DEPOSITION 113 



or the indentations impressed on the drying surface by rain. Thus 

 shallow water deposits may retain an impression of the sunshine, 

 the showers, and the breezes of the period when they were formed. 

 Examples of all these features may be seen in the rocks of the crust. 



There is one class of deposit which is conspicuous, because in 

 it there is neither sorting of ingredients nor stratification. This 

 used to be known as drift or diluvium, and it was at one time 

 thought to be the deposit found by a tumultuous deluge. The most 

 characteristic form of the deposit is called boulder-clay, and it is a 

 mass of tough clay in which are embedded angular stones of all shapes 

 and sizes (Fig. 22). The stones have often been transported a long 

 way from their parent rocks, and the only sign of wear they generally 

 show is a certain amount of smoothing and polishing, and some- 

 times scratching and grooving of certain of their surfaces (Fig. 23). 

 There is no denuding agent now operating in Britain which is cap- 

 able of producing such material, and yet the boulder-clay is found 

 scattered all over the country north of the Thames, and is specially 

 abundant in the mountain districts. On comparison with de- 

 posits now being thrown down at the termination of Swiss and 

 Norwegian glaciers, there is found to be agreement in all the 

 points mentioned above. Glaciers are the drainage of the per- 

 manent snows of high altitudes and high latitudes, the snow passing 

 into the form of ice and being driven to flow down the valleys. Ice 

 flows infinitely more slowly than water, so it banks up in the 

 valleys and fills them to a great depth, unlike the water of a river, 

 which is confined to the waterway. Quantities of frost-riven 

 fragments fall on to a glacier, and are carried on its surface 

 or fall into its cracks. Armed with the last, the ice becomes 

 an important eroding agent, and rasps down, polishes, and 

 scratches its valley floor, wearing off quantities of fine rock flour. 

 All these materials are shot down when the glacier melts into a 

 pell-mell mass at its end, called a terminal moraine. During the 

 height of the Glacial Epoch, of which they give evidence, the 

 supply of ice was so great that it overflowed from one valley 

 system to another, and even crossed or ascended valleys and sur- 

 mounted watersheds which were opposed to its general course. 



Acting on a country already much disintegrated by atmos- 

 pheric action, it soon gathered and transported great masses of 



VOL. VI. 8 



