It was desirable to determine, if possible, the origin
of these materials, as some interesting points of
ancient physical geography hinged upon this point.
I therefore again closely examined these sands
and the subordinate shingle, and succeeded in find-
ing a not inconsiderable number of subangular frag-
ments of chert of distinct Lower Greensand ori-
gin, together with a few pebbles of Lydian stone,
derived probably from some of the Portland beds,
and some pebbles of sandstone which might also
have come from the Boulonnais. I also found two
pebbles of a red granite, which may have been de-
rived from the granitic districts either of the Co-
tentin or of the Channel Islands. Still it is possible
that these granite pebbles may have been derived
indirectly from the Lower Greensand, which we know
to contain in places pebbles of granite and of other
older rocks, or from the more distant Boulder-clay or
beds of that age. I unfortunately lost my speci-
mens in returning to Calais before I had had the
opportunity of carefully examining them. The pre-
sence, however, of chert from the Lower Greensand,
and of pebbles of Lydian stone and of other rocks
from the Boulonnais district, suffices for our object,
which is to show the additional probability these
facts afford of the existence of a channel between
France and England anterior to the low- and pos-
sibly the high-level valley-gravels. For to the east-
ward of a line drawn between Cape Blanc-nez and
the South Foreland there is no coastline whence this
Lower Greensand chert could have been derived-
the area consisting of Tertiary strata-whilst a chan-
nel
open
to the westward would, after passing through
the belt of Chalk, intersect the Lower Greensand
Folkestone and the Portland and Purbeck beds
the Boulonnais, although that channel might have
been much narrower than the one now existing. At
the same time there is another explanation possible,
although I consider it much less probable.

of

of

I have mentioned in my last paper in the Phil. Trans.' that the old rivers of the Wealden area debouched, as do those of the present day, outwards into the Thames valley, but that they were of much greater size and extent. I have evidence to show that in the very centre of the Wealden area there are remarkable cases of valley-excavation and old river-action. I have even there found beds of highlevel gravel. Now these rivers, in their passage across the Lower Greensand, always took up considerable quantities of chert and ragstone fragments,

E.

General Section of Sangatte Cliff, along the Coast (about half a mile).

which they carried out into the valleys traversing the North Downs and into the valley of the Thames. It is therefore possible that somewhere between Calais and Dover there may have been an old river flowing eastward from the Wealden area, and debouching in this part of the English Channel, into which it might have transported the chert and other pebbles we find in the old beach at Sangatte. I am disposed, however, to consider the transport by sea-action along a channel open from west to east, as at present, although much narrower, the more probable.

Above the old beach is a mass, from twenty to eighty feet thick, of angular flint and chalk-rubble, identical in general characters with that in the same position at Brighton, in which Dr. Mantell discovered the remains of the Elephant, Horse, &c., but presenting some features of much interest, that have not yet been observed at Brighton. At both places irregular seams of fine chalk-rubble and marl are intercalated with the flint débris, but at Sangatte there are in addition some beds which are not to be distinguished from ordinary Loess. Not only do they resemble that deposit in general characters, but I likewise discovered in some of these loams, as also in some of the more marly beds, several species of land-shells common in the Loess. I found also, what I before discovered in the Loess of the Somme valley, numerous small, semitransparent, calcareous concretions, the probable remains of land-slugs. The following is a list of the shells from the Sangatte cliff, revised by my friend Mr. Gwyn Jeffreys:

Helix concinna.
pulchella.

Succinea oblonga.

Pupa marginata.
Arion ater.
Limax agrestis.

I found no freshwater shells, or Mammalian remains.

There is no intercalation of sea-beach or sea-bed in this mass of overlying rubble. The two beds are perfectly distinct, and the old beach must have been raised so as to form dry land before the accumulation of the land débris above it. It is an interesting problem how it could have been formed in such a position-whether by the mere melting of the winter snow on the adjacent range of chalk hills, or by the flooding of some adjacent river. It is not, however, my intention on this occasion to discuss this subject, but merely to make known additional facts which I consider of importance in their bearing on a very interesting theoretical question, and again to direct 'attention to a remarkable section-one requiring further examination -with the view especially better to determine the old rock-pebbles and the organic remains that are to be found in it.

2. On the SUPERFICIAL DEPOSITS of the VALLEY of the MEDWAY, with REMARKS on the DENUDATION of the WEALD. By C. LE NEVE FOSTER, B.A., D.Sc., F.G.S., and WILLIAM TOPLEY, F.G.S., of the Geological Survey of Great Britain.

Introduction.

CONTENTS.

Part I. Description of the superficial deposits.

a. General description of the valley of the Medway.

6. Superficial deposits.

1. Subaërial beds.

2. Modern alluvium.

3. River-gravel and brick-earth.
a. The Medway.

b. Tributaries of the Medway.
4. Pipes of gravel and brick-
earth.

5. Disturbances.

Part II. Denudation of the Weald.
a. Short sketch of previous theories,
with objections to the theory
of fracture and to the marine
theory.

