266 
MR. PRESTWICH ON THE GEOLOGY OF THE DEPOSITS 
has a depth of 120 feet ; and taking as a mean a width of 8000 feet, we have a sectional 
area of 960,000 feet, or as 1 : 400. The greatest flood of the Seine on record is that of 
the year 1658, when it rose to a height of 29 feet. Even in this case a flood of nearly 
60 times that magnitude would be required merely to fill the valley to the level of the 
high-level gravels, without taking into consideration the more rapid discharge. But 
neither in this nor in the other cases of modern times, are we aware of an increase 
in the volume of water, during floods in these regions, to many times the ordinary 
mean average, whereas we see that in a case such as is presented at Amiens a flood 
having a volume five hundred times that mean would be required to reach the beds of 
St. Acheul. 
This I conceive is sufficient to prove that the high-level valley gravels cannot be 
ascribed to floods of the present rivers, as has been, even of late, suggested. The only 
means adequate to fill, under existing conditions, the river- valleys of the Waveney, the 
Ouse, the Somme, and still more of the Seine, would be the ingress of the sea ; but such a 
supposition is at once refuted by the fact of the prevalence of land and freshwater 
shells in both the high- and low-level gravels, and the absence of marine remains unless 
immediately adjacent to the present coast. If therefore neither the supposition of river- 
floods, nor of a different relative level of land and sea allowing the latter to penetrate 
up the valleys, be admissible, how far are the facts in accordance with the hypothesis 
of these deposits being the alluvia of old rivers, and the valleys their excavated channels \ 
Let us in the first instance trace the direction whence the materials have come. 
The valley of the Waveney traverses a district formed of Boulder Clay, with under- 
lying sands and shingle, reposing on Chalk, which latter comes to the surface in 
the upper part of the river’s course, and is just visible at Scole. There is little in 
the valley-gravels to indicate a distant origin, as most of the debris composing them 
might generally have been derived from the surrounding hills. The only material in 
excess is the mass of subangular flint-fragments, derived probably in part from the 
more distant chalk area. Although not quite conclusive, still the evidence affords fair 
presumptive proof of the transport of the gravel from the watershed of the Waveney to 
the sea along the breadth of surface indicated by the valley-gravels (see Map, Plate IV.). 
A nearly similar uniformity exists in the case of the valley of the Ouse, which traverses 
a district of Boulder Clay overlying various members of the oolitic series. It is not, 
however, easy to determine in what proportion and to what distances the different 
materials composing the gravel of the valley of the Ouse have been transported from 
their parent rocks. They exhibit, as in the case of the Waveney, a local origin in con- 
nexion with the existing valley. All the materials found in the gravels can be referred 
to rocks or to older drift deposits traversed by these valleys and their tributaries, and 
in no instance do we find the introduction directly of any foreign element. Thus, 
besides the flint debris derived from the chalk in the valley of the Waveney, and the 
oolitic limestone and sandstone debris derived from the Oolitic series in the valley of 
the Ouse, all the superadded pebbles and boulders of the older rocks, as well as a 
