the conclusion that, strictly speaking, there are no Lower or Middle Glacial deposits within the area of the Cambridge valley[1].
Supplementary Notes.
Since the above notes and the conclusions therefrom were brought before the Society, the completion of the mapping of some of the gravels, mentioned as of doubtful age, seems to have thrown light upon the date and mode of their formation. It has afforded, also, additional evidence in support of the theory advanced respecting those gravels which were thereby asserted to be relics of an ancient river Cam.
There is a hill called "The Rivey," situated about ten miles S.E. of Cambridge; its height above the Cam is about 325 feet; it is, as it wore, an outlier from the escarpment, and it is similarly covered by Boulder-clay. This Boulder-clay rests directly on the Chalk, and is capped by a few feet of gravel; it is supposed that this is a patch of "Denudation gravel," and that the Boulder-clay beneath it retains its full thickness; or it may be a higher example of those to be described. In a direction running away from the escarpment are patches of gravel at a level several feet lower; then, further on, other patches at a still lower level, and others, still further away, with less elevation, the Boulder-clay being continuous beneath them all. At Hildersham, in the valley, 112 mile from the Rivey and 200 feet below the summit of the hill, the gravel and Boulder-clay occur in the same relative positions; and a little further on, the gravel overlaps the clay and rests on the Chalk.
This interesting and well-marked geological feature has been preserved, it may be, by its position in the bend to the north of the escarpment; and it affords a key to many other patches of gravel in similar positions, and heretofore considered as of doubtful age (p. 200). All the patches (many of which are small), and the indications of several other patches removed by recent denudation, seem to form parts of lines that once were continuous, and which are more or less at right angles to the long series ascribed to an old course of the Cam. They continue in a gradually descending order from almost the top of the 'scarp down to the level of that series—that is, from 325 feet down to 20 feet above the present river.
The explanation proposed is, that the slope of the Preglacial 'scarp was entirely covered by Boulder-clay, that the valley was, in fact, almost filled by that deposit, the patches of Boulder-clay which here and there remain marking out the line of that Preglacial escarpment (see ante, p. 198). This Boulder-clay has been removed
- ↑ Since the above paper was read the author's attention has been called to the fact that in the 'Geological Magazine' for February 1870, Mr. S. V. Wood, Jun., published a statement to the effect that the Middle Glacial beds in East Anglia do not occur at a greater elevation than 250 feet; also, that in a section illustrating a paper by him read before the Society, June 19, 1867, the overlap of those beds by the Boulder-clay, and the occurrence of the latter in the Cambridge valley without any Middle Glacial are represented. Although Mr. Wood did not draw the same inferences from these facts, he is entitled to priority in their observation.—W. H. P.
