248 
MR. PRESTWICH ON THE GEOLOGY OF THE DEPOSITS 
Somme valley was of intermediate date ; but I hesitated to adopt this view until facts 
could be obtained for a surer decision. The upper section at Montiers, however, which 
I discovered in 1861, was conclusive as to the relative ages of the two gravels. I 
had further considered that, supposing even this relation to be established, it was possible 
for the excavation of the valley to have been partly the result of some exceptional 
agencies, by which the interval of time between the formation of the beds of St. Acheul 
and those of St. Koch might have been shortened. But after repeated visits to the 
several districts during the last three years, and looking at the question from every 
point of view, I find myself unable to discover a sufficient explanation in the direction 
in which I first sought for one, and have been led to form conclusions respecting the 
causes in operation differing considerably on some points from those I at one time 
thought to be the more probable. 
[A few very brief remarks on the opinions hitherto held respecting the position and 
age of the deposits of this class may here not be out of place. In my former paper 
I showed that the flint-implement-bearing beds were of later date than the Boulder 
Clay, and that at Abbeville the latest of them passed directly under the recent alluvium 
of the valley of the Somme. They thus occupy a definite geological period, which yet 
remained to be studied as a whole. The quaternary deposits in general, of which these 
beds form part, had long been the subject of my special investigation. The various 
drift-gravels had been regarded, — 1, as being of marine origin ; 2, as due to cataclysmic 
action; and 3, as of fluviatile origin. In one place we had marine shells, at others 
freshwater shells. But as the greater number of the gravel-beds were without fossils 
and occurred at very different levels, it was a long-debated question how they should 
be correlated. Palaeontologists too were of opinion that the fossils indicated different 
ages, so that the freshwater deposits in a single valley, like that of the Thames, were 
held to be of independent and not synchronous formation*. On the palaeontological 
evidence, the beds of Grays were generally supposed to be pliocene or preglacial, a view 
maintained by Sir Charles Lyell f until 1857, when he expressed uncertainty as to 
their age. Other beds in the same valley, as those at Brentford, were considered by Sir 
Charles to be newer than those of Grays. On physical grounds I had long been 
satisfied of the contemporaneity of these deposits, and contended for their posteriority to 
the Boulder Clay. Professor Morris and Mr. Trimmer had also arrived at very similar 
conclusions, and were both in advance of me in attributing the phenomena to old river- 
action, but neither they, nor, as far as I am aware, any other geologists had attempted 
to make the rule general ; nor had they taken in the high-level gravels, or the Loess, as 
belonging to the same series and as part of the same phenomena. When, further, I found 
similar land and freshwater shells at Hurley Bottom and other places in the Thames 
valley, the different deposits, showing the same conditions, became readily correlated. 
* Forbes, Mem. Geol. Survey, vol. i. pp. 393, 395; Searles Wood, Trans. Palseont. Soc. for 1848, p. vi. 
and 1856, p. 304 ; Woodward, Manual of the Mollusca, p. 298 ; and others. 
t Manual of Elementary Geology, 5th edit. 1855, pp. 153, 154, and Supplement, 1857, p. 5. 
