676 REY. 0. FISHER ON A MAMMALIFEROUS 
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 interme- 
diate 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. 2037. 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 heen 
* Loc. cit. + Published at Washington, 1875. 
