Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/812

676 decided flint gravels, which are so conspicuous in the other tributary valleys, may be probably accounted for, as I have said, by the paucity of materials out of which such gravels could be formed; so that I doubt whether, from that circumstance, any conclusion can be drawn that this valley is more modern than the others.

The measurements referred to in the table of levels show three different levels, of which records still remain, at which the Rhee has run. Starting from the present stream, we find the next older level to have been lower than the present one; it is marked by the lowest-level fine river-gravel in a pit sunk through the present alluvium. The deposit next older than that is the mammalian gravel of Barrington. There may have been deposits of intermediate age, of which no records are left, unless the flint gravel of Foxton be such an intermediate gravel.

If we go lower down the river to Cambridge we find three gravel-levels; the highest and oldest the Barnwell gravel, the next lower the Chesterton gravel, the next the gravel beneath Jesus College and Midsummer Common, which I recollect being formerly largely extracted. These are all noted in Mr. Griffith's section.

Now I think there can be no doubt that a considerable spread of gravel marks a stationary level, or pause in the eroding action of a river in deepening its channel. Gravel is deposited when the river is occupied in meandering from side to side of the valley; and whenever an elbow of the stream reaches the side of the valley, it undermines it and widens the valley at that point. But when the valley is in process of being deepened, the river confines itself more closely to its course, and any gravels which it may then deposit are rearranged during the next stationary period. To what, then, are these stationary periods due, if such there be? It seems that they must be due to alterations in the relative level of sea and land. This is clearly put by Prof. Powell in his Report on the Exploration of the Colorado River, p. 203. He says, "we may consider the level of the sea to be a grand base-level, below which dry lands cannot be eroded; but we may also have for local and temporary purposes other base levels of erosion, which are the levels of the beds of the principal streams which carry away the products of erosion."

I would inquire, then, can we find any records of marine base-levels corresponding to these river-terraces? It is obvious from the silting up of our estuaries, such as the Fens, and the clean gravels to be found below the bottoms of our present streams, that the present sea-level is not so low as it was at a not very distant period. Now we have an indication of such a former lower level of the sea in the submarine forests. Thus we may account for the low gravels beneath our present alluvium.

The high-level gravels, on the other hand, simply intimate that the fall of the stream at the place where they were deposited was not, at the time, too great, and not so great as it became afterwards, when they were cut through. The former condition may have been