;)44 llie American Geologiaf. December, is<95 
terraces had been traced up over tlie Kinderliook shales onto 
the crinoiihil liinestone of the Burlington period, it became a 
certaint}' that the variable hardness of the rock had nothing 
whatever to do with the formation of the terraces. After the 
upper trough or main portion of the stream valleys had been 
excavated to approxinuttely its present size and form, the 
entire region in which these terraces occur, which includes 
the Ozark plateau and much of the surrounding country, was 
elevated by an epeirogenic movement, which lowered the base- 
level relatively to the general surface, and permitted the 
streams to cut new valleys in the bottom of the older ones. 
Whatever movements may have subsequently affected the 
region, this elevation was to a certain extent permanent. 
Isolated remnants of what appear to be terraces occur at 
various hights along the hillsides far above the prominent 
terrace here described, but they are indistinct and unimport- 
ant, and no attempt has been made to correlate them. Some 
probably are due to difference in hardness of the rocks ; 
others may record slight movements of the region, while the 
remainder may have originated through variability^ in erosive 
])ower, to which all streams are liable. 
I'he problem of locating, among the periods of geologic 
time, the date of the epeirogenic elevatory movement to which 
the lower troughs owe their origin, is rather dithcult at the 
present stage of the study. I will, however, endeavor to indi- 
cate my opinion on the subject, and will make use of several 
methods for determining the age of a valley. The first is by 
a comparison of valleys in the Ozarks with valleys of a known 
age in other regions: and the second is by the stratigraphic 
relations of the inclosed deposits. 
It is generally conceded that streams of nearly equal size, 
flowing with ecpuil drainage gradients, through strata pre- 
senting the same resistance to erosion, will erode vallej^s 
approximately eqnal in size. Hence, also, nearly eqnal val- 
leys (all conditions governing their erosion being about alike) 
may be considered to have been formed in essentially the 
same length of time. I wish to apply this principle in roughly 
determining the age of the valleys in the Ozarks, and shall 
compare the valleys of three streams in Illinois, namely, the 
Hock and Pecatoniea rivers and Yellow creek, with four 
