154 ILLINOIS STATE ACADEMY OF SCIENCE 



to 1 mile in width. Its total fall in Ogle County is only 

 some 50 to 60 feet. It is this narrow portion that has 

 given rise to the river bluffs. 



At Byron there are deposits of glacial gravel some 50 

 feet above the low water level of the Rock. At Oregon 

 such deposits also occur, being some 40 feet here, and they 

 extend thence to the southern end of the county and be- 

 yond. Remarkably small excavation by the river has taken 

 place since the deposition of this gravel. The rock exca- 

 vation has been interglacial and the gravel excavation 

 postglacial, the period of the rock excavation having been 

 the longer and for the greater part in limestone. Today 

 the outcrops seen along the Rock in Ogle County are mainly 

 St. Peter's sandstone, which outcrops for some 14 miles in 

 banks from 25 to 200 feet in height. It is this that forms 

 the bluffs for some two and a half miles above the town 

 of Oregon, near the middle of the county, to below Grand 

 Detour at the southern end. In color this sandstone is from 

 nearly white to golden yellow and dark brown, from the iron 

 once held in solution by the water. Sometimes these bluffs 

 are capped by limestone, as on the east side of the Rock 

 north of Oregon where the Black Hawk statue stands. 

 Liberty Hill, west of Oregon, is also of this sandstone capped 

 by limestone. A few miles north of Oregon, and south of 

 the mouth of Pine Creek near the southern boundary, the 

 outcrops rapidly decline. In the St. Peter's sandstone there 

 are sometimes ferruginous layers which, being more resist- 

 ant to erosion, are often left as brown to almost black paral- 

 lel or circular ridges. This is well shown at Hotel Rock, 

 on the west shore about four miles south of Oregon. Again 

 the sandstone may occur as an almost white, non-ferru- 

 ginous variety, consisting of almost pure silica, as at Castle 

 Rock on the west shore just north of Hotel Rock, where the 

 stone is soft, friable, and very porous. In the ravines about 

 the town of Oregon buff limestone occurs, and it is this 

 that caps the sandstone of the river bluffs upon which the 

 statue of Black Hawk stands opposite Oregon. Certain 

 creeks emptying into the Rock show St. Peter's sandstone 

 at their mouths, then, farther up stream, the buff lime- 

 stone, and still farther up blue limestone, and finally Galena 



