404 



B. SHIMEK AFTOXIAX SAXDS AXD GRAVELS IX 10 W" A 



They vary in total thickness up to 40 feet, and rise to a height of 10 

 to 40 feet above the latest alluvial plain. But where they have been 

 plowed and crowded by the overlying Kansan they are frequently mingled 

 with joint clay, till, etcetera, and are often piled up to a greater height, 

 as in Murray hill, northeast of Little Sioux, where they rise irregularly 

 to a height of more than 100 feet above the general level of the valley. 



DISTRIBUTION' IX HARRISOX AXD MONONA COUNTIES 



In the two counties under consideration the Aftonian beds are distrib- 

 uted along the bluffs bordering the Missouri valley, 31 and along all the 

 principal tributaries — the Boyer, Soldier, Maple, and Little Sioux rivers. 

 Sometimes they are present on one side of the valley, and again on the 

 other. Thus along the Boyer, below TToodbine, they appear on the west 

 side of the river, while in the lower course of the same valley they are on 

 the east side. More rarely they appear on both sides, as along the Maple 

 river, near Mapleton. This is consistent with the distribution of sand 

 and gravel bars along modern streams. 



EviDEXCE THAT THE BEDS ARE AETOXIAX 

 STRATIGRAPHIC POSITION 



That these sand and gravel beds are Aftonian is clearly shown by their 

 stratigraphic position between the Kansan and sub- Aftonian drifts. This 

 is well illustrated in the county line exposure near Little Sioux, the first 

 exposure in which the writer definitely determined the stratigraphic posi- 

 tion of these deposits, where a great bed of sands and gravels, not less than 

 15 feet in thickness, lies between the sub- Aftonian till exposed at the base 

 of the bluff and the typical Kansan till above, both of which it meets un- 

 conformably. 32 



Both sub-Aftonian and Kansan, with the intervening Aftonian, are 

 exposed at a number of points in this region. Such exposures were found 

 in Monona county, in Woodward's glen, in section 17, township 84 north, 

 range 44 west, and in a well near Castana, in section 13, township 84 

 north, range 44 west; in Harrison county, in section 5, township 81 

 north, range 44 west (the county line exposure already noted), and on 

 Murray hill near the corner of sections ?, 8, 17, and 18, in the same 

 township, and in the bluff above Loveland, in Pottawattamie county.* 



31 Sand and gravel beds at several points on the west side of the river, in Nebraska, 

 are also Aftonian. 



33 For the contact line in this exposure with the sub- Aftonian, see plate 34, figure 1 ; 

 with the Kansan, plate 33, figure 2. 



* More recently the writer discovered several additional similar exposures on both the 

 Iowa and Nebraska sides of the Missouri. 



