On British Drifts and the Interglacial Problem. 363 



find freshwater marl and peat that were deposited hi the many 

 lakelets and marshes that dotted the Holderness plain ; and in 

 the lower layers of certain of these freshwater deposits the 

 leaves of the arctic birch {Betuhi nana) have been detected, t 

 indicating- a climate colder than at present. 



In East Yorkshire, then, we appear to have a continuous 

 record of the events from the beg-inning- to the end of the 

 Glacial Period ; and yet, if I read the sections aright, we can 

 find no place into which a single mild interglacial epoch can be 

 intercalated. 



Let us now more briefly consider certain glaciated areas 

 within the influence of the ' West British ' ice-lobe which I have 

 personally investigated. 



Drifts of the Isle of Man. — From its isolated position in 

 the midst of the Irish Sea, the Isle of Man constitutes an 

 excellent gauge or glaciometer, on which is recorded the course 

 of events within the basin occupied by the West British ice- 

 lobe. In carrying- out the geological survey of this island I 

 made a close examination of its glacial deposits in every part, 

 and have stated the results rather fully in a recently published 

 memoir. 



We find here, as in Yorkshire, that prior to the glaciation 

 there was a sea-margin at approximately its present level, and 

 where the coast is composed of ' solid ' rocks, in approximately 

 its present position. In this sea, marine deposits indicative of 

 cold conditions were accumulated, and were afterwards dis- 

 placed and mingled with the boulder-cla}^ of an ice-sheet that 

 g-radually filled the basin and swept southward, or south-south- 

 eastward, over the very summit of the island. At its maximum 

 the surface of this ice-sheet stood more than 2000 feet higher 

 than present sea-level. The difference between the altitude 

 attained by this ice and that of the East British lobe in the same 

 latitude is especially noteworthy. In Yorkshire the eastern ice 

 did not reach much above 800 feet on the flanks of the Cleve- 

 land Hills, declining to 500 feet or under off" Flamboroug-h 

 Head. The higher land which surrounds the Irish Sea Basin 

 may be in part responsible for this difference, but I think that it 

 must have been mainly due to the heavier precipitation in the 

 west. 



Then followed a declining stage in the glaciation, during 



* By Dr. A. G. Nathorst, at Bridlington ; and by C. Reid, at Holnipton. 

 ' Geolog-y of Holderness,' pp. 78 and 85. 



1906 October i. 



