26 Dr. D. Woolacott—The Interglacial Problem. 
Norway '—the Scandinavian Drift being a “pure culture” of 
Norwegian rocks, together with such material as it obtained in its 
passage across the North Sea.? 
(4) The course of events after the first oncoming of this ice- 
sheet is difficult to decipher (as the reading of it depends on 
whether Interglacial periods are accepted or not, and also on the 
manner in which it is thought certain beds, etc., were produced),? but 
of the final period of glaciation we can form a fairly clear picture. 
During this epoch the coastal area of Northumberland and Durham 
was invaded by ice from several regions. There were four main 
ice-streams * from the west, all of which received a southerly trend 
along this area due to the pressure of the invading sheet from the 
Continent. These four western flows were (a) one down the Firth 
of Forth (Forth ice),’ (6) one down the Tweed which swept round 
the Cheviots (Tweed—Cheviot ice), (c) a broad stream from the south- 
west of Scotland and the Lake District which came over the 
Northumberland moors, was confluent with a small Alpine glacier 
down the South Tyne valley, and filled up the Tyne valley (Lake 
District ice, south-west of Scotland ice, Northumberland ice or Tyne 
valley ice) and (d) one from the Lake District over Stainmoor, 
bringing with it the Shap boulders and confluent between Cotherstone 
and Middleton ® with a glacier down the Tees valley (Lake District _ 
(Shap) ice,’ Stainmoor ice or Tees valley ice). The indicator 
boulders and the direction of the glacial striations prove that these 
main flows, following the general contour of the country, moved 
eastwards in more or less parallel lines until they came under the 
influence of the Scandinavian ice-sheet,® which diverted them 
1 Professor Bonney (Presidential Address, Brit. Assoc., Sheffield, 1910, 
p. 23) has endeavoured to show that the Norwegian deep off the south-west 
coast of Norway would offer a difficulty to the passage of an ice-sheet, but Iam 
in agreement with Dr. Trechmann when he states that this deep need not be 
considered as presenting a difficulty, and I also consider that the evidence 
(distribution of boulders, marine shell-bearing gravels and sands pushed inland, 
etc.) proves that the Scandinavian ice did actually move over the North Sea 
area, and that a lobe impinged on the Durham coast near Castle Eden. 
* Trechmann, op. jam cit., p. 186. 
3 e.g. the deposit at Horsebridge Head. 
4 Minor glaciers in the Cheviots and Pennines are not considered here, 
e.g. a glacier came down the Wear valley and was confluent with the 
Northumberland ice on the north and Lake District (Shap) ice on the south. 
> Boulders of olivine-basalt, most probably from the Firth of Forth, occur 
in the clay at Fulwell Quarries and of St. Abb’s porphyrite in that near 
Tynemouth. 
5 Three miles and 8 miles west of Barnard Castle respectively. 
7 It appears highly probable that boulders of Shap granite do occur in the . 
Drift of the Tyne valley. The evidence for their occurrence is very strong. 
8 Along the coast of Northumberland from Bamborough to the Wansbeck 
there are two boulder clays of different origin, a lower typical bluish clay 
mainly of western derivation and an upper reddish clay of Tweed valley and 
northern origin. Smythe, Glacial Geology of Northumberland, p. 94. In a 
depression at Hartlepool there is a boulder clay with only Western erratics 
(Trechmann, op. jam cit., 1920, p. 191), and at Horsebridge Head there is a 
deposit containing Scottish rocks beneath the Western derived boulder clay. 
