212 



wholly composed of drift, and is probably a remnaut uf the moraine 

 which dammed the valley and compelled the stream to cut the gorg? 

 through sandstone, perhaps by the retreat t)f a waterfall. 



The Bridgeton terrace is about 30 feet above the present divide 

 between the Kaceoon and the Wabash at Rosedale and demands some kind 

 of a dam of corresponding height at that point. Such a dam would also 

 account for the turning of the Raccoon over a col at Coxville into the 

 valley of a northward flowing tributary of Ijeatherwood, and solve the 

 problem of the lower Eaccoon and its reversed tributaries. Such a dam 

 once existed and does not now exist. Very large dams which vanish at a 

 convenient time are usually made of ice. I therefore postulate an Ice aam 

 across the space between Atherton Island and the east bluff of the Wabash. 

 The lake held up by such a dam found its lowest outlet to the north near 

 Coxville. The dam lasted long enough to permit the aggradation of the 

 valley to the height of the Bridgeton terrace and the subsequent cutting 

 down of the outlet to its present level or lower. The whole valley is filled 

 with sand and gravel to depths which l)orings alone can reveal. Wells on 

 the valley floor are usually very shallow. One is reported near Bridgeton 

 lo have passed through 150 feet of gravel, wliich is not improbable. The 

 ice dam may have been a part of the Illinoisan ice sheet and the valley 

 cutting of the lower Raccoon may have proceeded during the long inter- 

 glacial interval. Among all the changes and chances of two glacial periods 

 and one interglacial, a pai-t of the Wabash water may have followed that 

 valley, making Atherton Island truly insular. The present fall from 

 Rosedale to the mouth of Raccoon is 75 feet, which could be easily reversed 

 by removal of filling. 



During the climax and retreat of the Wisconsin ice sheet, the valleys 

 of the Wabash and its tributaries were aggraded to the present terrace 

 levels. For a time the present mouth of the Raccoon was Itlocked by ice 

 and the stream flowed over the terrace directly to the Wabash. By the 

 final withdrawal of the ice margin from the Shelbyville moraine the 

 northern outlet was left a little lowei' than the Ros«^lal«' divide, and tlie 

 present conditions came into existence. 



III. 



I A NATURAL BRIDOE IN PARKE (OINTV, TNO. 



Aliout two miles nortlieast of Mansfield. I'ari^c County, there is a nat- 

 ural bi-iilge of suthcieiit interest to lie noticed. .\ small trilmtary of Rae- 



