270 
MR. PRESTWICH ON THE GEOLOGY OE THE DEPOSITS 
gives rise to the heaping of the gravel, nor are the depressions in its surface marked by 
the presence of any such interfering masses ; yet the disturbance has been from above, 
and sometimes before the deposition of the brick-earth ; for the upper part of the gravel 
(c) and the overlying sand-bed (d) are often affected independently, without there being 
any corresponding disturbances in the lower gravel or overlying brick-earth. The irre- 
gularities in the bedding of the gravel do not depend upon the presence of blocks, which 
occur in all levels through it ; and the lamination of the marly sand (d) is continued 
through the curved planes of contortion in a manner and at angles which such a sedi- 
mentary bed could never have assumed in process of deposition by the mere action of 
water. The tertiary blocks come from a distance of twenty to forty miles above Amiens. 
Low-level Valley-Gravels . — It may not be possible to draw an exact line of demarca- 
tion between those gravels which I have designated as the high-level valley-gravels and 
these low-level gravels. They are the extremes of a series, marking a long period of 
time and probably formed under analogous but not identical conditions. The higher- 
level terraces are generally, however, so distinct, so broad, and so clearly separated from 
the low-level gravels, by bare slopes of the underlying rock formations, that, although 
they may differ amongst themselves to the extent of several feet, the space between the 
two groups is usually sufficiently distinct to make it not difficult to refer each bed to its 
right relative position. In places there are passage beds following the more gentle 
slopes, and at other places there are intermediate terraces, though commonly of little 
importance and trifling width. The broad distinction consists in the one being on hills 
of various heights flanking the valley, while the other occupies the immediate river- 
valley, always following its main channel and constantly rising on its sides to the height 
of several feet, and, where the valley is broad, forming low terrace platforms on its sides. 
Unlike the high-level gravels, of which the interrupted and local occurrence, compa- 
rative isolation, and unequal levels render the course and connexion indistinct, these 
low-level gravels, from their general continuity and their slight difference in level, leave 
us in no doubt as to their relation to the existing valleys. 
The main points of difference to note are the greater thickness of the low-level gravels, 
their more uniform bedding, the more common presence of beds of sand and fine gravel 
with oblique lamination, and the absence generally of contorted strata. When we speak 
of bedding or stratification in these deposits, it is not continuous and persistent seams 
that we refer to, but to the greater or lesser extension of lenticular masses of sands and 
gravel of various thickness, which gives to a small section the appearance of bedding ; but 
none of the beds are persistent. Large blocks are often common in these lower gravels. 
They are generally more worn than in the upper gravels, and the question arises to what 
extent they may be derived from the former. A great number occur in the lower 
gravels of the valley of the Somme, though they are not so numerous as in the upper 
gravels. In the valley of the Seine, on the contrary, they are extremely numerous in 
the lower gravels at the Gare d’lvry, at Grenelle, and in various pits between Paris and 
Rouen. In the valley of the Thames I know of but very few. On the whole these 
