R. M. Deeley—The Thames Valley Gravels. = 1138 
erosion have removed this easterly watershed of the Thames and 
given it its present direction of outlet to the sea. Probably the 
initiation of the Straits of Dover dates from the advent of the first 
ice-sheet which crossed the North Sea to Britain, and if the Straits 
existed in Chalky Boulder-clay time they were probably very narrow. 
Indeed, the reason why the early ice-sheets of the Pleistocene period . 
were able to cross the North Sea was the absence there of the 
volumes of warm water which now pass from the south through the 
Straits. The erratics in the raised beach of the south coast of 
England probably reached their present positions at a later period; 
for there is much evidence which favours the view that the last 
cold period occurred long after the Tilehurst Terrace Gravels were 
laid down. 
It may be that the Tilehurst fluvio-glacial aeatels did not at any 
one time quite fill the valley between the heights shown by the line 
BB and CC, Fig. 2; but rather that terraces of very considerable 
thickness were formed between these limits, for the grading and 
aggrading of the valley varied in accordance with the varying 
quantities of material thrown into it from time to time by the 
glacier streams. 
Some of the gravels which stand at a higher level than those of 
the High Terrace remain to be noticed. Monckton } describes some 
gravels which rest on the Ashley and Bowsey Hills. He considers 
that some of the pebbles found in them point to a glacial origin. 
Mr. White found a pebble of grey chert with casts of detached joints 
of erinoids at Bowsey Hill and another of similar character at Ashley 
Hill. Both the black and grey cherts found may well have been 
derived from the Carboniferous Series to the north. These gravels 
also contain a great abundance of white quartz pebbles. On the 
north side of the Thames, opposite the Bowsey and Ashley Hills, 
there are also gravels at a similar height, and as shown on the Map, 
Fig. 1, higher-level gravels run along the south-eastern slopes of 
the Chiltern Hills, above the fluvio-glacial deposits of the St. Albans 
Ice Lobe. They also form a well-marked terrace resting on a platform 
about 400 feet high in the Kennet Valley and on the high land to the 
north-west of Bagshot. Here, however, they do not contain Northern 
Drift. Their extent and boundaries are very uncertain and the 
method of their formation is obscure. The Bowsey and Ashley Hill 
gravels. are shown thus + in the diagram, Fig. 2. They seem to be 
- much too high to have been formed when the St. Albans Ice Lobe 
was in existence. Some of the plateau gravels lower down the 
Thames may also be of the age of the Higher Terrace Gravels. 
A possible explanation of the existence of such high-level gravels 
containing Northern Drift erratics may be as follows The position 
of the Thames Basin is a very interesting one from the point of view 
of stratigraphical glacialogy ; for its southern, central, south-western, 
and western portions were outside the area occupied by the ice-sheet 
which advanced to some extent over the northern and north-eastern 
watershed ; and owing to its position on the edge of the ice-sheet, it 
felt the full effect of any variations in the position of the ice margin. 
1 Q.J.G.S., vol. xlix, p. 314, 1893. 
DECADE VI.—VOL. III.—NO. III. 8 
