Agecroft Geology. 191 



turn, come the Triassic rocks; and over all (except on 

 the higher hill-ranges) there is sand or clay, or gravel, 

 both stratified and unstratified. This last, in the aggre- 

 gate, is technically termed "Drift." The whole of this 

 great surface was unquestionably once covered by salt 

 water. At the latest period of that marvellous marine 

 dominion, blocks of ice containing boulders floated in it; 

 and wherever great heaps of sand now occur, we have 

 the remains of ancient beaches or sand-banks, many of 

 which were cut through by the water, while others are 

 charged with pebbles that had been rounded by rolling 

 over and over upon some primeval shore, rattling while 

 on their journeys, just as at Walney Island we may hear 

 the pebbles of to-day. The lofty eastern edges of this 

 great stone basin are, as would be anticipated, quite free 

 from deposits of drift. But everywhere else, westwards, 

 drift covers up all the underlying rock, the latter showing 

 itself only where rivers in cutting their channels have 

 slowly worn it away. 



The Agecroft valley participates with all the rest of the 

 district in the possession of drift. Here, however, is well 

 shown, in addition, how the first settlings of gravel and 

 sand often themselves became covered at a later period 

 with yet another new deposit material brought down 

 and diffused by shallow and tranquil streams, then of 

 considerable breadth, but which in course of time shrank 

 into relatively narrow ones, and continue as such to the 

 present moment. That the lower Irwell, as we have it in 

 the Agecroft valley, was once a broad flood of this descrip- 



