IOWA ACADEMY OF SCIENCES. 67 



to be used as ballast have crumbled to line sand under the action 

 of the weather. Finally the gravel is exceedingly ferruginous 

 in places, and is everywhere much stained and weathered, 

 particularly near the top of the deposit, the weathered portion 

 taking on a characteristic reddish-brown color. 



At the typical locality the Buchanan gravel rests on blue till 

 of Kansan age, and is overlain by a bed of lowan till varying 

 from two to eight feet in thickness. The lowan drift, as is 

 usual in this vicinity, contains very many large, light-colored 

 granite bowlders. Some of these are perched on the very 

 brink of the pit, some were undermined in taking out the gravel 

 and have fallen to the bottom, others lie scattered in great 

 numbers over the adjacent fields. Within a radius of one-fourth 

 of a mile bowlders may be found ten, fifteen, or even twenty 

 feet in diameter. 



It is clear to the most casual observation that the gravel 

 bed near Independence lies between two sheets of till, and that 

 the weathering, oxidation and extensive decay that the mate- 

 rials have suffered constitute in some degree a measure of the 

 great length of the interglacial period. At the time the gravels 

 were first studied, only two years ago, remember, it was the 

 current belief that the Pleistocene deposits of Iowa, except in 

 the area occupied by the Wisconsin lobe, contained a record of 

 two ice invasions, and of two only. Accordingly the Aftonian 

 gravels and soil beds which had been previously observed in 

 Union county, were assumed to lie between McGee's lower and 

 upper till; and since the Buchanan gravels plainly occupied 

 what seemed to be a similar position, they were first referred 

 to the Aftonian stage. Our knowledge of Pleistocene geology 

 has moved with tremendous strides during the past two years. 

 A review of its progress would occupy more space than can be 

 given to this paper. A few points, however, must be noted. 

 First, Bain showed that the till overlying the Aftonian beds 

 was Kansan, the lower till of McGee, and not the lowan, or 

 upper till as had been assumed. This observation rendered 

 necessary a series of adjustments in views previously enter- 

 tained. A new drift sheet was added to the glacial series of 

 this region, and the Aftonian and Buchanan beds were shown 

 to lie at different horizons. Before that adjustment Chamber- 

 lin* had published his classification of American glacial depos- 



*Journalof Geology, Vol. Ill, p. 370, April-May, 1895. 



The Great Ice Age, James Gelkie, third edition, 1895, Chamberlain contributes the 

 chapters on Glacial Phenomena In N. A., pp 734-774. 



