NOTES ON THE GEOLOGICAL SECTION AT CHELMSFORD. 11
capped by Boulder Clay and intersected by valleys, in which the sand
and gravel of the Glacial Period, which underlies the Boulder Clay,
appears. Here and there in these valleys may also be seen London
Clay, the oldest formation anywhere visible within a radius of more
than a dozen miles round Chelmsford. East of that town, towards
Maldon, there is a considerable tract of Glacial Gravel uncovered by
Boulder Clay, and south of it the highest ground is capped by out-
lying patches of Bagshot Beds, Boulder Clay, or Post-Glacial
Gravel.
Diagram Section across the Valley of the Rives Can below its Junction
with the Wid. (Length about 11/4 miles.)
L.C.—London Clay ; G.G.—Glacia Gravel ; B.C.—Boulder Clay
a. — Alluvium River Deposits
b. e.—Brick-earth. 
But besides the beds just mentioned there are certain strata con-
fined to the various river valleys and consisting of deposits of gravel,
sand, brick-earth, and alluvium, the material composing which has
been brought down by the streams themselves from the higher parts
of their valleys. It is evident that, as regards these valley deposits,
we shall almost invariably find that the higher their position above
the stream the older they are, the highest having been formed when
the valley had been but slightly excavated, the lower when its
present depth had been more nearly attained.
At and west of Chelmsford the Geological Surveyors have mapped
no river deposits but the brick-earth already mentioned and the
marsh alluvium on the banks of the present streams. The Glacial
Gravel appears to occupy the space between the Boulder Clay
capping the plateau and the alluvium at the bottom of the valley,
except where the brick-earth lies between them and rests presumably
on the Glacial Gravel. The highest point at which the brick-earth
is seen is about 100 feet above ordnance datum, and 20 feet above
the alluvium of the marshes.