THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 17 



southeast across the Dakotas; from the Gulf of Mexico northward on either 

 side of the Ozarkia; and finally, broadly eastward from the Pacific deep, 

 the waters of the oceans crept over the lands, at times spreading widely, 

 at others confined to relatively limited channels. The determination of this 

 plan of submergences is one of the greatest steps that has been made in 

 the preparation of a logical geological history of the continent. 



III. INTERPRETATION OF THE ADJACENT LANDS. 



The paleogeography of any unit is far from completely made out, even 

 when the constituent rocks and fossils are thoroughly known. The composi- 

 tion and arrangement of the material in any bed deposited on an ocean 

 littoral or in a smaller body of water is influenced in large measure by the 

 nature of the land from which it was derived. The temperature of the 

 water and the food-supply of aquatic faunae are no less closely influenced 

 by the condition of the bordering lands. Terrestrial deposits reflect even 

 more closely the character of the adjacent degrading land. It is obvious 

 that to understand the paleogeography of any unit of time it is necessary to 

 know the condition of the land areas which have contributed the sediment 

 to the observed stratigraphic unit. Such knowledge is gained only with 

 great difficulty in many cases. The debris from the land which forms the 

 observable record has in most cases undergone decided changes and must 

 be interpreted in the light not only of all possibilities of weathering and 

 alteration of the original material, but from the method of transportation. 



a. Where direct contact can be established the land surface may in part 

 be made out from the suggestions given above and below. Where, as in 

 the great majority of cases, the littoral zones have been destroyed, the 

 source of material may be recognized by the character of the sediments or 

 inferred from possible sources of supply such as elevated regions. In such 

 cases the nature of the contact between the eroded surface and the overlying 

 beds may tell the extent of its degradation and the character of the surface. 



h. The physical character of the deposits may reveal the greater or lesser 

 degree of weathering, erosion, and transportation, and hence the ruggedness, 

 gentleness, the velocity of the streams, the amount of protecting vegetation, 

 climatic variations, etc. The included fossils of a land vegetation may 

 show something of the nature of the soil and climate. Volcanic ash, loess, 

 and wind-blown sand all help to restore the condition of the land. A typical 

 case of the interpretation of wind-drifted sand is to be found in Grabau and 

 Sherzer's discussion of the Sylvania sandstone of southeastern Michigan.^ 



Wind-blown sand, volcanic ash, and loess, however, carry far less informa- 

 tion concerning the adjacent areas than would a water deposit. More easily 

 carried and far more thoroughly sifted from all foreign substances, these 



' Grabau, A., and W. H. Sherzer, The Monroe Formation of Southern Michigan and Ad- 

 joining Regions. Michigan Geological Survey, series i, Pub. 2, p. 6i, 1910. 

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