EROSIONAL HISTORY OF DRIFTLESS AREA 113 



Area. At Waukon, Elon, Seneca, and at some points in 

 Minnesota, the gravel is firmly cemented with iron, so that 

 a conglomerate exceedingly resistant to erosion is formed. 



Some of the pebbles contain fossils which are of Ordo- 

 vician and Niagaran age. Some of the pebbles and fossils 

 collected from the Sparta quadrangle are shown in Fig. 35. 



The conclusion seems unavoidable that streams were once 

 nearly at grade on the Dodgeville plain ; that they deposited 

 extensively in their beds, and that deposition ceased as the 

 dissection of the summit plain was inaugurated by uplift. 

 The coarse texture of the fluvial deposit, their apparently 

 local origin, and their association at Devil's Lake with 

 potholes all suggest that they were not deposited by the 

 largest and oldest streams of the time, but rather by second- 

 ary streams w^hose gradients were still appreciable and 

 whose sources were not far distant, and yet by streams 

 which have long since ceased to exist. Presumably the 

 larger streams deposited also, but the material probably 

 consisted of sand and silt rather than of gravel, and these 

 non-resistant deposits have been entirely removed or 

 mingled with the upland soils so thoroughly as to be indis- 

 tinguishable. The gravel was probably deposited on those 

 portions of the Dodgeville plain which were somewhat 

 above the lower valley bottoms, and by subsequent erosion 

 they have come to stand as the very highest points because 

 of their superior resistance. 



Whatever may have been their detailed origin and dis- 

 tribution these fluvial gravels, occurring at widely separated 

 points on the Dodgeville plain, hundreds of feet above pres- 

 ent drainage, go far to prove that the surface of the Drift- 

 less Area has not been formed by erosion in a single cycle. 

 Certainly there is no provision in the cuesta single cycle 

 theory for the occurrence of these deposits on the Dodge- 

 ville plain. 



Con elusion 



As was brought out in Part I, the most satisfactory proof 

 of more than one cycle of erosion is to be found in certain 

 combinations of evidences. A combination amounting to 