6. Bearing of the river-gravel on the
question.

7. On the mode of deposition of beds of gravel and loam, and on the action of streams and rivers in modifying their channels. d. On the origin of escarpments. Conclusion.

INTRODUCTION.

DURING the last few years the subject of river-gravel has so much occupied the attention of geologists, that a short description of the gravel and brick-earth of the valley of the Medway will not be without interest, especially as those deposits have a most important bearing on the denudation of the Weald. In the present paper we propose, firstly, to describe the superficial deposits of the valley of the Medway, and, secondly, to show what light those deposits throw on the theory of the denudation of the Weald.

PART I. DESCRIPTION OF THE SUPERFICIAL DEPOSITS.

a. General Description of the Valley of the Medway.-Before describing the superficial deposits it will be well to devote a few lines to a concise account of the basin of the Medway; the position of the beds will then be more readily understood. As we intend to treat of only so much of the basin as lies within and south of the Chalk escarpment, we can confine our description to that part. The escarpment of the Chalk forms on the north a well-marked boundary to our district. On the east the line of watershed separating the valley of the Medway from that of the Stour passes south from the Chalk by Lenham to Pluckley and Shadoxhurst; thence the watershed turns westwards, and, passing Cranbrook, Ticehurst, Wadhurst, Crowborough, and West Hoathly, divides the waters of the Medway from those of the Rother and the Ouse. From West Hoathly a line passing northwards by Copthorn Common and Bletchingley to the Chalk escarpment, north-west of Godstone, separates the Medway basin from that of the Mole; the boundary of our basin then follows the Chalk past Titsey, turns south-east and runs eastwards along the high ground of the Lower Greensand to Ightham Common, and then *This high ground forms part of the prominent chain of hills known as the Forest Ridge. The highest point, Crowborough Beacon, is 804 feet above low-water mark.

adjacent District.

(Reduced from Sheet 6 of the Map of the Geological Survey of Great Britain: the Cretaceous beds surveyed by Mr. F. Drew, the gravels by Messrs. Foster, Top

ley, and Dawkins.)

northwards to the Chalk near Wrotham, and is thus separated from the basin of the Darent.

The Chalk forms a steep escarpment facing the Weald to the south, but to the north and north-east the grounds lopes down gradually; the dip is everywhere into the escarpment, lower beds rising to the south. Through this escarpment the Medway flows at Burham. To the west of the transverse valley thus formed the strike is E. and W.; on the eastern side it is nearly S.E. and N.W. South of the Chalk we come upon the Gault, forming a flat of low ground averaging three-quarters of a mile in width. The Upper Greensand is here very thin, and makes no feature on the ground; springs often flow out at the base of the Chalk. The Lower Greensand rises gradually from beneath the Gault, and ends, like the Chalk, in a steep escarpment to the south. The upper part of the Lower Greensand is sandy (Folkestone Beds)*; this division is underlain by a thin bed of clay and sandy clay with fuller's earth (Sandgate Beds); and the lower part of the Lower Greensand (Hythe Beds) consists mainly of beds of limestone and sand, known as "Kentish Rag" and "Hassock." Here the valleys, which do not reach down to the Atherfield Clay, are often dry, like those of the Chalk. The Kentish Rag country east of the Medway is known as the Quarry Hills. Springs flow out at the junction with the Atherfield Clay below. This clay is the lowest member of the Lower Greensand, and rests immediately on Weald Clay, which occupies a low and broad plain, varying from four to seven miles in width. The Hastings Sand†, subdivided into beds of clay and sand, rises up on the south from beneath the Weald Clay towards the high land of the Forest Ridge. All the streams to the north of this ridge run into the Medway, those to the south drain into the Rother and the Ouse; the former enters the English Channel at Rye, and the latter at Newhaven.

The Medway is formed by the junction of a number of small brooks coming down from the high land near East Grinstead; it flows down past Hartfield to Penshurst, where it receives a large tributary, the Eden, and passing Tunbridge, arrives at Yalding. Here the Beult and the Teise fall into the Medway, which now enters the gorge cut in the Lower Greensand; it soon reaches Maidstone, receives the Len, and then flows on in a general north-north-westerly direction towards Snodland, where it is joined by the Snodland Brook. The Medway now takes its course along the gorge through the Chalk, passes by Rochester, and finally reaches the Thames at Sheerness.

6. Superficial Deposits.-The following different kinds of superficial deposits are found in the Medway valley :

1. Subaërial beds.

2. Modern alluvium.

*For a description of the subdivisions of the Lower Greensand, see Fitton, Trans. Geol. Soc. 2nd ser. vol. iv. 1836, p. 103, and Drew, 'Memoirs of the Geological Survey,' Sheet 4, 1864.

+ For a description of the northern part of the Hastings Sand country, see Drew, Quart. Journ. Geol. Soc. vol. xvii. 1861, p. 271.

VOL. XXI.-PART I.

2 H