24 Dr. D. Woolacott—The Interglacial Problem. 
thickness or extent; e.g., the local deposit of sands and gravels at 
Horsebridge Head, north of the mouth of the Wansbeck,! resting 
on the bed-rock beneath typical boulder-clay, which has been 
described by one writer as a Preglacial shore-line and by another 
as the deposit of an early Glacial stream, may have been deposited 
during an Interval-in-the-Glaciation of Hastern Northumberland or 
may even be Interglacial; while, on the other hand, the bed of loess. 
at Warren House Gill lately described by Trechmann as an Inter- 
glacial deposit 2 may not be such, but may be an Interval-deposit 
marking an important episode in the glaciation of Kast Durham 
between the deposition of the Scandinavian and British drifts. (No 
deposits of peat, plant beds, etc., with a temperate flora are yet 
known in the Glacial deposits of Northumberland and Durham, 
and so need not be discussed in this paper.) 
There is another point to which the greatest attention should be 
paid if the Interglacial problem has ever to be solved, and that is 
the exact use of the terms Pre-, Inter-, and Postglacial. A bed 
beneath boulder-clay is not necessarily Preglacial, as e.g. the deposit 
at Horsebridge Head or the Sewerby raised beach at Bridlington 
(both of which have been considered to be Preglacial; the only thing 
certain is, however, that they were formed prior to the Glacial 
deposit under which they lie). In the same way a bed occurring in 
the boulder-clay which was not formed or not directly formed by 
ice is not necessarily Interglacial, and beds resting on typical 
boulder-clay with only recent deposits above are not always 
Postglacial. 
The difficulties met with in the working out of the sequence in 
the Drift of an area are due to several causes, the principal of which 
are (a) the irregular surface on which it rests, (b) the possibility of 
the moraine profonde of one ice-stream being overlain by that 
of another from a different region during one continuous period of 
glaciation, (c) the possibility of the moraine profonde, etc., of one 
ice-stream being entirely removed by another ice-flow without an 
Interglacial period or even a “‘ Glaciation-interval ”’ between, (d) the 
possibility of some of the deposits being laid down by water without 
even an “ Interval-in-the-Glaciation”’, and (e) (as a result of these) 
the complex manner in which the beds change from point to 
point so that it is exceedingly difficult to correlate them. Every 
worker in the field on the Drift must have been impressed by the 
complexity of the deposits, the detailed and exact observation that 
is necessary to determine the mode of origin of certain beds, or to 
work out the sequence of events, and by the great need there is for 
the exact use of terms in describing the formations. 
There are certain features of the Glacial and Postglacial phenomena 
of the two north-eastern counties on which most glaciologists would 
1 A fuller description of this bed is given later. 
2 Op. jam cit., 1920, p. 187. 
