504 W. UPHAM STAGES OF THE ICE AGE 



Mississippi from the Wisconsin driftless area south to northeastern Mis- 

 souri. An ensuing return of a more temperate climate melted the 

 Keewatin ice far back previous to the culmination of the illinoian glacial 

 stage, when an eastern icefield advanced into southeastern Iowa, cross- 

 ing the present valley of the Mississippi along a distance of over 100 

 miles and covering a tract in Iowa about 10 to 20 miles wide. Kem- 

 nants of a black soil, peat, branches of trees, and bones of the rabbit and 

 skunk, found there on the Kansan drift, overlain by the western border 

 of the Illinoian drift, show that for a considerable time this tract had a 

 land surface before it was again covered by ice. This time of glacial 

 recession from the farthest eastern limit of the Kansan glaciation, fol- 

 lowed by the most western advance of the Labradorian icefield in its Illi- 

 noian stage, is named by Leverett the Yarmouth interglacial stage, from 

 Yarmouth, Iowa, about 20 miles northwest of Burlington. At that vil- 

 lage interglacial beds 43 feet in thickness were penetrated by a well, with 

 30 feet of overlying Illinoian till and 33 feet of underlying Kansan till. 



Illixoiax Glaciation 



Although the glacial drift has a width of about 300 miles between 

 the driftless area and the boundary of glaciation in Missouri and south- 

 ern Illinois, the Keewatin and Labradorian icefields were not confluent 

 there. An ice-dammed lake would thus have been held on the large 

 unglaciated area of southwestern T\ lsconsin and its smaller adjacent 

 parts in Minnesota, Iowa, and Illinois, whether such confluence occurred 

 during the Xebraskan stage or in Ivansan and Illinoian time. The 

 absence of lacustrine beaches and deltas testifies that the Mississippi 

 flowed through the driftless area during all the Ice Age without being 

 obstructed to form a lake. 



By the Illinoian ice-advance into Iowa when it attained its greatest 

 westward extent, the Mississippi was displaced and caused to flow tempo- 

 rarily outside the ice-border, crossing the lower courses of the Maquoketa, 

 AVapsipinicon, Cedar, Iowa, and Skunk rivers. Along this part of the 

 boundary of Illinoian glaciation its drift is amassed in a wide but low 

 marginal ridge, 40 to 60 feet above the adjoining surface. 



Stream erosion on the Kansan drift sheet has much exceeded its 

 amount on the Illinoian drift, indicating, as estimated by Leverett, a 

 greater interval between their times of deposition than from the latter 

 to the present time. The difference in contour, however, may be partly 

 due to an originally patchy condition and inequalities of the Kansan 

 deposition for the outer region of attenuation of its icefield, in contrast 



