30 Dr. D. Woolacott—The Interglacial Problem. 
lakes or by streams flowing from the margins of the ice-sheets, yet 
when the ice had finally left the country it was the relative contour 
of the surface of the bed-rock and drift that was the determining 
factor in valley development. The valleys of the Tyne and Tees 
and Upper Wear were superimposed on the partly filled Preglacial 
channels, but many of the smaller of these had been entirely 
obliterated, and the direction of the Postglacial streams bear no 
relation to them. When the direction of ice-flow had been across 
the pre-existing valley, there was left after the ice had vanished 
a thicker deposit of Drift on the lee-side of the valley than on the 
other. This contour of the glacial deposits forced the new streams 
to erode their courses on the comparatively driftless side of the valley. 
The most-marked channel thus produced is that of the Wear from 
Durham past Finchale Abbey towards Chester-le-Street,! which is 
here developed in rock and boulder-clay on the eastern side of the 
“Wash”. The Wear valley thus is a composite one, being a com- 
bination of three types—in its higher reaches it is superimposed on 
the pre-existing channel, from near Durham to Chester-le-Street its 
course was determined by the Drift-contour, but the gorge near its 
mouth was originally eroded by an overflow stream from ice-dammed 
waters, and was finally developed since the uplift of the Durham 
coast in late Glacial or Postglacial times. 
(10) The dry valleys of Northumberland and Durham which form 
such a distinct feature in its physiography have been produced 
in three distinct ways, viz.: (a) some are Preglacial channels which 
have not been filled up by drift, (6) some are the forsaken overflows 
from glacier-lakes, (c) some were formed by streams that flowed 
from the margin of the ice-sheet—on to country which was not 
occupied by lakes—and are thus associated with Kaims. 
(a) The dry valleys of the Permian area round Sunderland, of 
which Tunstall Hope is a typical example, are mature, comparatively 
driftless valleys, whose outlets were blocked by material deposited 
by the Tweed—Cheviot ice as it moved along the coastal region and 
by the marginal deposits of the sea as the raised beaches were 
being formed. (6) The valleys which were eroded by the over- 
flow waters from temporary lakes held up by ice are singular in 
form and position and bear no relation to the present drainage or to 
any possible river drainage of the past.2 They are usually trench- 
1 There are several of this type in Northumberland. Smythe, op. jam cit., 
De EA 
Py Professor Bonney, discussing the glacier-lakes of Cleveland (Pres. Address, 
B. A. Sheffield, 1910, p. 29) has suggested that ‘“‘ the so-called overflow 
channels much more closely resemble the remnants of ancient valley-systems 
. and they suggest that the main features of this picturesque upland 
were developed before rather than after the beginning of the Glacial Epoch ”’, 
but a detailed examination of most of the more important dry channels 
of Northumberland, Durham, and Cleveland has convinced me that—with 
certain exceptions, as for example some on the Permian area of East Durham 
—it is impossible for the great majority of them to be the remnants of old 
drainage systems. 
