OF MARLBOROUGH. Xl 



Hungerford Boads, show the power of currents of water to 

 abrade or wear away the surface. 



Boulders and fragments of rock become firmly fixed in 

 river-ice or icebergs during winter, and when the ice breaks 

 up, are carried off to be deposited in the sea. Glaciers also 

 carry with them blocks of stone and the debris which they 

 grind away as they slowly glide down from the summits 

 of snow-capped mountains. Every year immense icebergs 

 are brought down from the shores of Greenland and Iceland, 

 where they have been partially formed by glaciers running 

 out into the sea and tearing up the bottom as they slide, 

 until parts of them become detached and are borne along 

 southwards by the cold current from the north which flows 

 along the eastern coast of North America. As these ice- 

 bergs approach the warmer waters of the Gulf-stream* they 

 melt away and deposit their load of boulders and clay. 

 The boulder- clay of the east of England, and of which we 

 have some remains in this neighbourhood, was deposited in 

 this way at a time when the greater part of what is now 

 Great Britain was under water, and when there was no 

 Gulf-stream to prevent the sea and the land in this part of 

 the earth from being covered by icebergs and glaciers. In 

 some of the valleys of North Wales and of Scotland, glaciers 

 and icebergs have left their traces behind them in the 

 parallel striaa or grooves which they have hewn out in the 

 solid rock. 



There is another external agency, but one of quite a 

 different character from these, which is very important, as 

 by means of it beds of limestone and silica or flint are 

 formed. 



* The Gulf-stream is a current of the ocean which takes its rise 

 in the Grulf of Mexico, where its waters have been warmed by a tropical 

 sun, and flows towards the north-east at the rate of five miles an hour, 

 reaching across the Atlantic to the shores of England and the north- 

 west of Europe, and by its warmth preventing our climate from being 

 severe and our shores ice-bound. 



