186
THE ESSEX FIELD CLUB.
Afterwards, the party commenced a pleasant walk, under the guidance of Mr.
Geo. Morris, through low meadow-land, where several interesting plants were
observed ; notably, Equisetum maximum, which was seen growing in the wet
shady groves, in dense masses over four feet high, giving one a good impression of a
forest of the Carboniferous period. In the long grass near by, the elegant little
Adder's-Tongue Fern, Ophioglossum vulgatum, was abundant and the Wild
Guelder Rose, Viburnum opulus, the ancestor of the familiar garden shrub, was
seen flowering in the thicket. Other interesting plants noticed were the
Twayblade (Listera ovata) and the Mares-tail (Hippuris vulgaris).
Passing under the viaduct of the G.E.R. main line, the party noticed a pit
worked in the recent or post-glacial valley-gravels. These have yielded, within
BLOCK OF "CALCRETE" FOUND NEAR AUDLEY END.
recent years, Bronze Age, Late-Celtic, and Romano-British burial urns, which
may point to a continued occupation of the valley by a population partly
supported by fishing and wild-fowling at a time when these upper reaches of the
Cam were a series of wide swampy lagoons.
The main road was then followed for some distance, towards Saffron Walden.
Near the Convent Plantation, a recent exposure of a large mass of indurated
or solidified gravels (see illustration) was inspected. This consolidation is
brought about by the small nodules of chalk included in the gravel becoming
partly dissolved under the action of percolating water charged with carbonic
acid. The lime thus obtained, being re-deposited around the other stones,
binds the whole into a natural form of concrete, which has received