14
mapped out broadly by the Geological Survey,* but there is
yet much work to be done in the way of filling in details,
especially with regard to the Drift and other superficial
deposits. Geologically considered, our district is compara-
tively modern, the oldest formation being the chalk which
crops out on the Essex shore of the Thames about Purfleet,
and extends to just beyond Little Thurrock, a distance of
some five miles in an easterly direction. Overlying this strip
of chalk at its eastern extremity there is a detached patch of
Thanet sand. A line drawn across from Grays Thurrock to
Stifford, the northern limit of the chalk at about its widest
part, would be nearly one mile and three-quarters in length. At
Bishop Stortford the chalk again appears. The thickness of this
formation in the London Basin is from over 600 to more than
1,000 feet; a boring carried down into the Gault at Loughton
Station gave a thickness of about 690 feet, and at Harwich a
boring to a depth of 1,042 feet carried down into strata below
the Gault showed the chalk to be 888 feet thick. Cretaceous
fossils have been obtained in some abundance from the chalk
pits at Grays and Purfleet. By far the larger portion of our
county stands on the tertiary formations above the chalk. Of
the Lower Eocene series the Thanet Sands are present in a
broken band of about one mile in width at its widest part,
and of an average thickness of about thirty feet, which crops
out to the north of Purfleet, and following the chalk extends
eastward along the valley of the Thames. The chalk pits at
Purfleet and Grays show well the junction of the two forma-
tions. Next in order above the Thanet beds we have the
Woolwich and Reading beds following the former, as a narrow
strip commencing about Wennington and extending eastwards
to Stifford, where the strip commences to broaden out, and
another patch of the same beds is found about Stratford and
West Ham, to the east of the alluvium of the Lea valley.
The Woolwich and Reading beds have an average thickness of
about fifty feet. The uppermost member of the Lower Eocene
* In making the following rough sketch of the geology of the county I
have largely availed myself of the admirable publications of Mr. W.
Whitaker, of H.M. Geological Survey, as well as of the maps published
by the Survey.