Geoscience Reference
In-Depth Information
and ice caused widespread river flooding in England. This extreme situation may
provide an analogue for the pre-glacial Great Ouse when it was powerful enough
to carve out its present over-size valley before its upper catchment area was
blocked by an advancing ice sheet from the north.
When Harmer discussed the glacial origin of some gorge-like valleys in Eng-
land, he suggested that such cases must be typical and not anomalous. Harmer
continued that in all glaciated regions, whether in Britain or abroad, the invasion of
any region by an ice sheet, especially where its movement was upstream, would
obstruct the natural drainage, producing lakes in which levels would have risen
until an outlet for the water was established in some new direction. In this key
paper, Harmer presented the hypothesis that certain gorges in England might have
originated as spillways due to the blocking of the previously normal line of
drainage by an ice sheet (Harmer
1907
).
Harmer's hypothesis was later referred to by Leonard Wills when he suggested that the
Upper Severn drainage was added to that of the Lower Severn due to the impounding of a
glacial lake in Shropshire and Cheshire which rose until it flowed over the former
watershed
between
the
Shropshire
Plain
and
the
headwaters
of
a
tributary
of
the
Worcestershire Stour (Wills
1924
).
In 1947, Sherlock also made reference to Harmer's hypothesis, when, in discussing
gaps in the Chiltern Hills, he suggested that an ice-sheet advancing from the north
during the Pleistocene epoch impinged on the escarpment but was not able to
override it in the Wendover-Tring section. With the waning of the ice, massive
floods of water were released forming a lake between the Chiltern Hills and the
retreating ice. The waters of this lake rose until their level reached cols in the
escarpment where Wendover and Tring stand today. Here they overflowed into
valleys with torrents of great erosive power on the dip-slope of the Chiltern plateau
cutting deep steep-sided gorges very rapidly into the soft chalk—fully vindicating
the hypothesis suggested by Harmer 40 years earlier (Sherlock
1947
).
Although the geological history of the Thames remains the subject of con-
flicting hypotheses, in 1907 Harmer drew attention to an interesting parallel
between the basin of the Middle Thames around Oxford and the Vale of Pickering
in Yorkshire (Harmer
1907
). The latter, as is well known from Kendall's 1902
paper, was occupied by a glacial lake which was due to the Derwent River having
had its outlet to the sea south of Scarborough closed by a dam of ice; as a result the
water in this lake rose until it overflowed at a gap near Malton; the river thus
formed cut a gorge through which the drainage from the Vale of Pickering now
flows southwestward into the Yorkshire Ouse, and reaches the sea through the
Humber.
According to Harmer the Upper Thames originally discharged northeastward
through the Fens into the Wash; however when this outlet was blocked by ice, the
waters of the Upper Thames collected as a pro-glacial lake (Lake Oxford) which
were discharged by several overflow channels cut through the Chiltern Hills;
eventually, as the lake-level fell, the discharge was only maintained through the
Goring Gap at the southwestern end of the Chiltern Hills.
