Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/573

 bourhood. This bed is that distinguished in the section by the letter a, and it consists of dark-blue clay full of chalk-debris, exactly resembling the Boulder-clay (6) in parts distant from Norwich, but different from the appearance presented by that clay in the neighbourhood of the section, where it is much more chalky. There is, we think, no question as to the identity of the sand which reposes on this bed, in the hole, with that numbered 5, in the group of beds out of which the valley is excavated, since it possesses the peculiar admixture of fine chalk grains possessed by this sand in the various sections of the immediate neighbourhood, as well as generally in the south-east of Norfolk, and offers a complete contrast to the postglacial gravel (7).

Assuming the bed a really to underlie No. 5, as represented in the section, the case is not without parallel elsewhere, though there are but two other localities of its similar occurrence at present known with certainty to us. One of these is Witham, in Essex*, and the other the vicinity of Hertford, mentioned by Mr. Hughes†, in both of which it seems to occur at the base of 5, in a trough which had been formed prior to the accumulation of that deposit, but which in those cases does not appear to be coincident with the existing valleys.

We forbear to discuss here the causes which seem to us to have given rise to this bed, or which have produced the intraglacial erosion of the hole, or trough, in which it has thus been preserved, as these will be more conveniently considered in connexion with the general glacial structure of the east of England, further than to observe that we trace in it the action of a glacier‡. The features disclosed by the section appear to show that after the bed 4 had been deposited, an erosion took place just here which swept out the beds 2, 3, and 4, and excavated the deep hole or trough (whichever it be) in the chalk in which we now find the bed a. Whether this bed was deposited only in the hole or trough thus formed, or whether it spread generally over 4 in these counties, and then was denuded prior to the deposit of the sands 5, we have not yet formed a decided opinion ; but the physical break which the case before us seems to show, occurred here between the termination of the uppermost deposit of the Lower Glacial period, represented by bed 4, and the commencement of the Middle, represented by bed 5, concurs with what we find in this respect in many places in Norfolk and North Suffolk, since there is generally an unconformity between the bed 4, or its marl representative, and beds

O. Fisher, in Geol. Mag. vol. v. p. 98. We have since found it exposed in section about a mile from Witham, in the bottom of the valley.
 * The first notice of it at this place was in a well-section given by the Rev.

† Quart. Journ. Geol. Soc. vol. xxiv. p. 286.

‡ In the discussion on this paper I admitted that the erosion in question might have been produced by a grounding berg. I withdraw that admission altogether, believing, from a study of the respective formations over East Anglia, that after the beds forming the Lower Glacial series had been converted into land, this land became extensively occupied by ice before it was again depressed to receive the sea, which deposited the Middle Glacial sands ; and that to such land-ice the erosion in question is due. — S. V. W., Jun.