R. M. Deeley—The Thames Valley Gravels. 61 
fragments of the old deposit. Since these gravels were laid down the 
Thames has greatly deepened and widened its valley, and in doing so 
has destroyed the larger portion of the old deposits. 
When first the water thrown off by the ice passed over the cols and 
entered the Thames Valley, it may have been muddy; but was free 
from gravel and sand, these latter being deposited in the valleys up 
which the ice was advancing, and were subsequently covered up by 
or converted into boulder-clay. The large volume of water thrown 
into the Thames Valley would first result in increased erosion and the 
destruction of much of the old Thames gravel of pre-Chalky Boulder- 
clay age. However, when the ice reached the watershed, then 
immense quantities of gravel and sand would be thrown into the 
valley, and the stream being unable to carry it away the gravel and 
sand would collect in the valley bottom and form a thick bed. 
When a river is thus aggrading its valley it does not form one wide 
stream, but breaks up into a form known as a “‘ braided stream”’. 
The Thames Valley may be divided into an upper or Oxford Basin 
above the Goring Gap, and a lower or London Basin below the Goring 
Gap, the Chiltern Hills and the high ridge to the south-west 
separating them. } 
When the ice margin stood on the north-east watershed of the upper 
basin, gravels were formed at heights of 100 feet or more above the 
present level of the Thames. However, in the Oxford Basin very 
cold climatic conditions existed even outside the Chalky Boulder- 
clay ice margin, and there is a marked absence of stratification in the 
Plateau Gravels of the region. Even if the ice did not override the 
watershed at one time, it is probable that heavy snow-drifts collected 
_on the valley sides in sheltered positions, destroying older gravel 
terraces and disturbing gravels then forming. 
In the Geological Survey Memoir for Oxford it is stated that the 
stones in the high-level gravels are a very mixed collection; among 
_ them occur numerous partially rounded flints a few inches across, 
liver-coloured quartzites doubtless from the Bunter Pebble Beds of 
the Midland Counties, white vein quartz, various coloured quartzites, 
hard sandstones resembling greywethers, soft sandstones with felspar 
of the Millstone Grit type, and black lydian-stone. The highest 
outlier on the Oxford sheet is at Bower’s Hill, 540 feet O.D., but this 
deposit is doubtless much older than the Chalky Boulder-clay. 
In the Oxford Basin the Plateau or Fluvio-glacial Gravels are not 
well developed, having suffered very considerably from the erosive 
action of the rivers and streams, and perhaps from the collection and 
movement of snow masses. However, there are some outliers of 
gravel, etc., at levels of 300 feet above O.D., which would appear to 
belong to the series formed whenthe Chalky Boulder-clay ice stood on 
the watershed to the north-east. The patches between Ipsden and 
_ Lewknor, extending along the edge of the Chilterns at heights of 300 
to 400 feet above O.D., may be partly of this age; but as they are 
almost wholly flint gravels formed of materials derived from the 
weathering of the Chiltern Hills, they may be of various ages. 
In the basin of the Thames at.Chiselhampton and Great Milton, 
there are patches of gravel at heights of about 290 feet above O.D. 
