256
ON THE OCCURRENCE OF RHAXELLA-CHERT
IN EPPING FOREST GRAVELS.
By PERCY G. THOMPSON.
[Read 29th November 1913.]
SOME years ago I devoted a considerable amount of attention
to the rock-constituents of the gravels of the Epping Forest
district, with a view to obtaining some light on that fascinating-
geological problem—the places of origin of the heterogeneous
pebbles which occur, in greater or lesser numbers, in all the
surface drifts of southern England. I gave special attention
to a gravel-pit in the Forest, lying to the south of Monk Wood,
from which the Loughton Urban District Council dug road-
metal from time to time, and my almost constant watch upon
this pit during the years 1906 to 1908 was rewarded by a very
respectable assemblage of "travelled stones" from diverse beds.
The Monk Wood gravel is a remnant of the highest terrace
of the Roding Valley Drift, at about 280 feet above O.D., laid
down prior to the invasion of the district by the Chalky Boulder
Clay, which latter occurs (within three miles), lying on the slopes
of the river valley at various levels down to about 150 feet
above O.D. The pit shows inconstant layers of current bedded
sand interbedded with the gravel, and a capping of some two
feet of brick-earth containing a few stones occurs wherever the
original surface has not been disturbed by shallow diggings
for sand and ballast—a common practice in the past all over
the higher portions of the Forest.
The constituents of the Gravel in this pit include various
igneous and metamorphic rocks, usually in a very friable and
decayed condition, such as granites, mica-schist, garnetiferous
schist, vesicular purple trap, and a few basalt fragments.
Carboniferous sandstones and chert containing crinoid-casts
are frequent, many Bunter-Sandstone and quartzite pebbles
occur, but there is an almost total absence of Jurassic rocks
or of fossils derived from them. Radiolarian chert and rhyolite
of unknown derivation are found. Cretaceous debris is repre-
sented by nodular, unworn chalk-flints (the largest being over
13 inches long), which cannot have been exposed to much rolling
during their journey from the nearest outcrop of the Chalk to
the north-west. Various kinds of chert occur in this gravel, but.