98 ILLINOIS ACADEMY OF SCIENCE 



CHARACTERISTICS OF THE ROCKS 



Shales are the dominant rocks among those exposed in this 

 area. Except for a few sandstones, they are nearly all shales 

 with thin sandstone beds interspersed. The sandstone beds 

 in the shales increase both in number and in thickness in the 

 older formations, so that in the oldest formation, the Portage, 

 flaggy sandstones constitute an important part of the rock, 

 and some of the beds are a foot or more in thickness. 



NATURE AND EXTENT OF DEFORMATION 



The deformation includes folds and faults. Of the folds 

 there are two types — those with longer axes parallel to the 

 valleys and those with their axes transverse to valleys. These 

 two types are found not together but in different areas. The 

 faults, so far studied, are all thrusts, with possibly one slight 

 exception. 



FOLDS PARALLEL TO VALLEYS 



The first type of folds noted above consists of small anti- 

 clines in the bottoms of valleys that trench the uplands (Fig. 

 1). These folds involve only a few feet of strata, and rarely 

 affect the walls of the post-glacial valleys in which they are 

 most commonly found. Some of these anticlines are narrow 

 and close, while others are broad and open. They are var- 

 iable also in length, ranging from a few feet in some in- 

 stances to over 100 feet in others. While the axes of the 

 folds vary somewhat in direction, most of them trend nearly 

 with the valleys in which they occur. Though the folds seem 

 to have been formed as the valleys developed, no definite 

 evidence of age has been noted in connection with them, and 

 they are small and unimportant when compared with many of 

 the folds of the second type. 



FOLDS TRANSVERSE TO VALLEYS 



In the second type of folds the axes are chiefly transverse 

 to the valleys, and they affect not only flood-plains, terraces, 

 and walls of valleys, but they occur also in the lake cliffs east 

 of Erie, Pennsylvania. (Fig. 2). Some anticlines occur alone, 

 but frequently several are associated with synclines in a series. 

 These folds commonly involve from 10 to 20 feet, and in some 

 instances 60 to 80 feet, of strata exposed above the stream bed 

 and an unknown amount below it. The folds vary in width 

 from a few feet to over 500 feet. Many folds may be seen dis- 

 tinctly on both sides of a stream. 



