143 



ing ouly a vvorked-over sandy surface, and, along tlie valley 

 margins, occasional gravel terraces. 



The re-excavated expansion below Peoria is now about 

 twenty miles wide between Pekin and Havana, narrowing 

 gradually l)elow Havana to about twelve miles near the mouth 

 of the Sangamon, and to seven miles at Beardstown. then 

 widening to about twelve miles in western Cass county. At 

 this point the expansion suddenly narrows, and the valley, now 

 in Mississippian limestones, assumes a fairly uniform width of 

 three or four miles. The eastern border of the expansion is in- 

 dicated l»y a low liluff. often ol)scured by subsequent wind ac- 

 tion. Teheran. Mason county, lies at the foot of this bluff. 



After the ice had retreated from Illinois l)ut still occupied 

 the lower lake-region and St. Lawrence valley, the upper lakes 

 discharged for a time over the low divide at Chicago, along 

 present drainage lines, the valley in this stage of its history be- 

 ing called the Chicago Outlet. The clear and abundant tlow ex- 

 cavated the present river tlood-plain along the western side of the 

 valley to a depth of fifty to seventy feet below the glacial flood- 

 plain, about thirty feet below its present level. It also spread 

 out to some extent over the glacial flood-plain. Peoria Lake 

 was scooped out in the comparatively narrow opening through 

 the terminal Wisconsin ridges. Leverett ('99, PI. VL) has in- 

 dicated the existence at this time of an eastern channel below 

 Pekin, occupying a depression now approximately followed by 

 the line of the Chicago, Peoria and St. Louis railway from the 

 Mackinaw Kiver below Pekin to Havana. The Mackinaw enters 

 this depression about ten miles south of Pekin, and, turning 

 abruptly northward, reaches the Illinois by way of the upper 

 end of the channel. (Quiver Creek enters it near Forest City, 

 and follows it down to the Illinois. The railroad runs along 

 the ea.stern side of the ancient island included between the two 

 channels. The topography along this line has probably been 

 altered to a C()nsideral)le extent by wind action upon the loose 

 surface deposits since the formation of the eastern channel. 

 With the establishment of the St. Lawrence drainage the Hli- 

 nois assumed its present size and [losition. Tlie lessened flow 



