RIVER DEVELOPMENT IN ESSEX. 245
succeeded by a phase of refrigeration culminating in the Ponders
End Stage—a phase of truly arctic or sub-glacial conditions.
During this long period the changes in the course of the Thames
were slight and all took place within the boundaries of the
southern drift belt. Any large changes of course must be
assigned to a date earlier than that of the Boyn Hill (100 ft.)
terrace, which is the oldest deposit represented in this southern
belt. Our attention is therefore drawn to the northern belt,
as likely to throw light on the former course of the Thames.
As regards the deposits in this northern belt, confusion has
undoubtedly arisen from the practice of classing them as glacial
deposits. They comprise coarse gravels and sands, locally
well-bedded, and yield much Triassic debris in the form of
quartzites and other stones from the Bunter Pebble Beds. It
is this fact which seems responsible for the vague, and in some
cases, mistaken, views as to the nature of the gravels. Prestwich
supposed that Goring Gap was cut as an "overflow-channel"
after the waters of the Upper Thames, formerly discharging
to the Wash, had been ponded back by the ice-masses advancing
from the north-east. He supposed that the escaping waters
introduced the Bunter quartzites into the gravels of the Lower
Thames basin. This was a perfectly consistent and, at the
time, tenable theory, but later writers have, for the most part,
found no evidence of such a lake above Goring Gap and have
agreed in rejecting the general conception and the chronology
of events which it imposes. In spite of this, the habit of regarding
the presence of Bunter quartzites in a gravel as indicative of
glacial origin has lingered, though the theory on which it is
based is dead. It is clear that the Chalky-Boulder Clay ice,
in its southward course, left the main outcrops of Bunter pebble-
beds far to the west and though it may have picked up a few.
in the vicinity of Nottingham, it is fairly certain that the quartzites
in the glacial gravels around London were derived at second-
hand from pre-glacial, or at least pre-Chalky Boulder Clay
deposits. The presence of such quartzites in the Red Crag of
East Anglia supports such an idea. We have seriously to
consider the view that the quartzites were introduced by normal
fluviatile agency. It is no new one, for it has been advocated
by Salter, H. J. O. White and others in the west of the Basin