CONTAINING FLINT IMPLEMENTS, AND ON THE LOESS. 
299 
— and that it may be brought down to the time when our valleys and plains began to 
receive their tranquil and inequality-levelling deposits of silt and peat, and the modern 
order of things commenced. 
To estimate the time to which we have to carry back the high-level gravels, we have 
to consider what may have been the duration of their accumulation, and that of the 
subsequent excavation of the valleys with the resulting low-level gravels*. A difficulty 
here meets us at the onset. The accumulation of sand, gravel, and shingle along the 
course of rivers is so irregular (sometimes very rapid, at other times slow, what is done 
one year being undone another) that we are entirely without even the few data by which 
we are approximately guided in ordinary sedimentary strata. The thickness of the 
deposits affords no criterion of the time required for their accumulation. They rarely 
exceed 20 feet, and are more frequently not above 10 to 12 feet thick. It is well known 
that recent inundations have covered valleys with sand and gravel to the depth in places 
of 4, 6, or even 10 feet in the course of a few days ; and therefore there are no high-level 
gravels which, so far as thickness is concerned, might not have been deposited in the 
course of a few weeks or even a few days. But the evidence of time lies in their length, 
breadth, and extent, — in the life existences of the period, — and in the physical changes 
in progress, such, for example, as the subsequent valley-excavation, and the wide 
distribution of the resulting debris. There is also another phenomenon connected 
with this period which I would point out as containing some elements for an approxi- 
mate estimate of the duration of time. We have as yet no data to judge of the rate 
of progress of the operation ; but it is one which admits, to a certain extent, of time- 
measurement, and may hereafter, perhaps, be employed with some chance of success. 
At present it will merely serve to give us some idea of the time employed ; but that even 
is a step gained. I allude to those cylindrical perforations in calcareous strata filled by 
the sand and gravel beds overlying or formerly overlying them. These are of various 
dates, but a large proportion of them commenced, 1 believe, with the high-level 
gravel period, or with that of the gravel which immediately preceded it. 
These cylindrical and funnel-shaped holes, or gravel and sand pipes as they are 
termed, vary usually from 5 to 50 feet in depth and 1 to 10 feet in width, though they 
are sometimes much larger. I have seen traces of them in the chalk skirting the S.E. 
side of the hill of St. Acheul. Near Picquigny, on the road from Amiens to Abbeville, 
there is an escarpment of the chalk in which there are the remains of several gravel 
(high-level) pipes from 10 to 20 feet in depth. Others are to be seen near Mareuil, and 
again near Yonval ; the gravel itself in most of these cases has been denuded from the 
surface, and remnants only preserved in these natural funnels. The outlier of high-level 
gravel on the hill above Mautort presents a section of one measuring 15 feet in diameter, 
but the depth is not shown. The most remarkable instance that has come under my 
* I am speaking now of the postpliocene valleys. Where the land, as in Auvergne, was earlier raised above 
the sea, we may have valley gravels going hack to pliocene, or miocene periods, and continued in uninterrupted 
succession to the recent period. 
