136 PROF. PRESTWICn ON THE AGE, FORMATEON, AND 



in subsequent pre-Glacial times this plain was scored by streams 

 flowing off a central mountain-axis*. The smaller streams, gradually 

 becoming tributary to the larger one, centred in the Darent, and 

 the excavation of the present valley then commenced. There is a 

 gap or break in the sequence between the pre-Glacial drifts and 

 the earliest of the so-called post-Glacial drifts of the valley, which 

 is probably covered by the extreme Glacial epoch. It was then a 

 time of erosion and denudation, and the record of it in this area is 

 to be found, not so much in beds of drift and gravel, as in bared 

 broad valleys and scarped ridges. 



Of the earliest drift of the Darent Valley there is little that has 

 escaped later denudation. The bank of coarse gravel on the brow 

 of the hill on the west side of the valley between Eyusford and 

 Farningham — a gravel abounding in a large proportion of Lower- 

 Greensand debris and extending from the height of 280 to 360 feet 

 — affords probably the best example. 



The traces of flint drift scattered, in the upper part of the valley, 

 on the slopes of the Lower-Greensand range, may possibly belong 

 to this epoch, but I speak with doubt. Thus at Kent Hatch, 

 above Westerham, at the level of 600 feet, there is a sprinkling of 

 unstained subangular flints, with a few worn brown-stained flints, 

 and there is a similar patch on the same level in the field to 

 the west of the Union Workhouse, above Sundridge. At Fawko 

 Common, near Sevenoaks, the same thing occurs. At the latter 

 place, Mr. Crawshay has found a small, well-shaped flint imple- 

 ment. To these may be added the instance before mentioned 

 of a thin covering of flint drift with some flint implements f at 

 Bitchet and Stone Street, near Ightham, at the height of 530 feet J. 

 Although these drifts are mere handfuls, they are significant, 

 inasmuch as their materials are foreign to the area where they 

 are now found. 



Another minor fact pointing to an early stage in the erosion of 

 the valley is the indication, which exists on the bare sides of the 

 hill west of Shoreham, of an old line of water-level, at a height of 

 400 feet above O.D., or of about 200 feet above the present stream. 

 At the point * (fig. 4) is a band of a compact breccia, about 10 

 to 12 feet broad, extending horizontally for some distance along the 

 brow of the hill. It consists of angular fragments of chalk con- 

 solidated by a calcareous infiltration, and rendered so hard that it 

 requires a smart blow with a hammer to break it. It appears to have 

 been originally a talus of chalk fragments, such as would accumulate 

 at the foot of a chalk slope or cliff, and to have been concreted by a 

 calcareous cement into this brecciated rock by a spring charged with 

 carbonate of lime in the manner of an ordinary travertine. But at this 

 spot there is no impervious stratum to give rise to a spring. Tho 

 Chalk, which rises to a height of 120 feet above the point (*), 



* Quart. Journ. Geol. Soc. vols. xlv. (1889),p. 291, and xlvi. (1890) p. 171 et^eq. 

 t They are those which I have placed in the second or * Hill ' division in 

 the Ightliam paper. 



+ Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 276. 



