PLEISTOCENE AND RECENT DEPOSITS. 209 



placed in any one of these groups. Such is the angular flint- 

 gravel of the Downs, which has probably been in process of 

 formation from the time when the Chalk was first exposed to sub- 

 aerial denudation up to the present day, and therefore runs 

 through all three groups. But inasmuch as it provided the 

 materials from which the Plateau Gravels were constructed, we 

 may conveniently take its description first. The following table 

 gives the sequence of the groups in descending order, the numbers 

 indicating the order of their descriptions in the following 

 pages: — 



IV. Deposits now in course of formation or of recent date 

 (Alluvium, Peat, Blown Sand, Tufa, Chalk Talus, &c.). 

 III. Deposits formed after the present valleys came into 

 existence (Valley Gravels and Brick Earth). 

 II. Deposits formed before the present valleys existed (Plateau 

 Gravels). 

 I. Deposits partly earlier than, partly contemporaneous with 

 Groups II., III., and IV (Angular Flint Gravel of the 

 Chalk Downs). 



I. — Angular Flint Gravel of the Chalk Downs. 



This is a deposit of very indefinite age. It occurs on the tops 

 of all of those Downs in which the Chalk dips at a small angle, 

 probably because of the expanse of nearly level ground being 

 greater than in the narrower Downs, where the dip is high. The 

 deposit is unstratified, and closely packed with unworn flints or 

 fragments of flints, imbedded in a loose gritty or sometimes a 

 brown clayey matrix. In three instances near Brading, it contained 

 a large proportion also of perfectly rounded fl.int-pebbles, mixed 

 with angular flints, but pi'obably derived from some Tertiary 

 pebble-bed. 



This deposit is no doubt of svib-aerial origin, the flints, together 

 with a portion of the matrix, representing the insoluble residue of 

 a great thickness of Upper Chalk. But there occur also materials 

 in the matrix which could not have been derived from any part 

 of the Chalk, viz., the grains of quartz and other rocks, which 

 give the gritty character to the gravel; and also the comp^.etely 

 rounded pebbles alluded to above. The occurrence of such 

 materials makes it certain that other beds besides the Cnalk, 

 presumably some of Tertiary Strata, have been laid under 

 contribution. 



The thickness of rock that has bean removed since this sub- 

 aerial deposit began to form has undoubtedly been very great. 

 The gravel not only oversteps the present limits of the Chalk-with- 

 flints, but occurs on hills in which no beds so liigh even as the 

 Middle Chalk now occur, as, for example, on St. Catherine's 

 Hill. In such cases, the gravel seems to have been gradually 

 lowered by the slow solution of the chalk beneath it. 



If this view of its origin be correct, some portion of the gravel 

 must date back from a time when all the strata, both Tertiary and 



E .56786. O 



