188 ILLINOIS STATE ACADEMY OF SCIENCE 



surrounding territory. This is an area of advanced 

 stream dissection in the midst of an expansive, slightly 

 dissected till plain. Flattish upland rock ridges, thinly 

 mantled with glacial drift, whose preglacial surfaces 

 truncate the underlying strata, are present. Rock River 

 flows for some distance in a youthful valley) locally a 

 deep gorge, whereas Leaf River, its tributary of much 

 less volume, occupies a mature valley in the same type 

 of formations. Some other streams flow in places in 

 broad open valleys but elsewhere their courses are en- 

 trenched in narrow rock gorges. A conspicuous topo- 

 graphic element, without homologue in this area, is a 

 broad east and west depression, the Oregon Basin, in 

 the vicinity of Oregon. 



It is obvious from this brief outline of the salient topo- 

 graphic features of the quadrangle that its physio- 

 graphic history has not been simple, but that the present 

 surface has been produced by the successive action and 

 interaction of several geologic processes. In brief, the 

 present topography is dominantly that of surfaces 

 stream-carved from rock of variable resistance and di- 

 versified structure, which throughout much of the area 

 has been only slightly modified by deposition of glacial 

 drift, with some conspicuous features due to drainage 

 changes that resulted from the later invasion of the bor- 

 dering territory by ice-sheets. Certain details have been 

 impressed upon the major features by post-glacial activi- 

 ties. 



Stratigraphy. — The rocks which crop out in this quad 

 rangle consist of indurated sedimentary formations of 

 the Ordovician system, and unconsolidated deposits of 

 Pleistocene and post-Pleistocene age. The former range 

 from lower to upper Ordovician, and the latter from Illi- 

 noian to the present. In descending age order the ex- 

 posed formations are : 



Recent : alluvium, dune sand, talus. 



Late Wisconsin: valley trains, alluvium. 



Early Wisconsin: alluvium, loess ( ?)■ 



Post-Illinoian : loess. 



Illinoian: till, fluvio-glacial deposits. 



