PAPERS ON GEOGRAPHY AND GEOLOGY 191 



tain numerous shell fragments or poorly preserved shells 

 or imprints of shells. Much chert is present in portions 

 of the formation, as scattered nodules along the bedding 

 planes or in thin well-defined nodular beds. Certain 

 strata are characterized by pores and pits up to a few 

 inches across, which give rise to a rough, peculiarly 

 fretted surface. Rusty calcareous sand, in separate par- 

 ticles or loosely adherent, partially fills most of the pits 

 on weathered surfaces or throughout the fresh rock. In 

 places these beds have disintegrated into a friable mass 

 that resembles loosely cemented sandstone. 



The youngest indurated formation is the Maquoketa 

 shale, which occurs in this area as a single outlier on the 

 summit of Bunker Hill, in the extreme northwestern 

 corner of the quadrangle. About 70 feet of shale with 

 intercalated thin beds of more or less fossiliferous lime- 

 stone constitute the exposed section. 



Beyond doubt the Niagara dolomite, which caps a 

 series of hills to the west, formerly covered this area, but 

 the last vestige of its outcrops has been destroyed. 



The mantle rock immediately overlying the bed rock is 

 very largely glacial drift, which is almost entirely of Illi- 

 noian age. Xo till of an earlier or later epoch is known 

 to occur in this quadrangle. Some outstanding char- 

 acteristics of the Illinoian drift are, 1) the large amount 

 of gravel and sand in many parts of it, '2) the depth of 

 leaching and oxidation, 3) its thinness throughout much 

 of the area, and 4) the rough conformity of its surface 

 in many places to the contour of the underlying bed rock. 



The drift is not uncommonly a sandy and gravelly till. 

 Large masses of stream-deposited gravel and sand are 

 locally present in it. Numerous other large bodies of 

 sand and gravel occur upon the surface of the till as 

 kames and eskers. Both the included gravels and the 

 superficial deposits are especially abundant in the Ore- 

 gon Basin and in Leaf Biver Valley. The eastern portion 

 of the largest esker in the state, the Adeline esker, is in 

 the western part of Leaf Biver Valley in this area. 



Since its deposition the till has undergone extensive 

 chemical change?. It is deeply oxidized, and limestone 

 fragments as well as the calcareous portion of the rock 



