INDIVIDUAL CHARACTERISTICS OF THE IOWAN 147 



in some localities, lie on the surface or are but partly embedded, and they 

 contribute striking and diagnostic features to the Iowan landscapes 

 (plate 4, figure 2). Their prominence above the surface in a region 

 which has suffered no erosion has led at least one geologist to conclude 

 that they represent superglacial material carried by the Iowan ice and 

 gently lowered on the surface as the glacier melted. Setting aside the 

 obvious difficulties in the way of accounting for superglacial drift on the 

 surface of a continental ice-cap traversing a region of low relief, there is 

 evidence in the large proportion of planed specimens, great and small, 

 that the Iowan boulders were transported beneath the ice. To meet a 

 demand for suitable blocks for bridge piers and heavy foundations, many 

 of the larger boulders were actually quarried in Buchanan, Black Hawk, 

 and other counties; and in a majority of cases, when the last pieces were 

 lifted out of the shallow pits in which the granites lay, the low r er surface 

 showed a face, flattened and scored, many feet in diameter. These 

 boulders were probably embedded completely in the lower surface of the 

 ice ; the ground moraine otherwise was very thin ; the granites now stand 

 out conspicuously above the surface, because the subglacial drift, as a 

 whole, was insufficient in amount to conceal them. 



DRAINAGE COURSES IN THE IOWAN 



A discussion of the general surface features of the Iowan should in- 

 clude some reference to the drainage courses. These add some special 

 characteristics to the young drift plain. In many instances, owing to 

 the meagerness of the drift, the streams recovered their old, pre-Iowan 

 valleys after the disappearance of the ice; but some were obliged to seek 

 new courses across the drift plain, and these now flow in shallow trenches 

 almost at the level of the cultivated fields (plate 5, figure 1). There are 

 here no valleys in any proper sense, no river bluffs, no floodplains ; the 

 Iowan streams are young; they have barely commenced to cut their val- 

 leys; they have very few tributaries; the lateral drainage is effected by 

 flow along the broad sags of the original surface, instead of being limited 

 to definitely cut channels; over extensive areas the drainage is sluggish 

 and imperfect. 



AGE OF IOWAN COMPARED WITH KAN SAN 



After this review of its general characteristics we may compare the 

 Iowan drift with the Kansan, giving especial attention to criteria indica- 

 tive of age. The differences seem almost immeasurably great. With 

 the exceptions already noted — namely, low-lying areas or the axes of 

 very broad divides — the whole of the original Kansan plain has been 

 completely carved and sculptured by flowing water and a miniature type 



