E. Blackwelder — Yakutat Coastal Plain of Alaska. 459 



Art. XL. — The Yakutat Coastal Plain of Alaska:* A 

 combined terrestrial and marine Formation; by Eliot 

 Blackwelder. 



The reconstruction of the climatic, physiographic and bio- 

 logic conditions of past ages has long been one of the chief 

 objects of geologic study. Many lines of search have been 

 pursued with various degrees of success. Some facts have 

 been deduced from the fossils, others from the constitution of 

 the atmosphere, and still others from the rocks themselves. 

 Like most other questions, these have been investigated now 

 with caution and a full realization of the complexity and 

 uncertainty of the factors, and now with easy confidence and 

 blindness to difficulties. Some interpretations of sedimentary 

 deposits made in an off-hand way have been little better than 

 guesses. An encouraging number of others, however, are 

 firmly based so far as they go, and, best of all, the facts and 

 inferences are duly separated. 



In the interpretation of the older sedimentary rock forma- 

 tions we have to deal with what may be called fossil products 

 of aggradation. There are two ways in which one may 

 approach a given problem of this sort : (a) having studied 

 the many facts carefully, we may reason out, from our 

 knowledge of physiographic, climatic and biologic principles, 

 what conditions must have prevailed at that time and place ; or 

 (b) we may compare the fossil deposit with various modern 

 deposits of known origin and decide to which of them it 

 corresponds. Both of these methods are used and often in 

 combination. In order to put the second into practice it is 

 necessary for us to know intimately the characteristics of 

 many modern deposits, and also all the conditions under the 

 influence of which those deposits are now being made. 

 The careful description of modern formations in many 

 parts of the world is, therefore, essential to progress in this 

 study, and this paper is presented as a small contribution to 

 the mass of information needed. 



From near the mouth of the Alsek River to Controller 

 Bay, the southern coast of Alaska is fringed by a narrow 

 plain. From Yakutat Bay westward it is partly covered by 

 the Malaspina piedmont glacier. The part with which I am 

 familiar is that between Yakutat and the Alsek River. Ocean - 

 ward the plain clips beneatli the water level, leaving a tolerably 

 regular shore line. On the north it is hemmed in by the Bra- 

 bazon range, the front of which rises abruptly 2000-4000 feet 

 without either foothills or fringing talus-slopes. 



* Published by permission of the Director, U. S. Geological Survey. 



