LOESS AND THE IOWAN DRIFT. 355 



tify such a change, and certainly none appear in the recent 

 contribution in the American Geologist. 



So far as the writer has been able to determine, no one has 

 yet presented direct evidence of such lateral blending of loess 

 and Iowan drift as would prove their contemporaneousness. 

 According to Leverett's own testimony the loess is sometimes 

 above and sometimes below the Iowan drift. Referring to the 

 Iowan on p. 131 (1. c), he says that "the loess overlaps the 

 drift sheet only a short distance," and on p. 147 he reports a 

 drift, evidently Iowan, at the base of a thick loess deposit. 

 On p. 137 he states that "in places the Iowan till is covered 

 to a depth of several feet by loess," and on p. 138, that low 

 Iowan drift knolls at Polo "are capped by 4 feet of loess-like 

 silt." In the foregoing cases the loess caps the Iowan drift, but 

 in other sections Leverett found the loess under the Iowan. On 

 p. 138 (1. c.) he says: "At the village of Stratford, . . . the rail- 

 way exposes a bed of fossiliferous silt at the base of the Iowan 

 drift, resting on an old land surface formed on the Illinoisan. . . . 

 In two other localities fossiliferous silts have been found at the 

 base of the Iowan, one being . . . west of Irene, . . . and another 

 . . . one mile east of Belvidere. Here as at Stratford, the fos- 

 sils are mainly one species." * 



This fact, that loess is found both above and below the Iowan, 

 suggests that in the territory under discussion there were two 

 non-glacial periods of loess deposition, the one preceding and 

 the other following the advance of the Iowan glacier. 



Leverett evidently placed some reliance upon the greater 

 thickness of the loess along the Iowan drift-border in conclud- 

 ing that the two deposits were contemporaneous, for on p. 155 

 he says: "On the margin of the Iowan ice-sheets, especially 

 the one which occupied eastern Iowa, there is a thicker deposit 



*The species is Succinea avara, a common loess land-snail, of which 

 Leverett says (1. c, p. 165): — "a form which is now found in swampy 

 places as a rule, but which occasionally occurs in dry places. Shimek 

 regards this as strictly terrestrial rather than a semi-aquatic form." 

 Succinea avara is not found in swampy places as a rule, a fact which 

 the writer will demonstrate to anyone who will go to the field with him. 



