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NOTES ON THE GEOLOGY OF THE DISTRICT
AROUND CHELMSFORD, WITH A LIST OF
MOLLUSCA FROM THE ALLUVIUM AT
ROXWELL, ESSEX.
By REGINALD W. CHRISTY.
[Read February 9th, 1889.]
The geology of the immediate neighbourhood of Chelmsford is no
doubt devoid of any great general interest, as it exhibits nothing older
than the Eocene Beds which are represented by the London Clay
and the unfossiliferous Bagshot Beds. The former, although covered
in many parts by Drift, forms the real rock of this county, using the
term "rock" in its technical sense. The Drift thins out as it
approaches Chelmsford from the north and west until it practically
disappears south of the town, where, with the exception of a few
outlying masses of Boulder Clay, the London Clay takes its place.
The Bagshot Beds occur as outliers only on the highest ground ;
for instance, at Galleywood, Stock, Fryerning, and part of Writtle.
I have already mentioned the Drift overlying the London Clay ;
there remains, therefore, only the Post Glacial Beds, of which many
interesting instances occur, to complete our scanty list of surface
strata.
I do not propose saying anything about the London Clay, having
had no opportunity of working at it, but will pass at once to the
Drift, which in our district is composed of two beds, viz., the Mid-
Glacial Gravels overlying the London Clay and the Chalky Boulder Clay
above them. The former are gravelly and sandy beds, varying in thick-
ness from 6 ft. to 20 ft. or more, and only appear in the river valleys from
which the Boulder Clay has been washed away. They are extensively
worked throughout the district north and west of Chelmsford, and
about the town, for road-metal; and as they contain many varieties
of rock, such as flint, quartzite, limestone, &c., they are very useful
for this purpose. Occasionally too, veins of clean sharp sand occur
in the gravel, which is used by the builders.
It is in this stratum that most of our drinking water is found, there
being very rarely a good supply in the Boulder Clay; but the gravels
and sands beneath never fail to furnish water in abundance, resting
as they do on the impervious London Clay. At intervals all along
the river valleys springs issue from this stratum and run continuously,
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