Problems, Middle Mississippi River Region, Pleistocene Time 185 
chronous and that some subsequent event has produced the ap- 
parent discrepancy in altitude. It may be significant that the 
thrust fault described by Flint? between Red Rock, Missouri, 
and Grand Tower, Illinois, practically separates the higher from 
the lower set of terraces. The number of observations on the 
lower set of terraces is too few and the total horizontal distance 
involved is too short (only 36 miles) to give a definite answer 
as to the cause of the discrepancy. Further study down stream 
Should clear up the matter. Until it is proved otherwise, the 
terraces between Wittenberg and Hubbel Creek will be consid- 
ered to be a part of the Festus Terrace. 
Hubbel Creek flows in a mature valley for a distance of more 
than 12 miles. The lower portion of the valley is bordered by 
terraces composed of sand and silt. Then, just before it reaches 
the flood plain of the Mississippi, it enters a narrow, steep-walled 
rock gorge, totally out of harmony with the character of its 
valley farther upstream. This rock gorge is scarcely a quarter , 
of a mile long and is incised in the St. Peter sandstone. Study 
of the region shows that there is a broad silt-filled opening in 
sandstone cliffs a short distance to the east of the present mouth 
of Hubbel Creek. This helps explain the anomaly. Hubbel 
Creek was a mature stream flowing in a mature valley. The 
Mississippi then became an aggrading stream. It filled its own 
channel to a height of approximately 50 feet above the present 
channel.. The old course of Hubbel Creek also became filled 
With sand and silt. On this new and flatter surface Hubbel 
Creek flowed and, as is common in the case of tributary streams, 
its mouth was deflected a short distance down the course of 
the master stream. When the Mississippi later became an erod- 
ing stream, it cleared the sediment in its own trough. As this 
occurred, Hubbel Creek became incised in the flat plain. Its 
upper course was essentially in the same place it had been before 
deposition occurred and the rejuvenated stream found no dif- 
ficulty in reexcavating its valley in the alluvium. But the lower 
Portion of Hubbel Creek, which had been deflected, was now 
flowing over the sandstone belt which formed the rim of the 
Mississippi River trough. The sandstone being much more re- 
