REVIEWS 605 



and in these remaining localities only two species of fresh-water pulmonates appear. 

 In closing his report Shimek remarks that "the collection as a whole adds emphatic 

 evidence of the fact, no longer to be doubted, that the loess was not of subaqueous 

 origin." This unqualified statement appears rather singular in view of the fact that, 

 in the report on this county, Udden calls attention to the presence of a water-deposited 

 loess beneath the part of that formation thought to be wind-deposited. 



Udden, J. A. Loess with Horizontal Shearing Planes. Journal of 



Geology, Vol. X, pp. 245-51, 1902. 



The shearing planes here discussed were observed in Pottawattamie county, Iowa 

 They occur at the junction of the typical or pebbleless loess with a slightly pebbly 

 deposit of loess-like material which underlies it. They appear to indicate a differential 

 motion of the layers which they separate. The direction of movement is prevailingly 

 from northeast to southwest, in harmony with the direction of general slope of the 

 upland, but not in harmony with local topography. To account for the motion two 

 hypotheses are suggested: (l) Tundra conditions may have prevailed, and the annual 

 temperature changes caused creeping on a sloping plain; (2 ) there may have been 

 glacial conditions. With a sudden onset of arctic climate, an extension of snow far 

 beyond the margin of the ice seems probable, and an extra-morainic neve might 

 develop. Such a snow-covered region would furnish ideal conditions for loess accu- 

 mulation. A slow creeping of the entire field might take place in the direction of 

 general slope of the land. 



Udden, J, A. Geology of Jefferson County. low^a Geol. Survey, Vol. XII, 

 PP- 355-437, 1902. 



The physiography is discussed on pp. 359-67; well records, on pp. 419-22; Pleis- 

 tocene deposits, on pp. 422-29 ; glacial scorings, on p. 430 ; water supply and soils, 

 on pp. 436, 437; while the remainder of the report treats of hard-rock geology. The 

 drift of this county appears to have been left as a very smooth plain, with a gentle 

 southeastward slope of 3 to 5 feet per mile. In this plain the rivers, creeks, and 

 smaller streams have sunk their valleys 50 to 150 feet. It is estimated that from one- 

 fourth to one-fifth of the total area of the county stands either at the original level of 

 the plain or is included in the very gradual slope on the border between the plain and 

 the steeper valley slope. The slopes are one-eighth to one-half mile wide, and show 

 a rate of descent of 50 to 150 feet per mile, with the greatest pitch somewhat below 

 the middle of the slope. The slopes facing the south are more gradual than those 

 facing the north, and this is thought to be largely due to the rapid alternations of 

 freezing and thawing to which the south-facing slopes are subjected. (See review of 

 paper by B. L. Miller above, for other possible factors.) Terraces are found on Skunk 

 River up to 125 feet above the stream, or 40 feet below the upland, and on Cedar 

 Creek to 80 feet above the stream. The highest terraces have been shown by 

 Leverett to be referable to drainage at the time of the Illinoian ice invasion. Udden 

 doubts if they indicate a lower elevation of the land than now, for they seem, in Jef- 

 ferson county at least, to have been dependent upon obstructions produced by rock 

 sills in the valleys. 



A list of twenty-seven wells shows a variation of but no feet in the altitude of 

 the rock surfaee. If no deep-buried valleys occur which have been escaped in the 

 borings, the reliefs of the rock surface are no greater than those of the drift surface. 



The occurrence of weather-stained, leached, and partially indurated gravel beds 

 at the base of the drift is noted. As these contain Archn^an rocks, they appear to be 



