Ui 



joined the Rock River somewhere in Bureau count}-, and the 

 lower Illinois valle}' was then part of one of the main drain- 

 age lines of the continent. The question natural!}- arises as to 

 the effect of this former relation on geogi-aphical distribution. 

 Apparently it had none, since the entire Illinois valley was 

 subsequently covered by the Illinoianglaciation. at which time 

 our river systems were shifted to about their present lines. 



The exact rock limits of the preglacial Illinois valley are 

 but imperfectly known. Apparently in obedience to the gen- 

 eral law of the westward shift of southward-flowing streams, 

 the present river flood-plain from Peoria down i PI. V. i follows 

 the west border of the immense valley, never very far from the 

 west bluff, which has a rock core throughout, and exhibits 

 practically all the few rock exposures of the lower valley. Be- 

 low Meredosia the valley is excavated in limestone strata and 

 is comparatively narrow, usually three or four miles wide, nar- 

 rowing to a minimum of two and a half miles. From Meredo- 

 sia up to Beardstown i fifteen miles i it widens out to six miles 

 or more. Above Beardstown the general rock surface was ap- 

 parently rather low originally and composed of softer strata, 

 and the valley is enormously expanded. The low east bluff is 

 completely concealed by subsequent deposits, yet fi'om the 

 Sangamon to the Mackinaw the original valley is probably not 

 far from twenty miles wide. Above Peoria the limits of the 

 rock valley are largely undetermined. forrea.sons given further 

 on. 



The deposits of the Illinoian glaciation are not very deep. 

 The -water action seems to have been moderate, and the sur- 

 face sand deposits caused by it in Illinois are of little conse- 

 quence. The larger preglacial valleys, although usually tilled 

 %vith these deposits to a depth of about one hundred feet, are 

 as a rule still traceable, and are usually reoccupied by the main 

 streams, although slight detlectious are not uncommon. The 

 terminal ridges pu.shed up by the ice in this period are now 

 discontinuous, fragmentary, or entirely wanting, although 

 often still prominent and massive. Probably the greater part 



