36 lee Maries in North Wales. [Jan.; 



pebbles, whereas many of tbe rocks and stones found in the drift 

 are as sharp, angular, and irregular as the blocks and masses 

 which are detached by the winter's frost, and lie under an inland 

 precipice. 



The solution of this curious problem of the origin of the drift, is 

 to be found in the history of glaciers and icebergs. When a valley 

 is filled with ice, the rocky debris from its slopes and precipices fall 

 upon the surface of the glacier. A quantity of the earth and stones 

 of the bottom of the valley is also forced into the crevices or frozen 

 to the bottom of the icy mass. Now when the ice-filled valley 

 terminates in the sea, large fragments of the glacier break off and 

 become icebergs, and floating away carry with them their load of 

 earth and rocks, which are deposited where they melt, or topple 

 over, or are stranded. In the North Atlantic as far as icebergs 

 float, there must be an annual deposit of matter on its bottom 

 exactly of the same nature as the drift, while in Hudson's Bay and 

 the Gulf of St. Lawrence it must be accumulating still more rapidly. 

 When North Wales was one or two thousand feet lower than at 

 present, it must have formed a group of islands, among which ice- 

 bergs would frequently become entangled and deposit their loads of 

 foreign matter. At the same time Snowdon and Cader Idris would 

 have been sending down glaciers into the sea, which would spread 

 the debris of their precipices and valley bottoms on what are now 

 the upland slopes and low valleys, but which then were submerged 

 banks and ocean straits. As the land rose above the sea to its 

 present elevation, rivers, floods, and glaciers would more or less 

 furrow and clear away the drift from the valleys, and leave it dis- 

 tributed in the irregular manner in which we now find it. The 

 mere presence, therefore, of this unstratified mass of earth, rocks, 

 and boulders would of itself prove a recent glacial period ; since it 

 clearly indicates the existence of icebergs and glaciers in seas and 

 countries where they are now never found. 



2nd. Moraines. — Every modern glacier carries upon its surface 

 more or less of the debris of the rocky valleys through which it 

 passes. As the glacier moves downwards, these are carried with 

 it, and at its termination, where its waste by melting exactly 

 balances its downward progress, this debris must necessarily be 

 deposited, and form a more or less regular heap of rock and earth 

 called the terminal moraine. These moraines are sometimes 

 destroyed almost as fast as they are formed by the streams which 

 issue from the glacier itself or by torrents from the adjacent moun- 

 tains, but under favourable conditions they remain, and long after 

 the glacier has entirely disappeared tell the tale of its former 

 existence. If owing to a steady change of climate a glacier retires 

 regularly, the moraine-heaps will be distributed over the whole surface 



