486 PLEISTOCENE. 



been formed by Coast-ice, as well as other Boulder Clays, in North Wales, 

 Lancashire, etc. 



The subject of milder Interglacial periods is not yet one on which definite views 

 can be expressed ; the evidence of the shell-fauna of the Middle Glacial sands of 

 East Anglia is open to question (see sequel) ; and Sir J. W. Dawson has shown, 

 on the evidence found in Canada, that the occurrence of marine shells, land plants, 

 and insects in the Glacial deposits of that country, indicates not so much the effect 

 of general Interglacial periods, as the local existence of conditions (favourable 

 to animal- and plant-life) like those of Grinnel-land and Greenland, in proximity 

 to each other at one and the same period, and depending on the relative levels of 

 land and the distribution of ocean currents and ice-drift. ^ 



The Glacial deposits scattered over the country have been 

 examined in detail in many areas,.- and the general sequence of 

 events has been marked out. At thfe same time the deposits them- 

 selves are so changeable, that the attempts made to correlate 

 divisions in different parts of the country are very far from 

 satisfactory, and it will be better to describe the principal local 

 deposits, without attempting to indicate to which particular portions 

 of the Glacial period each subdivision may belong ; and without 

 trying to explain its precise method of formation. It is con- 

 sidered that at the climax of cold (during the accumulation of the 

 Chalky Boulder Clay and its equivalents), the northern parts of 

 England and Wales were smothered up in ice, which filled also the 

 shallow seas adjoining, while the south of England was never 

 subjected to such glaciation, and the scattered Drifts and 

 occasional boulders there met with are due to marine currents and 

 occasional ice-bergs. The great accumulations of ' Head ' and 

 other subaerial detritus in the southern counties are no doubt 

 memorials of the ' Great Ice Age.' But it is difficult to frame a 

 scheme that will fairly exhibit all the changes that have taken 

 place even in Glacial times. In certain areas in the north we find 

 evidence of three or even more beds of Boulder Clay, in other 

 areas we find but one or two beds. It is probable that in many 

 cases earlier Boulder Clays were re-constructed in later times, and 

 the sequence is obliterated ; thus creating great difficulties in the 

 correlation of the beds. Where, as is too often the case with 

 Glacial deposits, there is room for much diversity of opinion, 

 geologists fully avail themselves of it. Hence it is best to picture 

 the Glacial period in a general way, and to admit that Glaciers and 

 Ice-sheets, Ice-bergs and Coast-ice, have all had their share in the 

 production of the phenomena, although we cannot always localize 

 their action. 



NORTHERN AND WESTERN COUNTIES OF ENGLAND AND WALES. 



Glacial Drift occurs over a great part of Northumberland ; and in that county 

 and in Durham the Boulder Clay largely conceals the Coal-measures. There are 

 also great accumulations of gravel and sand, with occasional bands of unctuous 

 clay that till up a number of old Pre-glacial valleys. Thus, according to Sir A. C. 

 Ramsay, the miners, while mining a bed of coal, sometimes find the seam crop 



1 Address to Brit. Assoc. iSS6 ; Major H. W. Feilden, Address to Norfolk 

 Nat. Soc. 1886. 



