246 C..P. BERKEY PALEOGEOGRAPHY OE SAINT PETER TIME 



size in the Saint Peter, those most rounded, as noted above, would not 

 be picked up and carried far by the wind, but could easily be rolled along 

 the ground. The smaller sizes abundant in the rock fall within the 

 lifting capacity of the wind and might be carried long distances.* These 

 smaller sizes are prevailingly less worn and more angular than the larger 

 sizes, and this again is consistent with such agency. Of all agents of 

 transportation, wind is the best assorter. From a given supply of sand 

 of reasonable area! limits the wind is able to remove the finer dust very 

 perfectly. The finest and the coarsest particles are widely separated. 

 If, therefore, the supply area is not so large as to outreach the assorting 

 competence of prevailing winds, and if the winds involved are not so 

 changeable as to undo one day what may have been accomplished on the 

 previous one, the result is sure to be a perfectly cleaned sand. 



Conclusions as to Origin 



The writer is well aware of the incompleteness of an argument of this 

 kind. To find that certain phenomena are consistent with a certain 

 explanation is far from proving its correctness; but, in view of the diffi- 

 culty acknowledged by workers in the Saint Peter, it is at least worth 

 noting that wind agency will account for the purity and textural character 

 of this formation under very reasonable conditions. 



The surface over which the Saint Peter sands were deposited was ap- 

 parently very uniform. If it had departed far from a low-lying plain, 

 we should doubtless have many marks of it in erosion forms characteristic 

 of such elevation. On that plain, on its retreat, the sea spread great 

 quantities of sand and left the marginal supply (Basal sandstone mar- 

 gin) exposed to all the transporting agencies. This the wind began to 

 carry as dune sands along the shore. Into these sands the rivers sank 

 as they coursed toward the retreating sea, accomplishing little in erosion. 

 At the maximum retreat of the sea, it is the writer's belief that the Saint 

 Peter sands presented the aspect of a shifting-sand plain, perhaps akin 

 to a desert in this one feature at least, though not necessarily arid; so 

 the sands were washed out by the retreat of the sea and thereby assorted, 

 then worked many times over by the wind in the absence of the sea, and 

 thereby still more perfectly assorted, and finally, in the readvance of the 

 sea, much of it was again worked over a last time, thereby reaching its 

 present remarkable condition of purity. 



The Saint Peter therefore owes its constancy of grain and its purity 



* J. A. Udden : The mechanical composition of wind deposits, Augustana Library 

 Publications, No. 1, 1898. 



