80 THE ILLINOIS GLACIAL LOBE. 



Illinoian invasion or to an extension of the Iowan ice westward from the 

 Rock River Basin. 



pecatonica esker system. — In Stephenson County there are several gravelly 

 belts which have been studied in considerable detail by Mr. Oscar Hershey 

 and are discussed by him in a recent paper in the American Geologist. 1 

 These gravelly belts bear less resemblance to typical eskers than the Adeline 

 and Hazelhurst ridges. They are marked by frequent interruptions and 

 display a series of branches or spurs which give them greater complexity 

 than the ridges just discussed. The main belt follows the Pecatonica Valley 

 from eastern Stephenson County westward to the mouth of Yellow Creek, 

 about 3 miles east of Freeport; thence it passes up the south side of Yellow 

 Creek to the village of Bolton. The length of this belt is nearly 20 miles, 

 and the ridges are in places scattered over a width of 2 or 3 miles. Their 

 distribution may be seen on PI. XII. It will be noted that there are usually 

 two, and in places several, parallel ridges traceable for a few miles; they 

 then either diverge to form branch belts or die out altogether. Hershey has 

 called attention to points of special development in these belts where the 

 aggregate bulk of the ridges is increased to several times the usual amount. 

 One of these points of special development occurs opposite the mouth of 

 Yellow Creek, another 3 or 4 miles farther west, and a third at the western 

 end of the belt at Bolton. These are interpreted by Hershey to have been 

 formed successively from west to east, and to mark each the position of the 

 ice margin at the time it was forming. The ingenious hypothesis which 

 Hershey has presented may perhaps satisfactorily account for the develop- 

 ment of these belts, but the question can scarcely be decided in the present 

 state of knowledge of such phenomena. 



At the western end of this gravelly belt the ridges culminate in an 

 accumulation of greater strength than is displayed at any other point along 

 the belt. An area of more than a square mile is occupied by sharp ridges, 

 the highest of which rise 75 or 100 feet above the adjoining plain. The 

 plain immediately west of this system of ridges, though imperfectly exposed 

 to view, is apparently underlain extensively by sand, and is referred to by 

 Hershey as a sand plain. It bears only slight resemblance to the deltas 

 formed at the terminus of eskers in other localities, for the ridges do not 

 merge into the sandy plain. As suggested by Hershey, there was probably 



1 Am. Geologist, Vol. XIX, 1897, pp. 197-209, 237-253. 



