THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 13 



movement. A typical case upon a large scale is the great overthrust on 

 the eastern side of Glacier National Park.^ 



Erosional breaks are the most common and most looked for limiting 

 planes, but would be apt to be present upon only one side of a unit of small 

 stratigraphic extent. The various types of unconformity are explained and 

 illustrated in every textbook and in typical cases are easily recognized, but 

 increasing attention is being paid to the determination of minor uncon- 

 formities and unconformities obscure because of the position of the beds. 

 Where an exposed bed is nearly horizontal and is but little dissected by 

 erosion, or where long erosion has reduced the exposed surface of horizontal 

 beds to a near level surface, the succeeding deposit may be so nearly parallel 

 and conformable as to present the appearance of uninterrupted deposition. 

 Schuchert has described and illustrated typical instances of this condition,'' 

 and Ulrich has repeatedly drawn attention to the necessity of determining 

 even minor unconformities. 



Erosional periods followed by the deposits of transgressing and retreating 

 seas result in peculiarities of the unconformity which have been interpreted 

 by Grabau.^ 



The true meaning of an unconformity is not always fully realized by 

 paleogeographers, either as to the time involved, with all the possibility of 

 structural and surface changes and changes of environment, or as to the 

 amount of geological record lost by erosion and by lack of deposition during 

 the period of exposure. Blackwelder^ and BarrelP have called attention 

 to the importance and the significance of these erosional intervals. 



Changes in bedding, in material, or in color may mean much or little, 

 dependent upon conditions. In horizontal, persistent beds, evidently de- 

 posited under uniform conditions in quiet water, such changes would at 

 once attract attention as indicative of some considerable disturbance either 

 of surface or climate and hence of the life; but in delta, fluviatile, terrestrial, 

 or even shore deposits, frequent alterations of bedding, of material, or of 

 color are entirely consistent with unchanged conditions upon the land. 

 Small bodies of water may easily receive material from different sources 

 within a limited period of time; rivers may change their course or alter the 

 velocity of their currents in a capricious manner; occasional floods may 

 sweep together material not deposited in the same place under normal 



1 Campbell, M. R., The Glacier National Park— A Popular Guide to Its Geology and 



Scenery, Bull. 600, U. S. Geological Survey, 1914. 



2 Pirsson, L. V., and Chas. Schuchert, Text-book of Geology, pp. 586-587, 1915. 

 Schuchert, Paleogeography of North America, Bull. Geol. Soc. Amer., vol. 20, p. 441, 1910. 

 Barrel, Jos., Rhythms and the Measurement of Geological Time. Illustrations of Rhythms 



in Sedimentation. Bull. Geol. Soc. Amer., vol. 28, p. 798, 1918. 



3 Grabau, A., Types of Sedimentary Overlap, Bull. Geol. Soc. Amer., vol. 17, p. 567, 1907. 



Principles of Stratigraphy, chap, xvill. 

 * Blackwelder, Eliot, The Valuation of Unconformities, Jour. Geol., vol. 17, p. 289, 1914. 

 s Barren, Jos., Rhythms and the Measurements of Geologic Times. Illustrations of Rhythms 



in Sedimentation. Bull. Geol. Soc. Amer., vol. 28, p. 798, 1918. 



