NOTES ON THE GEOLOGICAL SECTION AT CHELMSFORD. 13
at Moulsham the Chelmer, the largest of the streams which col-
lectively form the river known by that name below Chelmsford,
joins the Can, its course above the junction being nearly north to
south, or at right angles to that of the Can. When these streams
were in flood the natural result of this disposition of the river-
valleys would be that the flow of water from the Can and its
tributaries would be checked by that of the Chelmer running from
north to south. This would cause an unusual amount of flooding
and deposition in the valley of the Can above the junction with
the Chelmer, or, in other words, just where the brick-earth now
exists.
Then, we have seen that the sandy clay forming the lowest bed
of the brick-earth contained small pebbles of Chalk, both in the pit
we visited and in that seen by the late H. W. Bristow. Mr.
Woodward does not appear to have seen any, and his opinion as to the
age of the brick-earth is consequently much more guarded than it
would otherwise have been. Now, the various streams mentioned all
rise within the Tertiary area and traverse country the surface of which
consists chiefly of Chalky Boulder Clay. Therefore, it can hardly be
doubted that these pebbles of Chalk have not been derived directly
from that formation, but are the result of the erosion of the Chalky
Boulder Clay, through which the valleys of the various streams have
been cut. The occurrence of these Chalk pebbles in the brick-earth
consequently implies the prior existence of the Chalky Boulder
Clay. Then, while the Boulder Clay has been so deposited that we
find it to consist of stones of the most varied age and composition,
shape and specific gravity, dispersed irregularly through a more or
less clayey matrix, the only pebbles in the sandy clay at the base of
the brick-earth are small and of one specially light material, Chalk,
and imply selection through the sorting action of running water.
It may be worth while, in view of the importance attached by
some geologists to the determination of the horizon of the Mammoth,
to say a few words on the curious alternations of very regularly-
bedded strata with thin, irregular beds of other material, as a cha-
racteristic, not merely of these Chelmsford deposits, but of river-
deposits in general. In this case, as in many others, we find that
the fine, compact, sandy clay towards the base of the section bears
every sign of tranquil deposition, while the overlying gravel occurs
in thinner and much more irregular beds. It would seem that at
Chelmsford, when the brick-earth was deposited, the rivers there