90 GEOLOGY OF THE ISLE OF WIGHT 



level than the present streams. Periods alternated during 

 which gravels were laid down by the river, and when the 

 river acquiring more erosive force, by an elevation of the 

 land giving its bed a steeper gradient, or a wetter climate 

 producing a greater rush of water, cut a new channel 

 deeper in the old valley. So our valleys in Southern 

 England are frequently bordered by a succession of gravel 

 terraces, the higher ones being the older, dating from times 

 when the river flowed at a higher level than at present. 

 Such terraces may be seen above the Eastern Yar and its 

 tributary streams. In the centre of the old gravels is the 

 alluvial flat of a later age. 



The Island rivers cut out their channels when the land 

 stood at a higher level than at present. The old channels 

 of the lower parts of the rivers are now filled with alluvium, 

 partly brought down by the rivers and partly marine. 

 The channels are cut down considerably below sea level ; 

 and by the sinking of the land the sea has flowed in, and 

 the last parts of the river courses are now tidal estuaries. 

 The sea does not cut out estuaries. They are the sub- 

 merged ends of river valleys. 



Some idea may be formed of the antiquity of our Island 

 rivers by observing the depth of the clefts they have cut 

 through the downs at Brading, Newport, and Freshwater. 

 But to this we must add the depth at which the old 

 channels lie below the alluvium. It would be interesting 

 to know the thickness of the alluvium. But it is not often 

 that borings come to be made in river alluvia. However, 

 in the old Spithead forts artesian wells are sunk ; and 

 these pass through 70 to 90 feet of recent deposits before 

 entering Eocene strata. Under St. Helen's Fort, at the 

 mouth of Brading Harbour, are 80 feet of recent deposits. 

 The old channel of the Yar, at its mouth, must lie at least 

 at this depth. 



Before it passes through the gap in the Chalk downs 

 the Yar has meandered about, and formed the alluvial 



