AND EIVER VALLEYS BORDERING THE BRITISH ISLES. 311 



IV. Submerged river channels. — The views I have just ex- 

 pressed receive remarkable confirmation from the existence 

 of old river channels, which may be traced on the Admiralty 

 charts by the soundings. It will be evident that during the 

 period when the British platform vs^as in the condition of a 

 land surface, the rivers descending from the adjoining land, 

 as well as the rain which fell upon its own sm-face, must 

 have had outlets to the ocean towards the west; and we 

 are, therefore, led to inquire, are such outlets, in the form of 

 river channels, to be recognised by the soundings ? I am 

 able to give a very decisive answer in the afKrmative to this 

 question. Notwithstanding that the submerged lands around 

 the British Isles have for thousands of years been covered by 

 water, more or less loaded with sediment, and during the later 

 glacial stages, laden with icebergs and floes carrying and 

 depositing stones and mud, two old river chaimels, at least, can 

 be clearly traced, one draining the lands now occupied by the 

 waters of the Irish Sea, and the other, by those of the English 

 Channel. The courses of these old rivers are indicated by 

 slightly irregular depressions in the soundings, varying in 

 depth from 2 to 20 fatlioms below the general levels adjoining, 

 but they become remarkably accentuated on approaching the 

 margin of the great escarpment, where they are converted 

 into gorges or caiions bounded by precipitous walls of rock, 

 and traceable down nearly to the base of the escarpment.* 



V. The English Channel River and the " Hurd Deep " (Figs. 

 5 and 6). — The course of the river Avhich drained the area of 

 the English Channel can generally be traced by a curving 

 line of depression from its source near the Straits of Dover 

 to the margin of the great escarpment, where it cuts deeply 

 into the rock in the form of a gorge or caiion. Owing, pro- 

 bably, to silting up by sediment the course is less evident 

 than it would have been had no sediment been deposited. 

 But at one part of its course its position is still clearly defined 

 on the chart for a distance of 70 miles under the name of the 

 "Hurd Deep." This is a nearly straight E. and W. gorge 

 about 4 to 5 miles across, and at its deepest part 354 feet 



mode of formation by marine action on emergent lands of such plateaux 

 as the British platform, p. 644. 



* In a recent paper read before the Geological Society Mr. T. 

 Codrington has described various river valleys in the south and west of 

 England and Wales in which the solid rock is found at various depths 

 below the sea-level, the original channel being filled in with "glacial 

 deposits," that of the Dart being 110 feet below low water mark. 



