22 Dr. D. Woolacott—The Interglacial Problem. 
diverse views that are still held regarding their origin are an evidence 
of the difficulty experienced in the interpretation of the phenomena. 
Accepting as definitely proved that the greater part of the Drift 
is the product of an ice-sheet, the main controversy now centres 
round whether there were true Interglacial episodes in the formation 
of the Glacial deposits ; some geologists asserting that the evidence 
proves there was a succession of Glacial epochs in the Pleistocene 
Period + and some that our era may even be an Interglacial one, 
while others affirm that there is no evidence of any such intervals 
in the Glacial deposits of Britain.2 As I propose to discuss the 
evidence that has been advanced recently in favour of Interglacial 
periods in the Drift of the north-east of Hngland ? it may be well to 
consider what is meant by such periods, and the nature of the 
evidence that seems to be necessary to prove that they occurred. 
The subject is specially complicated because of the difficulty of 
distinguishing in a limited area an interval of ice-retreat from an 
Interglacial period, also because there are no exact criteria by means 
of which Interglacial deposits can be differentiated from interval- 
deposits or even sometimes from fluvio-glacial beds. If an area 
has once been glaciated and the ice has retreated, it is obvious it 
may be reglaciated without a true Interglacial period intervening, 
for by such an epoch we imply not only the retreat of the ice from 
a limited area and its subsequent return, but the disappearance 
or almost total disappearance of the ice from the main gathering 
grounds (produced by the oncoming of a warmer climate over the 
whole glaciated region) and then the regrowth of the ice-sheet and the 
reglaciation of the area considered. Within the limits of the country 
studied it might be difficult to distinguish a temporary retreat of 
the ice from an Interglacial period, as they might be marked by the 
same phenomena, although to a less extent and over a smaller area 
in the one case than the other. Interglacial periods might be 
signalized on a marine area by the production of a shore-line and 
the laying down of deposits, on a land area by the formation of 
lakes and lake deposits, plant beds, loess deposits, and by a con- 
siderable amount of denudation ; and somewhat the same phenomena, 
although on a lesser scale, might mark in a limited area a retreat 
of the ice from it alone.. The only way in which an Interglacial 
epoch can be proved is by showing that the retreat of the ice took 
place over the whole glaciated region at the same time. It might 
obviously be impossible to distinguish in one area the phenomena 
of an interval of ice-retreat from those of a true Interglacial period ; 
thus the difficulty that has arisen’in connexion with the Interglacial 
problem is due in the first place to the non-recognition of the fact 
1 J. Geikie, The Great Ice Age. And other authors. 
2 Lamplugh, Address to the Geological Section, British Association, York, 
1906, etc. It has also been asserted recently by a Swiss geologist that there 
is no evidence of true Interglacial periods in the Glacial deposits of Switzerland. 
3 Trechmann, op. jam cit., Quart. Journ. Geol. Soc., 1920. 
