THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 11 



drawn attention to the similarity which exists between the deposits of deltas 

 and those of a transgressing sea.^ 



Aeolian deposits. — Dune sands, loess, and volcanic ash are the most 

 common wind-carried material. 



Dune sands are generally cross-bedded, but the line of the cross-bedding 

 is more concave than where it is formed by water-currents.^ The lower 

 part of the line is more nearly parallel to the surface below and then rises 

 in a sharp curve ; in water-laid beds the lines are straighter and at a sharper 

 angle. Aeolian sands are generally very pure, clean, quartz grains with 

 few or no traces of life in them. The character of the grains and the criteria 

 for distinguishing aeolian and subaqueous sands are given by Sherzer.' 



Loess is a light reddish, gritty clay, frequently splitting up in vertical 

 columns and marked by vertical tubules due to the decay of plant roots. 

 Not infrequently land and fresh-water shells are present.* Such accumula- 

 tions are the result of dust-storms or less violent but more persistent trans- 

 portation of the fine rock debris continued over great distances and in 

 enormous quantity.^ 



Volcanic ash is especially liable to be carried for great distances from its 

 source, as it is thrown high into the air and may be caught by strong and 

 persistent winds of high altitudes. Such drifted material frequently forms 

 large accumulations, especially where it has fallen into bodies of water. 

 Thick masses accumulated in the Rocky Mountain region in Tertiary times 

 were long regarded as lake clays from their fossil content, but the microscope 

 revealed their origin.^ 



Some accumulations have been found evidently formed upon land far 

 from their source, as in Oklahoma, when the nearest source was the north- 

 eastern New Mexican volcanic region. On the Pacific coast such accumula- 

 tions carry marine fossils, showing that volcanoes, perhaps located near the 

 margin of the continent, had contributed materially to the marine sediments. 



GLACIAL DEPOSITS. 



Aside from the surficial deposits of the last geological period, whose 

 recognition is a distinct study, the traces of glaciers of the past are hardened 

 and cemented till (tillite), scratched and soled bowlders, rounded and 



1 Dacque, E., Grundlagen u. Methoden d. Palaogeographie, p. 147. 

 ^ Grabau, A., Principles of Stratigraphy, p. 701. 



^ Sherzer, W. H., Criteria for the Recognition of Various Types of Sand Grains, Bull. Geol. 

 Soc. Amer., vol. 21, p. 625, 1910. 



* Shimek, B., Various papers, mostly in the Iowa Academy of Science. 



* Keyes, C. R., Deflation and the Relative Efficiencies of Erosional Processes under Condi- 



tions of Aridity, Bull. Geol. Soc. Amer., vol. 21, p. 565, 1910. Mid-continental Eolation, 



idem, vol. 22, p. 687, 191 1. 

 ^Sinclair, W. J., Volcanic Ash in the Bridger Beds of Wyoming, Bull. Amer. Mus. Nat. 



Hist., vol. 22, p. 273, 1906. 

 ^ Buttram, Frank, Volcanic Dust in Oklahoma, Oklahoma Geological Survey Bull. 13, 1914. 



