256 
ON THE OCCURRENCE OF RHAXELLA-CHERT 
IN EPPING FOREST GRAVELS. 
By PERCY G. THOMPSON. 
[Read 29 th November 1913.] 
S OME 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 
