200 ON THE BOULDER-CLAY IN ESSEX.
hills and ridges of Essex at least as far south as Warley Common.
In my survey of the evidence I propose to work from west to east,
beginning with the Epping hills, which form a crescent-shaped ridge,
stretching from within a mile of the River Lea on the west to Toot
Hill, near Ongar, on the east, varying in height from 300 to 379
feet above Ordnance datum. Upon this ridge stand High Beach,
Jacks Hill, Epping, Coopersale Common, the new fort at South
Weald, and Ongar Park Wood. About two miles north is another
ridge upon which Foster Street and Epping Long Green stand, and
that ridge was undoubtedly surmounted by the ice, for it is almost
covered with Boulder-Clay, and I saw a good ditch section at the
very top, 359 feet above O.D., and only twenty feet below the
highest point of the Epping ridge. According to the Geological
Survey map Boulder-Clay is now found on several parts of the
Epping ridge itself, and my explorations give me no reason to doubt
the correctness of the map, though I admit the sections now open
are not very satisfactory ; but I feel satisfied that Boulder-Clay does
attain a height of 340 feet above O.D. at Ongar Park Wood, and of
360 feet near Epping; and I have little doubt that a clayey
deposit which overlies the pebble gravel (Westleton Beds) at
Coopersale Common, and at High Beach, is decomposed Boulder-
Clay1 (see Whitaker's "Geology of London," vol. i., p. 272), and
that a number of large and but slightly rolled flints, which occur at
High Beach in the gravel, were derived from Boulder-Clay or
glacial gravel. The Epping ridge is crescent-shaped, and within the
horns of the crescent, which point southward, there is a very well
marked patch of Boulder-Clay at Theydon Bois. I noticed a satis-
factory section in it by the roadside three furlongs S.E. of the
station. Now, as the ice bearing the Boulder-Clay travelled from
north to south, this mass of material must have been carried over the
top of the Epping ridge before it reached its present position at
Theydon Bois. This gives some idea of the size and power of this
sheet of ice. Having shown, I think, that the ice did surmount the
Epping hills, I pass to Brentwood and Warley. Prof. Prestwich
records a considerable thickness of Boulder-Clay at the top of the
deep railway cutting (Prestwich, "Quart. Journ. Geol. Soc," vol.
xlvi., plate 7, fig. 5), and on the south of the Warley plateau, I have
seen a deposit of clay of a blue-grey colour, full of small flint frag-
1 It was well shown in a new reservoir at High Beach which I visited in company with Mr
T. V. Holmes and Mr. Whitaker.