POST-TERTIARY BEDS OF SCOTLAND. 95 



Floating ice ; dropping glaciated Boulders into shell-beds. 



Boulder Clay occasionally thrown, as before, over shell-beds. 



Gradual filling up with sand and silt of great sea-channels, 

 formed during period of greatest depression, as, eg. between 

 Firth of Forth and Firth of Clyde. During this process 

 Boulder G\diy?, possibly deposited by floating ice over sand and 

 silt — their alternation as now found being caused by seasonal 

 changes. 



Drifting ice breaking up shell-beds and mixing their contents 

 with Boulder Clay, especially along the newly forming coasts. 



C. Post-glacial Period. 



I. Eai-I^ Post-glacial Period. 



Upper Raised Beaches, formed as the land rose from the Glacial 

 Sea, during the gradual amelioration of the climate. 



Upper portions of Clyde clays, showing by absence of species 

 common in lower portions, the disappearance of an Arctic 

 fauna and the incoming of estuarine conditions. 



Land almost at its present level. 



IL Middle Post-glacial Period. 



Possibly milder climate. 



Peat with trees, showing conditions very favorable to growth. 

 Slight resubmergence of the land. 

 Pecten maximus bed at L^ine water, Colintraive, &c. 

 Lower estuarine mud ; possibly Scrobicularia bed of silt of east 

 coast. 



in. Final Post-glacial Period. 



Re-elevation of land to its present position. 

 Last Raised Beaches. 

 Upper estuarine mud. 



The final Post-glacial Period we regard as terminating with the last elevation of the 

 land, at however recent a date that may have taken place. 



