218 GEOLOGY OF THE ISLE OF WIGHT. 



patch o£ Blake Down and the series of patches which runs north- 

 ward to near Blackwater. 



Blake Down, forming the watershed between the Medina and 

 the eastern Yar, and the highest ground in what has been called 

 the Bowl of the Island, is capped with a deposit of gravel similar 

 to, though not so thick as, that of St. George's Down. The slope 

 of the plain on which it rests falls in this case towards the east, 

 that is down into the valley of the Yar, and, as before, the springs 

 break out at the lower margin of the gravel, and have cut it 

 back into a sinuous outline. 



The highest point of the gravel outlier occurs at its south end, 

 where it is 278 feet above the sea ; towards the north the plateau 

 slants down to a level of 230 feet. But the gravel runs down two 

 of the low ridges, which project eastwards, to a point 125 feet above 

 the sea, and only about 20 feet above the Valley Gravel of the 

 Yar. This is the nearest approach we get to an actual connection 

 between the Plateau Gravels of subdivision II., and the Valley 

 Gravels of subdivision III. 



Many gravel pits are dotted over Blake Down, showing 

 stratified flint-gravel with a few fragments of chert, and an occa- 

 sional band of gritty sand. Sometimes a layer of loam 1 to 3 feet 

 thick, lies above the gravel, but nothing that could be mapped 

 as brick earth. 



The series of outliers extending northwards from Blake Down 

 are clearly portions of a once continuous sheet. A line drawn 

 alono- tiieir western margins forma a regular curve, and probably 

 corresponds approximately with the original boundary of this 

 area of gravel. But on the eastern side the sheet has been deeply 

 eroded by the streams draining into the Blackwater. Two small 

 patches of gravel occur on the west side of the Medina, but they 

 lie at a lower level, contain more chert than those last described, 

 and are probably of later date. 



Excluding these two patches Ave find the level of the upper 

 maro-in of the series of gravel outliers falling northwards from 

 278 feet at Blake Down to 200 feet near Blackwater, and with 

 such regularity as to convey the impression that the gravel must 

 have been deposited along one continuous valley. Though the 

 present watershed between the Medina and the Yar passes right 

 across this line of gravels, yet it is so low, being only about 25 

 feet above the alluvial level of the Yar, that physically the valley 

 maybe said to run on continuously, along the line indicated. We 

 may suppose that the stream from Niton and Whitwell, which now 

 forms the head water of the Yar, formerly continued a northerly 

 course by Blackwater to the Medina, instead of, as now, making a 

 sharp bend across the normal direction of drainage at Budbridge. 

 Such alterations in the course of a river are not unknown else- 

 where, and have generally been brought about by the eating back 

 of one of the sources of the one river until it taps the waters of 

 the other. 



The date of the change must have lain between the deposition 

 of the Plateau Gravels and that of the Valley Gravels. For while 



