STREAM VALLEYS AND THEIR MEANING 495 



valley walls some 400 feet in height. The river swings from side 

 to side of the valley in broad, open curves which could scarcely 

 be dignified by the name of meanders. 



According to our deductions, an open valley of this type indi- 

 cates a great predominance of sweep over the other processes at 

 work in the formation of the valley. Such a valley might, as we 

 have seen, be brought about by either one of two conditions: {a) a 

 rapid deepening of the valley to grade, with subsequent widening, 

 or {h) a deepening so slow — -perhaps on account of a heavy sediment 

 load — that sweep predominated from the beginning, continually 

 shifting the locus of lateral cutting and thereby widening the valley 

 uniformly as down-cutting proceeded. 



In the valley of the Mississippi itself there appears to be nothing 

 to indicate which of these is the correct interpretation, but when 

 the tributary valleys are examined a clue presents itself. They are 

 found (so far as can be judged from the contour maps, which are 

 not so clear as could be desired) to enter through incised meander 

 valleys, most of the larger of them clearly of the intrenched type. 

 This form of valley indicates that the local base-level was lowered 

 quickly (the equivalent of rapid uplift), and that the first of the 

 two possible explanations mentioned is more probably the correct 

 one. Such a conclusion should be tested further by the examina- 

 tion of other tributary valleys entering the Mississippi above 

 and below this point. If all the larger tributaries are found to 

 agree in their testimony, the hypothesis of rapid down-cutting by 

 the main stream is strengthened. 



The possibility that the valley of the Mississippi, at the point 

 referred to, may, at one time, have carried a glacial stream much 

 larger than the present river, and the fact that the valley is 

 now silted up to a depth of from one to two hundred feet must 

 be taken into consideration, but it does not seem to me to 

 alter essentially the interpretation. Glacial waters may, how- 

 ever, be partly responsible for the remarkable sharpness of the 

 features of the valley walls. In all essential particulars, aside 

 from this sharpness, the valley is similar to that of the Kanawha 

 at Charleston (Fig. i). 



