490 GEOLOGICAL REVOLUTIONS. CHAP. 



effects of rain and snow, heat and cold, chemical and mechanical 

 agencies. Whoever will stand during a heavy thunder- shower on 

 the high land about Broadway or Birdlip, will require no further 

 evidence as to the power of falling rain to disintegrate the soil 

 and small stones, and move them little by little down the slopes. 

 In a few minutes a cyclonic storm may even clear away those 

 materials and expose the native rock. 



The hollows towards which they are, however slightly, urged, 

 though usually quite dry, and without trace of a channel, may 

 on such occasions be swept by short-lived currents ; and by a 

 repetition of these processes in a long period of time, the whole 

 surface will be lowered, and the waste be insensibly transported 

 farther down the valley. Insensibly, until we reach in descending 

 the hollow the region of springs and brooks. Even here the removal 

 of waste is almost insensible, since, except in particular cases, the 

 streams carry no stones only transport fine sediments in suspen- 

 sion, or roll small particles for small distances on their beds. 



But it was not always so. Flat meadows, formed by water 

 settling to rest and depositing sediment, appear on the sides of 

 the stream, and occasional gravel patches at their margins mark 

 the former force of stronger currents ; and when we pass downward 

 to the region of the Thames proper, the great receiving drain of 

 the country, the Cotswold stream flows amidst broad tracts of 

 pebbles which a more powerful river, rushing down the same valley 

 and subject to greater floods, deposited in the broad hollow which 

 had been filled by the sea at some earlier time. 



When was that gravel deposited ? The answer must be after 

 the last submersion and re-elevation of the tract where it lies. 

 For though gravels may have been formed in abundance in earlier 

 periods, none such could have remained lying as these lie, un- 

 disturbed by the rises and falls of the sea. By so much as the 

 amount of this gravel, and the finer sediments transported farther 

 on or carried to the ocean, have the higher grounds been wasted 

 and lowered in the course of the period which has elapsed since 

 the water, thought to be a cold or glacial sea, flowed over all but 

 the very highest of the Cotswold Hills. 



What is the length of that period, the latest of the steps in the 

 long scale of geological time? It is hard to say, because it is 

 difficult to find reliable evidence. When we stand by the conical 



