WOOD rN-TKAGLlCIAL EROSIOIT. 447 



bourhood. This bed is that distinguished in the section by the 

 letter a, and it consists of dark-blue clay full of chalk-debris, ex- 

 actly 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 postgla- 

 cial gravel (7). 



Assuming the bed a really to underhe No. 5, as represented in the 

 section, the case is not without parallel elsewhere, though there are but 

 two other locahties 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 t, 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 dis- 

 closed 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, repre- 

 sented by bed 4, and the commencement of the Middle, repre- 

 sented by bed 5, concurs with what we find in this respect in many 

 places in Norfolk and North Suffolk, since there is generally an un- 

 conformity between the bed 4, or its marl representative, and beds 



* The first notice of it at this place was in a well-section given by the Rev. 

 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. 



t Quart. Journ. Greol. 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. Y. W., Jun. 



