72 GEOLOGY OF THE ISLE OF WIGHT. 



due. seem to come in at a rather lower horizon, while the chert itself 

 is impersistent. The escarpment becomes conspicuous at Knighton, 

 where the dip is gentle, and is separated from the Downs by ^ 

 deep valley. Some old quarries on either side of the Knighton 

 valley have exposed friable green sand with cherty lumps. The 

 springs previously alluded to (p. 65), issue at the base of the 

 Chalk Marl. The sand is well exposed in a lane at the east end 

 of Knighton East Wood. 



This brinofs us to Yarbridge, where there is a fine section in 

 the Chalk Marl, ending, however, at its junction with the Chloritic 

 Marl. The latter can be seen in the sides of the lane which runs 

 along the foot of the Down Avestwards, while the sand below it is 

 shewn in the lane leading to Morton, 100 yards west of the High 

 Road. The Chert Beds are not distinguishable. 



East of the Yar the scarped ridge of the Greensand stands 

 out prominently, and excepting a break at Yaverland, continues to 

 do so till it presents on the coast the section which has already 

 been described. 



2. Around the Southern Downs. 



On the west side of St. Calherine's Down several small pits 

 occur along the scarped brow formed by the chert and freestone, 

 the former material being used for road-metal The outcrop of 

 the Upper Greensand is narrow, but steep, and on the broader 

 slope of Gault lie many huge masses of Greensand that have 

 slipped bodily down. The long flat-topped spur of St. Cathe- 

 rine's Down which juts out to the north, and marks the line of 

 strike, is ca})ped with a stri[) of Chert Beds, about 1,300 yards in 

 length, but only from 50 to 80 yards in breadth, and terminates 

 northwards in a remarkable semicircular hollow, which seems to 

 have been formed by a landslip. The chert is worked for road- 

 metal in small pits here, and on Head Down. West of Niton 

 some old quarries range along the outcrop o'i the chert and 

 freestone. 



Another fine brow, known as Gat Cliff, is formed by these 

 beds ill Appuldurcombe Park. The dip being southerly, the 

 boldest front is presented to the north. Here also a long line of 

 old quarries marks the outcrop of the freestone. 



In the valley south-east of Wroxall, along which the lailway 

 passes, several sections may be observed. The cutting by whi(*h 

 the tunnel is approached has been made in the Malm Rock, the 

 Gault, so far as can be seen, lying about the level of the rails. At 

 the south end of the tunnel the rails are about eight feet below 

 the freestone; the tunnel descends southwards at the rate of 1 in 

 173, and is about 1,300 yards in length. From these data it may 

 be calculated that the dip of the strata to the south amounts to 

 1 in 38 or an angle of rather less than 2°. 



St. Martin's Down which terminates noi-thwards in nearly as 

 bold a brow as that of Gat Cliff' contains chert bands of 

 exceptional thickness. 



