604 REVIEWS 



geology. This county is covered by a very thick deposit of drift which was appa- 

 rently built up to a plane surface, with a slope, toward the south-southwest, of one to- 

 three feet per mile. The subsequent erosion has been so great that the surface is now 

 nearly all embraced in the slopes toward the valleys. The valley bottoms are 50 to 

 200 feet below the upland plain, with a general depth of 100 feet. The grade of the 

 steepest parts of the slopes is estimated to be about 13 feet to the 100, or 7° from the 

 horizontal, but the average is much less. The drainage lines are apparently inde- 

 pendent of preglacial courses, and it is suggested that the streams may radiate in 

 conformity to glacial movements in this county. If the reviewer correctly interprets 

 the hypothesis here advanced, the streams are thought to have had their start on the 

 surface of the ice-sheet, and their courses determined by crevassing of the ice, but it is 

 not made clear why the same kind of drainage may not have been developed on the 

 drift surface after the ice had disappeared. 



The average thickness of the drift is estimated to be 140 feet, exclusive of the 

 loess, which has a thickness of about 60 feet. The surface of the till is not so 

 much leached beneath the loess as in the Kansan drift in eastern Iowa. A dark 

 blue-black till is found in the lower part of the drift which resembles the pre-Kansan 

 till of eastern Iowa in color, structure, toughness, and position, and in containing 

 vegetation, but the upper part of the till does not carry the distinguishing character- 

 istics found in the Kansan of eastern Iowa. Its erratics appear to be of the same class 

 as those of the blue-black till. The drift of this part of Iowa presents the rather 

 anomolous and contradictory condition of being more eroded than that of the Kansan 

 of eastern Iowa, and yet less weathered and leached. If one were to judge by the 

 amount of erosion, it would be referred to the pre-Kansan stage of glaciation, but, 

 judged by the amount of weathering and leaching, it would seem to be no older than 

 the Illinoian, and possibly not older than the lowan. It is well known that leaching 

 progresses very slowly in arid districts, but it seems doubtful if the degree of aridity 

 of western Iowa compared with eastern is sufficient to produce this difference in the 

 amount of leaching. The rate of erosion should also be less in an arid than in a 

 humid region, and yet the more arid western part of Iowa has the greater amount of 

 erosion. 



In the northern part of the county a "gumbo " separates the loess from the till on 

 upland tracts, and the upper or gradual portions of the slopes. In places it appears 

 to graduate upward into the loess, and downward into the till. It is more clayey than 

 the loess, yet a mechanical analysis shows that about 95 per cent, of its material may 

 be classed as loess. Interpretations concerning its origin are not reached, but it is 

 suspected to be of diverse origin from place to place. 



The loess is capable of separation into two distinct parts, especially the loess 

 found in the valleys. The lower part is pebbly and shows clear evidence of water 

 bedding. The upper part lacks these characteristics and appears likely to be a wind- 

 deposited material. The reviewer would call attention to a feature which seems to 

 indicate very clearly that the upper part of the loess on the borders of the Missouri 

 valley in Iowa is wind-deposited. The topographic sheet of the Omaha-Council Bluffs 

 district shows that the Iowa bluff stands considerably higher than the Nebraska bluff, 

 and as the p)evailing wind is from the Nebraska toward the Iowa bluff, it is thought 

 that the latter has received the extra accumulation through the agency of the wind. 

 A report on the loess mollusks made by Shimek (pp. 261-65) shows that the fossils are 

 strictly terrestrial in nine out of the thirteen localities where collections were made, . 



