154 8. V. Wood, Jun. — The Post- Glacial Period. 



drifted from the same mountainous districts during the late part of 

 the Glacial period itself, when, the continental ice to which the 

 Glacial clays are due having passed away, these districts had become 

 an archipelago in which the boulder-sands, boulder-beds, and boulder- 

 earth, which constitute the Glacial drift of these mountain districts, 

 were, according to my view, accumulated ; but we do not find evi- 

 dences of such a distribution of surface-blocks in East Anglia. It 

 is, however, quite conceivable that these last results of the ice- action 

 of the Glacial period were altogether removed by the considerable 

 denudation to which the sea-bottom must have been exposed during 

 its emergence. As a rule, all large areas from which the Glacial 

 beds have been removed by Post-glacial denudation are thus desti- 

 tute of boulders. Such, for instance, is the case with the Great 

 Valley of the Thames, the eastern part of which, having its northern 

 heights crowned by Glacial clay, is proved to have been once 

 covered by the Glacial sea ; ^ ^ but in which, as also in the wide sheet 

 of Post-glacial gravel that occupies the lower part of the valley, no 

 boulders, save those in the Grays brickearth alluded to in the sequel, 

 occur. It can hardly be denied that into such valley, so far as it had 

 then come into existence, some of the winter-formed ice issuing in 

 summer from the half-emerged valleys and glens of the mountain 

 districts, and drifting southwards, would be carried by the tide, and 

 distribute blocks, if at that time the climate of these districts was 

 such as to generate marine ice. 



It is very important, in this question, that the distinction between 

 Glacial-clay and Boulder-clay should be kept in view. Glacial-clay 

 may be Boulder-clay, or be nearly destitute of boulders, according 

 to the nature of the country over which the Glacial-ice travelled 

 before shedding at its seaward termination the moraine profonde, to 

 be distributed over the contiguous sea-bottom as Glacial-clay. If 

 that country be a rocky one like the North of England, or like 

 Scotland, boulders abound in the clay; but if an extensive area of 

 soft strata intervene between the sources of the ice- stream and its 

 seaward termination, such as was the case in East Anglia, the bulk 

 of the resulting Glacial-clay consists of the degraded material of these 

 softer strata, and the boulders in it form but a subordinate feature. 

 Where this kind of clay lies against the Chalk Wold of Lincolnshire, 

 it is nothing but reconstructed chalk, so pure as to be burnt for 

 lime ; and generally all over the counties south of the Humber and 

 east of the Trent, the Glacial- clay is principally formed of rolled 

 chalk, with boulders only sparsely scattered through it. On the 

 other hand, Boulder-clay produced by the dropping of boulders over 

 a sea-bottom from coast-ice, without the presence of any sea-termi- 

 nated glacier, with its submarine terminal moraine, is a different 

 thing, presenting none of that glacially degraded material which 

 constitutes the mass of Glacial-clay. In the East of England we 



1 Until the objections submitted by me at p. 92 of Vol. VIII. of this Magazine 

 to any other than a submarine origin for this clay are removed, I assume the existence 

 of such clay on the heights above the Thames Valley as proof of the Glacial sea 

 having covered them. 



