SURFACE DEPOSITS. . 89 



These regions of varying thickness of the Drift Deposit, and 

 varying distribution of its boulders, have a more or less 

 definite northerly and southerly extension, so that as one 

 goes westward across the State, he crosses them successively 

 For example, starting from the Mississippi river in Clayton 

 county, he has already crossed the boulderless region when 

 he reaches West Union, in Fayette county, and he there enters 

 upon the region in which boulders are more than usually 

 abundant. Reaching the valley of the Shellrock, at Clarks- 

 ville, he is in the midst of the region where the Drift Deposit 

 is comparatively thin. From this point the drift gradually 

 increases in thickness until he reaches the Great Watershed 

 beyond the Des Moines, where it is evidently some two hun- 

 dred feet thick. From the Great Watershed westward, it 

 continues of considerable thickness to the Big Sioux, but 

 crossing that river into Dakota, it is found to cover the Creta- 

 ceous strata there only to a very slight depth. 



Again, if he starts westward on the line of the Burlington 

 and Missouri River Railroad, he finds the drift to be of con- 

 siderable thickness all the way, and reaching the dividing 

 ridge in Union county, he finds indications that the drift there 

 is upward of two hundred feet thick. Continuing westward, 

 he finds it to diminish in thickness so rapidly and gradually 

 that less than a dozen feet of it intervenes, in many places 

 along the valley of the Missouri river, between the Bluff 

 Deposit, which there overlies it, and the underlying rocks in 

 situ. Occasionally in that region the drift is entirely 

 wanting, the bluff material resting directly upon the stratified 

 rocks. 



2. ORIGIN OF THE DRIFT. 



It is not proposed here to discuss at length the question of 

 the origin of the drift, but as reports of this kind are expected 

 to be more or less devoted to popular instruction concerning 

 the principles involved in the statement of facts observed, 

 brief explanations of that kind are here made. 

 V2, 



