98 THE BRITISH ISLES. 



the Severn and that of its afHuent, the Avon, are triassic, but there was a 

 time when ranges of carboniferous limestone extended right across the Bristol 

 Channel, connecting the hills of Somerset with those of Wales. The Mcndip 

 Hills (1,0G7 feet) are a remnant of this formation, and so are the three parallel 

 ridges near AVeston-super-^Iare, which jut out into the Bristol Channel. The 

 cape facing them in Wales belongs to the same formation, as do also the forti- 

 fied islands of Steepholra (240 feet) and Flatholm, which connect the fragments 

 of the ancient limestone range, which has disappeared through long-continued 

 erosive action. These islands, together with the sand-banks in their neighbour- 

 hood, form the natural boundary between the estuary of the Severn and the Bristol 

 Channel. 



The Severn, in comp:irison with the great rivers of continental Europe, is 

 only a feeble stream. About 80 inches of rain fall within its basin, and this 

 amount would be sufficient to sustain a river discharging 11,000 cubic feet of 

 water per second throughout the year, if large quantities were not absorbed by the 

 vegetation, sucked up bj' the soil, or evaporated into the air. It is only by the 

 construction of locks that the Severn, up to Worcester, has been converted into 

 a navigable river, having an average depth of nearly 8 feet. The Wye, Usk, 

 Lower Avon, and other rivers, which discharge themselves into the estuary of the 

 Severn, are usually looked upon as its affluents, though in reality they are 

 independent rivers, having their proper régime, and forming minor estuaries of 

 their own. Including these, the Severn drains an area of 8,119 square miles; it 

 discharges on an average 5,300 cubic feet of water per second, a quantity raised to 

 12,000 cubic feet when it is in flood.* 



In no other part of Europe does the tide rise to the same height as in the 

 Bristol Channel and the estuary of the Severn. In reality we have to do here 

 with three tidal waves, which enter the channel simultaneousl}', and increase in 

 height and vehemence in projiortion to the resistance they meet with on their 

 progress up the funnel-shaped estuary. One of these tidal waves originates in the 

 open Atlantic, and travels along the coast from the Land's End ; the second is 

 thrown back by the coast of Ireland, and enters through the centre of the channel ; 

 a third arrives from the northern part of the Irish Sea, coalesces with the former off 

 St. David's Head, and thus doubles its height. This enormous mass of water, 

 discoloured by the waste of the land resulting from its erosive action, rushes up 

 the channel with considerable velocity, producing a rise at ordinar}^ tides of 40 to 

 43 feet. At spring tides the rise at Chepstow, at the mouth of the Wye, is 60 



