49Q 



EUGENE WESLEY SHAW 



lake deposit about 80 feet thick at the lower ends and thinning out 

 to a feather edge up stream, accumulated on the old flood-plains. 



Nearly all the material deposited in the lakes was fine sediment 

 such as would be carried in suspension, and the lakes seem to have 

 been filled with this material up to certain concordant positions, 

 probably to the natural position of a flood-plain or just below the 

 high- water mark of the time. 



Fig. 4. — Diagram showing arrangement of principal deposits and surface features 

 along Beaucoup Creek, Perry and Jackson counties, Illinois. The filling thickens 

 and the flood-plain becomes narrower down stream. When the lake became extinct 

 the bed became a great swamp. The stream first cut into the lower end of the fill, 

 draining that part of the swamp and developing a narrow flood-plain below the sur- 

 face of the lake silt. With further downward cutting the new flood-plain was lowered 

 and extended up stream and the swamp area reduced. Meanwhile, stream deposits 

 continued to accumulate at the upper end of the lake bed. Many other valley bot- 

 toms are similar, having a peculiar swampy central portion. 



