OCCURRENCE OF CHALKY BOULDER CLAY AT CHINGFORD. 3
last, I noticed some half dozen bushes of Clematis vitalba growing
just below the crest of the hill, on the slope facing the Lea Valley,
a new locality to me for the plant; and the idea of a possible
local patch of Boulder Clay again recurred to my mind. No
section was available, but a steep bank beneath some trees
afforded an opportunity to dig a shallow hole, when what seemed
to be Chalky Boulder Clay was soon in evidence; it consisted
of tough brown clay with embedded fragments and pebbles
of white chalk.
A later visit to Yardley Hill, armed with a spade and the
necessary permission from the Forest Superintendent (whose
courtesy I here gratefully acknowledge), enabled me to dig a
trial-hole, 20 yards distant from the bank mentioned, and
2ft. 6in. deep, which proved very stony brown clay (probably
for the most part derived from the local London Clay) containing
rounded chalk fragments: the included stones being chiefly
flint, with many rounded flint-pebbles, a few quartz, quartzite
and sandstone pebbles, and small calcareous concretions and
ironstone concretions. I found also one small pebble of Car-
boniferous Limestone, containing a recognisable Productus.
The surface of the ground is very much broken by slips of
the hill-side, as is usual on clay slopes, and may have been further
disturbed by shallow surface diggings for gravel or marl.
This new patch of Boulder Clay is probably of very small
extent, perhaps not 100 yards each way. Its exact position
is at the very extremity of the ridge of Yardley Hill, just below
the crest of the hill, and on its western or Lea valley slope, at
about 200 feet above O.D., and some 150 feet above the floor
of the Lea valley.
The nearest point in the Lea Valley where Chalky Boulder
Clay has hitherto been mapped is at Monkham Hill, one mile
north of Waltham Abbey, and four miles distant northwards
from Yardley Hill. The new locality is four miles distant from
the patch of Boulder Clay at Enfield, five miles from that at
Bell Common, Epping, and a like distance from the patches at
Theydon Bois in the Roding Valley, and at Chigwell Row.
It is the most southerly patch of Boulder Clay in the Lea Valley,
and probably represents the final effort of the Great Ice Sheet
at or near its southernmost margin.
I may add that an inspection of the neighbouring locality