xv The Continental Area 305 



a gentle slope to the south-east overlooks the Severn Valley as 

 the Cots wold Hills. It is continued north-eastward to the 

 Humber and thence on the other side of the estuary north- 

 ward, where it swells up into the Yorkshire Moors, and 

 terminates in a line of cliffs along the coast. This edge is 

 the outcrop of a great belt of relatively hard oolitic limestone 

 (Jurassic period) which dips gently to the south-east, and is 

 separated by a line of older but less durable Secondary rocks 

 from the Primary system in the north and west. A similar but 

 more broken escarpment is formed farther south by an outcrop 

 of Cretaceous rocks, which also dip gently to the south-east. 

 This Chalk ridge reaching its greatest height in Salisbury 

 Plain, the Marlborough Downs, and the Chiltern Hills, is 

 continued in the lower East Anglian heights running north- 

 eastward through Norfolk. It appears north of the Wash 

 as the Wolds of Norfolk, and north of the Humber as the 

 Yorkshire Wolds, terminating in Flamborough Head. From 

 Salisbury Plain two low chalk ridges diverge : one runs east- 

 ward as the North Downs, the other south-eastward as the 

 South Downs, and both end in the Chalk Cliffs of Kent. 

 The River Thames rising on the southern slope of the Oolitic 

 ridge, flows through the Chalk ridge between the Marlborough 

 Downs and Chiltern Hills, and turns eastward to the North 

 Sea. Its triangular valley between the Chiltern Hills on 

 the north and the North Downs on the south is occupied 

 by tertiary rocks, consisting of clay, sands, and marls of 

 Eocene age. 



392. Ireland. The east coast of Ireland is compar- 

 atively low and unindented, while the west coast is cut into 

 many long inlets lined by lofty cliffs and fringed with islands. 

 The configuration of Ireland is entirely different from that 

 of Britain. A low plain occupies the whole interior, and 

 its elevation is so slight that a subsidence of 250 feet would 

 unite the Irish Sea and the Atlantic across the island. 

 Isolated groups of lofty mountains rise at irregular intervals 

 round the outer edge, the highest being Cam Tual (3400 

 feet) in the south-west. The Shannon, the largest river, 

 flows southward along the centre of the plain, and turns 

 westward into the Atlantic. Geologically the low plain of 



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