94 POST-TERTIARY ENTOMOSTRACA. 



A. Preglacial Period. 



" Crag" of Aberdeenshire. 



Beds beneath Boulder Clay at Kilmaurs, with remains of 

 Elephas primigenius. 



B. Glacial Period. 



I. Early Glacial Period. 



Elevation of the land above its present level. 



Land ice. 



Formation of imfossiliferous Boulder Clay, now found : 



(a) On the flanks, in the hollows, and on the summits of 

 Scotch mountains, as well as beneath more recent 

 deposits in the plains. 

 {h) Beneath the Arctic shell-clays of the Clyde. 



II. Middle Glacial Period. 



Depression of the land. 



Denudation of Boulder Clay. 



Formation of Arctic shell-beds in hollows of Boulder Clay at 



various depths, according to extent of local depression. 

 Floating ice over the sea increased in area by the depression. 

 Boulder Clay thrown over shell-beds : {a) by glacier reaching 



the sea ; or (^) floating ice. 

 Earliest Mytilus edulis beds, formed when depression began, 



followed by clays with shells of deeper range as depression 



increased. 

 Period of extreme depression. 



III. Final Glacial Period. 



Re-elevation of land. 



Gradual shallowing of the waters. 



More littoral character of glacial shell-beds. 



Later Mytilus edulis beds. 



