Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/271

Rh 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 neighbourhood 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 communication.

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 appearance 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. Inclined bedding is not unusual in river-gravels; and the included