R. M. Deeley—The Thames Valley Gravels. 63 
the Pebble Gravel. The pebbles are well water-worn, and consist of 
flint, quartz, and quartzite. They are certainly much older than the 
Chalky Boulder-clay, as can be seen from the relationship of the two 
deposits near Potters Bar and the region immediately to the north- 
east. On Fig. 2 the dotted line D D shows the general slope of these 
Pebble Gravels to be about 4:4 feet per mile as we goeast. However, 
the direction of the section is not quite along their dip slope. At 
one time the Pebble Gravels must have been a wide-spread deposit, 
but whether they are the remnants of the gravels of an old river and 
its tributary streams is uncertain. It is owing to the protection the 
gravels afforded to the soft rocks below that the deposit now occupies 
high ground above the softer surrounding rocks. 
At much lower levels, and sloping in somewhat the same direction 
as the Pebble Gravel, we have what are here considered to be the 
fluvio-glacial gravels and sands of the Chalky Boulder-clay ice-sheet. 
They stand at heights which lie roughly, as previously stated, 
between the levels shown by the dotted lines B B and CC, Fig. 2. 
In Fig. 1 the coarsely hatched area of the Tilehurst Gravels shows 
the ground which was probably occupied by the fluvio-glacial gravel 
of the Chalky Boulder-clay period. It is suggested that the bottom 
of the valley was then flat, denudation having reduced the level of 
much of the surrounding soft rocks to base-level, the whole area, 
where it was sufficiently low, being covered by a thick sheet of gravel. 
There may have been numerous ridges, low hills, and gravel terraces 
standing above the plain which were not covered by gravel, 
consequently the whole of the hatched area may not have been 
covered by the deposit. 
In the neighbourhood of Goring the base of the gravel stands at 
~ about 3800 feet O.D. Up the Kennet Valley it is a little higher, 
whilst it falls rapidly in an easterly direction. On this account 
it is not convenient to call the deposits the 300 foot terrace, 
and they will be referred to as the Tilehurst ‘errace Gravels for 
distinctness. 
At about ninety-four miles from the margin of the map, Fig. 1, 
there is a deposit of gravel and sand on the hill to the west of 
Tilehurst. Its highest point and the level of the rock terrace on 
which it rests are indicated by the vertical line. Monckton! describes 
this deposit, and some of the other gravels to be referred to, as glacial 
gravels, and states that they contain quartzites, large blocks of 
quartz, and igneous rock. By the Geological Survey they are 
described as being as a whole ‘‘distinctly current-bedded, though 
this character is more conspicuous in some pits than in others”’. 
In the Kennet Valley on the ridge to the north-east of Beenham, 
at nearly 100 miles, there is gravel and sand at about the same level 
as at Tilehurst. Other high-level gravels at Sonning, Finchampstead, 
Hurley, Bisham, and Winter Hill belong to this terrace. 
From Flackwell Heath to Harefield the deposits plotted are those 
of the St. Albans Ice Lobe, glacio-fluvial fan gravels and sands. In 
1 Q.J.G.S8., vol. xlix, p. 309, 1893. 
2 The Geology of the Country around Windsor and Chertsey (Mem. Geol. 
Surv.), p. 60. 
