210 GEOLOGY OF THE ISLE OF WIGHT. 



Secondary, extended far beyond their present limits, and must be 

 much older than any of the other gravels in the Island. On the 

 other hand, the formation of the gravels seems to be still proceed- 

 ing (though far too slowly to admit of observation), for it is 

 impossible to draw any hard-and-fast line between it and the 

 gravelly soil, which is being formed on the outcrop of the Chalk- 

 with-flints by weather and agricultural operations. 



The most important patch of this gravel is that which caps 

 the western end of St. Boniface Down, and v.hich supplies great 

 quantities of road-metal to Ventnor, But similar patches occur 

 also on Stenbury and Shanklin Do-^tqs, The patch on St. 

 Catherine's Hill is small, and interesting only from its position, 

 far away from, and far below the flinty Chalk ; small pockets of 

 gravel occur also here and there in the Chalk Marl at the edge of 

 the cliff. 



The extensive Downs between Calbourne, Chillerton, and 

 Carisbrook are very generally overspread by angular gravel, the 

 boundaries of the deposit following those of the flinty Chalk, but 

 always overlapping them. There are many shallow gravel-pits 

 along the southern edge of the Downs from Westover Down to 

 near Shorwell. 



The three patches above alluded to as containing many 

 rounded pebbles occur on Mersley and Brading Downs. No 

 sections can be seen there at the present time, but the gravel has 

 formerly been dug to a depth of about 2 feet for road-metal, 

 and the abundance of beach-pebbles is striking. Except in con- 

 taining these pebbles, which have probably been derived from some 

 Tertiary Bed, the patches do not seem to differ from the others 

 that have been described. 



II. — Plateau Gravels. • 



Their Age. 



These gravels are so called from their habit of capping flat- 

 topped hills. They occur generally as small patches, separated 

 by deep and broad valleys, and deeply cut into by the action of 

 springs, so as to present the sinuous outline generally found only 

 in beds of much older date. The complete alteration which 

 the features of the country have undergone since these gravels 

 were laid down indicates the great antiquity of the deposits. 



Though these outliers have clearly been isolated by denudation, 

 yet they do not seem to have belonged to one continuous sheet ; 

 for they occur at different levels. More probably they represent 

 successive stages in the process of development of the existing 

 system of valleys. In some cases even, the Plateau Gravels run 

 continuously down from the highest part of a watershed nearly to 

 the level of the Valley Gravel, thus tending to link together the 

 two groups. In the slopes of such outliers we have evidisnce of 

 the position of the lines of drainage at an early date. 



