126 GEOLOGY OF ILLINOIS. 



Big Bureau creek, in the south-eastern part of the county, is a prairie 

 brook, with no marked peculiarities. 



Several small creeks and brooks, such as Sugar creek, in the town- 

 ship of Palmyra, and Franklin creek, east of Dixon, are worthy of no- 

 tice. The latter is one of the most interesting little streams in the 

 county. It exhibits in its short course a fine section of the geological 

 formations in this part of the county. 



Qeolo gical Formations. 



These are varied and interesting. Below the superficial deposits, we 

 commence with the Niagara limestone, and go all the way down to the 

 St. Peter's sandstone. A section of the geological formations of the 

 county, in the order of their sequence, would be represented by about 

 the following figures : 



1. Drift deposits 10 to 75 feet. 



2. Niagara limestone ? 



3. Cincinnati group 30 ? 



4. Galena limestone 25 to 70 



5. Blue, or Old Trenton 20 " 75 



6. Buff limestone 18 



7. St. Peter's sandstone 150 



Eeversing this order, I propose to commence at the bottom, and de- 

 scribe these formations in the ascending order. 



The St. Peter's Sandstone. This very interesting rock underlies a very 

 considerable portion of the county, and outcrops heavily on Bock river 

 and Franklin creek. The heaviest outcrop in the county is opposite 

 Grand DeTour, just across the river. The base of the bluff, for thirty 

 or forty feet upwards, shows this rock. Here it has a solid, unstratified 

 look, and rusty-brown color,. On the Ogle county side of the river the 

 sandstone is whiter, and the outcrop is over one hundred feet in thick- 

 ness. For two or three miles the bluffs are mostly composed of this 

 material. Just below the mouth of Pine creek, the formation on Eock 

 river sinks rapidly out of sight; and is succeeded by the Trenton lime- 

 stones. On the Lee county side of the river the sandstone soon dis- 

 appears, after leaving the outcrop opposite Grand DeTour. Between 

 this latter place and Dixon fine outcrops of Trenton limestone occur. 

 The St. Peter's sandstone on Eock river, as will be seen by a reference 

 to my report upon the geology of Ogle county, is chiefly developed in 

 the latter county. For a distance of about fourteen miles, commencing 

 about two miles above Oregon city and terminating a short distance 

 below the mouth of Pine creek, it is a very marked feature of the Eock 

 river 'bluffs. The outcrop extends back but a short distance from the 

 bluffs. In some of the ravines and intersecting streams it can be traced 



