PREVIOUS WORK IN WESTERN IOWA 401 



trine 21 or fluviatile, 22 and as modified drift, referring to them as "stratified 

 drift," 23 "gravelly drift," 24 etcetera, but no effort was made to fix their 

 stratigraphic relations. 25 Udden 26 evidently included them in the till, 

 or at least considered them of glacial origin. 27 



The gravel beds of Harrison and Monona counties have heretofore re- 

 ceived but little attention from geologists. Saint John considered them 

 "modified drift," 28 and Bain found "gravelly drift" in the northern part 

 of Monona county; 29 but here also no attempt was made to definitely 

 determine either their geologic horizon or their extent, for Saint John's 

 observations were made in connection with a preliminary survey of that 

 part of the state long before the modern differentiation of glacial and 

 interglacial deposits was recognized, and Bain's studies in these counties 

 were made incidentally, in connection with the survey of an adjoining- 

 county, and were restricted to one locality. The recent investigations, 

 however, show not only that these sands 'and gravels occur in widespread 

 beds, but that they are unquestionably Aftonian. 



Description of the Beds 

 gravels containing mammalian fossils 



The beds consist of interbedded and cross-bedded gravels and sands, as 

 illustrated in plate 33, figures 1 and 2. 



The gravel is water-worn and variable in coarseness, sometimes con- 

 taining small boulders up to 4 inches in diameter, and consists largely of 

 foreign materials such as might be from the drift, and with occasional 

 fragments of fossiliferous limestones, evidently of far northwestern origin. 

 Occasionally large boulders, chiefly of Sioux quartzite, are also found. 



The beds are often strongly iron-stained, as in the exposure shown in 

 plate 34, 'figure 2, the iron sometimes cementing the gravel into plates 

 and masses of conglomerate, and occasional bands and wedges are almost 

 black with Mn0 2 . 



They also contain small "boulders" of light bluish gray silt or dark 

 blue black sub-Aftonian till, densely covered with sand and fusiform or 

 spherical in form, as if shaped by rolling on the bottom of a stream. 



21 H. F. Bain : Reports of the Iowa Geological Survey, vol. v, 1896, p. 277 ; J. A. 

 Udden : Ibid., vol. xi, 1901. 



22 J. A. Udden : Ibid., vol. xi, 1901, p. 255, and vol. xiii, 1903, p. 166. 



« 3 H. F. Bain : Ibid., vol. viii, 1898, p. 338. 



2 * H. F. Bain : Ibid., vol. v, 1896, p. 281. 



25 Except that Macbride referred the gravel beds of Ida and Sac counties, in the same 

 section of the state, to the Aftonian in general terms. Ibid., vol. xvi, 1906, p. 532, etc. 



28 Iowa Geological Survey, vol. xi, 1901, p. 251, etc. 



27 Ibid., p. 254. 



28 White's Report of the Iowa Geological Survey, vol. ii, 1870, pp. 177-184. 



29 Reports of the Iowa Geological Survey, vol. v, 1896, p. 281. 



