Geoscience Reference
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eroded a gorge through the Chalk escarpment south of Oxford to create the Goring
Gap. The proto-Thames now flowed south through this gorge and then east
towards what is now London, where it became a tributary of the Rhine; Britain at
this time was still connected to mainland Europe. Harmer continued:
The features of that part of the Jurassic plain which lies east and west of Oxford and those
of its outlet through the Chalk-escarpment, the well-known Goring gap, are so nearly
identical with the cases before discussed [that is, lake-like basins drained by gorges
excavated transversely across adjacent hill ranges] as to suggest primâ facia that they
originated in a similar way.
The basin in question [the Oxford plain] is bounded on the north-east, between Buck-
ingham and Leighton Buzzard, by a transverse ridge of comparatively high land, from 400
to 500 feet above sea level—partly composed, however, of Glacial Drift, that is the terminal
moraine of the Ouse branch of the great Eastern Glacier. At present, this ridge marks the
division between the drainage-system of the Upper Thames and that of the Fenland, but (as
before suggested) the watershed may have lain farther to the south-west in pre-Glacial
times, the Oxford plain then standing at a somewhat higher level than it now does.
Some deep borings at Hitchin and at Newport (Essex), revealing Glacial drift at a depth
of 68 feet in the one case, and 140 feet in the other, below Ordnance-datum, point to the
existence
in
pre-Glacial
times
of
valleys
descending
obsequently
from
the
Chalk-
escarpment.
In another boring 2 miles south-west of Sandy, in Bedfordshire, on low ground near the
Greensand-escarpment, Boulder-Clay 104 feet thick was found overlying the Oxford Clay,
about 100 feet below the level of the Bedfordshire plain and but little above that of the sea.
Such cases seem to indicate pre-Glacial valleys, subsidiary to that of an important river
flowing in a north-easterly direction towards the Wash, at a level considerably lower than
the River Ouse at present.
Further to the north-east borings at Boston, Fossdyke and Long Sutton in which
Boulder-Clay was shown to extend from 100 to 160 feet below sea-level, may represent
the seaward extension of such a drainage-system, excavated at a time when England stood
higher than it does now.
Approaching more nearly the Oxford region, we find a boring near Stony Stratford [North
Buckinghamshire], in the upper part of the Ouse basin, with 112 feet of Drift, resting upon
the sub-Glacial surface at about 115 feet above Ordnance-datum. The present level of the
nearest part of the Oxford plain is 100 feet higher, suggesting that the drainage of the latter
may have been originally towards the Fenland through this Drift-filled valley and some
channel now hidden by Glacial deposits, which are in that region of considerable thickness.
At Buckingham, for example, 6 miles south-west of Stony Stratford, a well was
carried for nearly 70 feet through glacial gravels to a level of about 220 feet O.D., without
piercing them; and another boring in the same locality showed 78 feet of Drift.
Assuming that the borings at Buckingham and Stony Stratford represent portions of the
same pre-Glacial valley, it is improbable that the former coincides with its maximum
depth near that place, as the longitudinal rivers of the Jurassic plain have no such fall as
100 feet in 6 or 7 miles; under any circumstances, the valley could hardly have terminated
abruptly near Buckingham.
If, moreover, the Glacial deposits could be removed, it would be found that the Oxford
plain was formerly connected with the basin of the Ouse by a valley of a more important
character, wider as well as deeper, than that which still exists. Such a valley seems to have
no meaning, and its origin is not easy to explain, unless we regard it as excavated by a
river flowing from the south-west; in fact, by the primeval Thames.
The late Sir Joseph Prestwich, indeed, maintained that the Thames (or Isis), originated
in pre-Glacial times, ran towards the Wash, having been diverted to the south during the
Glacial Period by the excavation of the gorge at Goring.
