38 



Stumps of what must have been immense trees, which from 

 their vertical position and interlaced roots, must have grown 

 in situ. They are now covered by the tide. The soil in which 

 they were growing consists of glacial till containing no marine 

 material, which must have at some earlier period formed part 

 of the settling basin of a glacial stream. 



The molluscs present in the ancient shell-bed, are conspe- 

 cific with those inhabiting Willapa Bay at the present time, 

 including Schizothaerus nuttalii Conrad, Macoma nasutaC ori' 

 rad, Fenerupis staminea Conr., Saxidomus giganteus\yts\i2LytSy 

 Clinoardium nuttallii Conrad, Cryptomya californica Conrad, 

 Ostrea lurida Carpenter and Thais lamellosa Graelin. The type 

 of Thais found in the shell bed is illustrated on Plate XVII. It 

 does not differ materially from some of the modern represent- 

 atives of this species in Willapa Bay. 



The openings in the till beneath the shell-bed are the exca- 

 vations of the burrowing crayfish {Upogebia) . 



Above the shell-bed there is an extensive deposit of terrace 

 material, presumably brought into this area by the Columbia 

 River when it was at its apex from the glacial melt. 



The cause of these changes in level has been variously inter- 

 preted, but the most plausible theory appears to be that of 

 "isostasy", which involves the concept that the continents 

 move up and down upon an elastic substratum, somewhat 

 after the fashion of ships loading and unloading ballast. 



Whatever the causes of these changes of level may have 

 been they must have profoundly affected the life of the inter- 

 tidal and shallow water zones. 



A second factor influencing the distribution of marine life 

 upon the western coast of North America, was the advance 

 and retreat of the great continental glaciers. According to 

 Charlesworth there were at least four such movements, with 

 interglacial periods having relatively warm climatic conditions, 

 during which many of the forms driven southward by the ice 

 returned to the north, as is indicated by the discovery of 

 Pleistocene deposit of temperate water molluscs at Nome, 

 Alaska, where arctic conditions now prevail 



