EAST ANGLIA DURING THE GLACIAL PERIOD. 199 



angle of about half a degree. It touches, as any other formation 

 would be expected to do, the outliers and hills within the larger 

 valley which are sufficiently high to reach its horizon ; and its 

 boundary-line conforms generally to the present features of the 

 district. 



As the surface of the Chalk, when gradually encroached on by 

 the glacial sea, became partly covered by a gravelly wash, so also 

 did the Boulder-clay on its emergence*. Of this gravel or loam but 

 little remains, and that invariably on the higher grounds, in 

 patches which once formed portions of extensive plateaux. I think 

 it can scarcely be assumed that so large a mass of material as the 

 Boulder-clay which once filled the valley could have been removed 

 by denudation, marine or subaerial, or by both combined, without 

 its component particles having been more or less reassorted and re- 

 deposited on the denuded surface. This may have been the origin 

 of certain deposits of doubtful age which occur, here in the form of 

 loam, there as an impure gravel. 



Besides the high-level deposits, there are in the valley broad 

 sheets of much newer valley -gravel, enclosing the remains of recent 

 shells and of extinct mammalia. They occur in three or more 

 terraces at different levels, marking as many points in the progress 

 of the valley's formation, the higher and older terraces having been 

 already reduced to mere patches, indicating a former extension, and 

 testifying to the rapidity of denudation. These gravels have been 

 made up mainly from the waste of the Chalk and superincumbent 

 Boulder-clay, and may be seen in many sections in the neighbour- 

 hood of Cambridge. Much has been already written about them ; 

 they have now been mapped by the Geological- Survey Officers, and 

 will be fully described by them also in their publications. The 

 same remark applies to the Fen-lands, which are of more recent 

 date than the Drifts proper that form the subject of this communi- 

 cation. 



But there are within the area certain other gravels upon which a 

 few remarks are necessary, owing to their resemblance to those of 

 Middle-glacial age. These deposits occur at an elevation of 20 to 

 60 feet, or thereabouts, above the level of the river, and consist of 

 gravels and sands with intercalated masses of loam and clay, the 

 latter having somewhat the appearance of Boulder-clay, or, rather, of 

 a wash from Boulder-clay in its immediate vicinity. The lines of 

 stratification are irregular, sometimes horizontal, more frequently 

 inclined, and, in the two most noteworthy sections, several miles 

 apart, dip north at an angle of 15°. In one of these sections are two 

 gravels, or, rather, a gravel and a loam, the latter being banked up 

 against a scooped-out edge of the former, thus presenting the ap- 

 pearance of a fault. 



There is a Middle-glacial appearance about the gravels ; but still 

 I think that all the phenomena might occur as well either in an 

 esker or in deposits left by a river of tolerable magnitude. In- 

 clined bedding is not unusual in river-gravels ; and the included 

 * Ante, p. 198 and Part 1. p. 196. 



