comparatively thin. While liouUlers were 

 large and numerous, the finer materials transported l)y the lowan ice 

 were decidedly scant. In many cases they were insufficient completely 

 to obscure the pre-Iowan topography. For example, the present drain- 

 age is very largely controlled by imperfectly disguised valleys which had 

 been previously eroded in the surface of the Kansan till; and in not a 

 few instances the hills which have to be cut in making railway grades 

 are pre-Iowan ridges with Kansan drift rising within a few inches of the 

 surface. In the great cut at Oelwein, Fayette county, the lowan till is 

 represented by a layer of loamy soil less than a foot in thickness, while a 

 short distance to the southeast, in Fairbank township, Buchanan county. 



