466 PREHISTORIC EUROPE. 



conditions, but in the meantime we must look a little more 

 closely at the leading characteristics of the postglacial beds. 

 The beds in question consist of three more or less well-marked 

 divisions or groups, which, beginning with the lowest-lying or 

 oldest, are as follow : — 



1. Dull brown and dark blue clay, with shells and associated banks of 



sand, gravel, and shells. 



2. Light gray and pale brown clay, with no organic remains. 



3. Sand unfossiliferous, with which are not unfrequently associated 



erratics. 



The lower shelly clay (No. 1) is well developed along the low- 

 lying maritime districts, where it seems to cover the ground as 

 a somewhat continuous sheet. It follows all the sinuosities of 

 the coast-line, and thus penetrates for a greater or less distance 

 into the interior of the country. It is likewise commonly met 

 with cloaking the bottoms of valleys that open directly upon 

 the sea, and can be traced along the margins of certain lakes 

 which in late glacial and postglacial times were arms and gulfs 

 of the North Sea and the Baltic. In some places this clay is 

 highly fossiliferous, and has yielded a very rich variety of shells, 

 in which respect it offers a striking contrast to the glacial clays, 

 characterised as these are by a much scantier assemblage of 

 species and individuals. Most of the postglacial species still 

 inhabit the neighbouring seas, but many of the shells are smaller 

 than those of the same forms that occur in the glacial deposits. 

 A few northern species, which are very common in the latter, 

 reappear in diminished numbers in the postglacial beds. Among 

 these are Yoldia pygmoea var. gibbosa, Tritonium Sabinii, Bhyn- 

 conella psittacea, Cardium elegantulum, and Margarita costulata, 

 none of which now lives south of Lofoten and Finmark. On 

 the other hand, the well-known species of the southern coast of 



1 The following species, which are very abundant and characteristic forms in 

 the Norwegian shell-banks of Glacial age, are feebly represented in the Postglacial 

 shell-banks : — 



In Glacial shell-banks. In Postglacial shell-banks. 



Pecten islandicus (Mull. ) . . . Becoming rare. 



My a truncata (Linn. ) Becoming thinner in the shell. 



Saxicava rugosa (Linn. ) . Do. do. 



