IOWA ACADEMY OP SCIENCES. 59 



granite boulders of massive size. Some of these boulders may- 

 be seen perched on the very margin of the pit, and some have 

 been undermined in taking out the gravel and have fallen to the 

 bottom. The surface of the whole surrounding region is thickly 

 strewn with lowan boulders. It is evident that the lowan drift 

 sheet was spread over northeastern Iowa after the gravels were 

 in place. 



These sands and gravels are now so incoherent that they may 

 be excavated easily with the shovel, and yet there is no evidence 

 that the glaciers that transported the overlying boulders and 

 distributed the lowan drift cut into them, or disturbed them, to 

 any appreciable extent. The lowan ice sheet was probably 

 thin , and all the loose surface materials in front of its advancing 

 edge were frozen solid. The thickness of the gravels is some- 

 what variable, owing to the uneven floor upon which they were 

 deposited, but it ranges from fifteen to twenty feet. The beds 

 have been worked out in places down to the blue clay of the 

 Kansan drift. 



Throughout the gravel bed, but more particular y in the 

 lower portion of it, there are numerous boulders that range in 

 diameter up to ten or twelve inches. These boulders are all of 

 the Kansan type. Fine grained greenstones predominate. Pro- 

 portionally large numbers of them are planed and scored on one 

 or two sides. Those that are too large to be used as ballast are 

 thrown aside on the bottom of the excavation, and in the course 

 of a few seasons many of the granites and other species crumble 

 into sand. The contrast between the decayed granites of tha 

 Kansan stage and the fresh, hard, undecayed lowan boulders 

 in the drift sheet above the gravels, is very striking. Many of 

 the boulders from the gravels are coated more or less with a 

 secondary calcareous deposit, a feature not uncommon among 

 boulders taken directiy from the Kansan drift sheet in other 

 parts of Iowa. 



As to their origin the Buchanan gravels are made up of 

 materials derived from the Kansan drift. As to age they must 

 have been laid down in a body of water immediately behind the 

 retreating edge of the Kansan ice. There are reasons for 

 believing that the Kansan ice was vastly thicker than the lowan, 

 but the temperature was milder, and so when the period of 

 melting came enormous volumes of water were set free. That 

 strong currents were developed is evidenced by the coarse char- 

 acter of the material deposited as well as by the conspicuous 



