212 



wholly composed of drift, and is probably a remnant of tbe moraine 

 which dammed the valley and compelled the stream to cut the gorg>5 

 through sandstone, perhaps by the retreat of a waterfall. 



The Bridgeton terrace is about 30 feet above the present divide 

 between the Raccoon and the Wabash at Rosedale and demands some liind 

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

 account for the turning of the Raccoon ov'?r a col at Coxville into the 

 valley of a northward flowing tributary of I^eatherwood, and solve the 

 problem of the lower Raccoon 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 uam 

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

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

 Coxville. The dam lasted loni^ enough to permit the aggradation of the 

 valley to the height of the Uridgeton terrace and the subsiKiuent cutting 

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

 with sand and gravel to dejiths which borings alone can reveal. Wells on 

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

 to have passed through IHO feet of gravel, which is not improbable. The 

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

 cutting of tlie lower Raccoon may have proceedtMl during the long inter- 

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

 and one interglacial, a part of the Wabash water may have followed that 

 valley, malting 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 blocked 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 lower than the Rosedale divide, and the 

 present conditions came into existence. 



III. 



A NATURAL BRIDGE IN PARKK COUNTY, IND. 



About two miles northeast of Mansfield. Tarke County, there is a nat- 

 ural bridge of sufficient interest to he not lied. A small tributary of Rac- 



