RHAXELLA-CHERT IN EPPING FOREST GRAVELS. 259
several pieces of the same rock, containing partial casts of a
Trigonia (perhaps referable to T. clavellata), of Pecten fibrosus,
and of (?) Ostraea, associated with basalt, Millstone Grit, Bunter
quartzites and sandstones, and crinoidal chert fragments, this
exposure, at just under 250 feet above O.D., being in a somewhat
lower terrace of the Roding Valley Gravel than is the Monk
Wood pit. I have also found fragments of Rhaxella-chert in
low-level Roding Valley Gravel at Bonner's Pit, near Abridge,
at about 80 feet above O.D., and near Hill's Farm, in Theydon
Garnon parish, at under 100 feet above O.D. ; and I have a
specimen taken in 1906 from mid-Glacial Gravel at Netteswell,
at about 230 feet above O.D.
It is therefore clear that this distinctive and easily recognisable
Chert, which occurs as pebbles in Valley Drifts at Dartford Heath,
in Epping Forest, at Loughton, at Coopersale, at Abridge, and
at Theydon Garnon, as well as in (presumably) Glacial Drift
at Cromer, and certainly at Netteswell, has been derived by some
agency from rocks of Corallian age which are in situ to the west
or north-west of the places mentioned, their nearest occurrence
to the Epping Forest district being over 50 miles distant in a
straight line. Whether the pebbles actually came from the
Arngrove district in Bucks, or from some other, more northerly,
point of the long and almost continuous strip of Corallian rocks
which outcrop across England from Weymouth to Scarborough,
but where as yet Rhaxella-chert has not been recognised in situ,
must remain for future determination.
One of two hypotheses is tenable. Either the Rhaxella-
chert, and the associated Triassic and Carboniferous debris,
together with the igneous and metamorphic rock fragments,
reached the Roding gravels, indirectly, by ice-agency in the
earlier stages of the Glacial Period, prior to the period of maxi-
mum glaciation of this country as represented by the Chalky
Boulder Clay, or, alternatively, the Roding must once have had
its head waters farther to the north-west, beyond the present
Chalk escarpment, so as to derive its materials from the Jurassic
plain beyond. The latter hypothesis is, however, exceedingly
unlikely, in view of the fact that the parent rock at Arngrove
lies at under 350 feet above O.D., whilst at Coopersale, more
than 50 miles distant, Rhaxella-chert pebbles have been found at
340 feet, and in Monk Wood at 280 feet above O.D., a gradient