RELATION TO THE " FOREST BED " 113 



the farm wells show 30 feet of the yellow, calcareous till peculiar to this 

 age resting on highly oxidized beds of Buchanan gravel.* What the 

 maximum thickness over the pre-Iowan valleys may be has not been 

 determined, but the thickness in general is less than 20 feet, and over 

 large areas the average may be less than 10 feet. 



ReLATIOiN of the IoWAN TO THE " FOREST BeD " OF NORTHEASTERN loWA 



Remains of a forest bed which was overwhelmed and buried by ad- 

 vancing glaciers are conspicuous in many of the drift sections in north- 

 eastern Iowa. The abraded and splintered wood is distributed through 

 a zone a number of feet thick, but it is most abundant in connection with, 

 or just a little above, a definitely marked soil band and peat horizon. 

 The principal belt through which forest material is distributed lies above 

 the soil and peat. The soil band, peat beds, and forest remains are all 

 evidence of an interglacial age of longer or shorter duration, for there is 

 a heavy underlying till sheet that is older than either soil or forest. Ac- 

 cordingly, in dividing the Pleistocene deposits of the region under con- 

 sideration into a lower and an upper till, it was most natural that the 

 soil, peat, and forest horizon should be adopted as the line of separation. 

 For some time after the sheet of till we now call Kansan had been dif- 

 ferentiated from the true lowan by evidences of erosion and weathering, 

 the belief in a forest bed below the lowan and above the Kansan was 

 still entertained. The differentiation of the lowan and Kansan till sheets 

 was made as the result of studies carried on along or near the margin of 

 the lowan drift. The two-deposits were strikingly different. That they 

 were separated by an immensely long interglacial interval was as clear 

 as noonday. It was the unquestioned belief when the names Kansan 

 and lowan were applied to the two drifts we are now considering that 

 one was the upper till and the other the lower till of McGee, and that 

 they were the formations to which the names lowan and Kansan had 

 been applied by Chamberlin. If there was a forest and soil bed, it must 

 be between these two deposits, for as yet there was no evidence that there 

 were more than two deposits to be taken into consideration. Gradually, 

 however, as available sections were multiplied and opportunities for study 

 were enlarged, it was found that the forest bed and soil band were inva- 

 riably located beneath the drift we had been calling Kansan, and that 

 no section anywhere revealed the presence of forest material immediately 

 beneath the typical lowan. A ferretto zone is there, with most convinc- 

 ing evidence of prolonged and profound weathering, but no soil band, 

 no buried forest — at least, none in any way comparable with the wealth 



* Iowa Geel. Survej^, vol. iii, p. 245. Des Moines, Iowa, 1898. 



