THE AGE OP THE LAST UPRISE OF THE BRITISH ISLES. 177 



Ireland. Tliis level being indicated by raised beaches with sea 

 shells of existing species in Denbighshire, Carnarvonshire 

 (Moel Tryfaen), and the AVicklow Mountains.* From this 

 maximum level of depression the amount diminished both to 

 the north and south. In Lancashire these gravels rise along 

 the banks of the Irvvell to about 600 feet, in Scotland to about 

 the same. On the other hand, in Gloucestershire, the level of 

 submergence was determined by myself many years ago to be 

 600 feet in the Cotteswold Hills, being that to which rolled 

 quartzite pebbles are to be found scattered over the tableland 

 formed of Jurassic limestone, these pebbles having been derived 

 from the New Red conglomerate of the Midland Counties and 

 drifted to their places by marine currents. This determination 

 was fully confirmed by the late Professor Phillips in his 

 Geology of Oxford.'\ Extending our observations still 

 further south, we hnd the gravels of this period forming 

 plateaus resting on the Bagshot Sands at Englefield Green and 

 Windsor Forest southwards, and finally, in the Isle of Wight, 

 forming the terraces of St. George's Down and Headon Hill, at 

 a level of 400 feet above the sea. Such, in brief, are some of 

 the localities at which the interglacial gravels may be observed. 

 They are everywhere later than the newest Tertiary strata, and 

 the deposits are consequently referable to the Great Ice Age. 



The rudely stratified clays with glaciated pebbles and erratic 

 blocks to be observed along the valleys of the Irwell and 

 Kibble and other parts of Lancashire resting on the inter- 

 glacial sand and gravel indicate a recurrence of sub-glacial 

 €onditions, when the waters of the sea were clouded with 

 glacial mud, and floats of ice carrying blocks from the glaciers 

 •entered the sea. This epoch need not detain us, as it was 

 probably of short duration ; and the deposits resulting from it 

 •do not appear to have extended into the centre and southern 

 parts of England. We, therefore, pass on to the consideration 

 of the subject which more immediately concerns us, and to 



* The fact of these beaches occurring at nearly the same level along a 

 west to east tract of about 100 miles is clear evidence of their marine 

 origin, although attempts have been made to prove they owe their 

 formation to " the great ice-sheet " which filled the Irish Sea. An ice- 

 sheet never could have produced beds of stratified sand and gravel with 

 shells, some of which are but little injured ; an ice-carriage would have 

 ground them to powder. 



t Page 457. The gravels were first described by Dr. Kidd and Dean 

 Buckland. Phillips gives the extent of submergence as 1,500 feet — some- 

 what excessive. 



