THE FIELD NATURALIST'S KNOWLEDGE.
69
An outlier or an inlier of harder rock in a clay-formed valley will
give an isolated hill or elevation, as Church Down Hill in the valley
of the Severn, and Windsor Castle Hill in the valley of the Thames.
An inlier of Silurian rock in a Coal Measure country will give a
wooded hill, while all around is a sombre expanse of blighted land
and smoky chimneys, as is conspicuously seen at the Castle Hill and
famous Wren's Nest of Dudley, in the South Staffordshire coal-fields.
A pervious bed overlying an impervious bed will produce springs
giving rise to valley-cutting streams as at Hampstead Hill. A
pervious bed overlying an impervious bed while both have a dip
towards a hill-side or sea-coast cliff may produce a landslip which
may quite alter the local features. So was produced the well-known
under cliff of the Isle of Wight.
THE RELATION OF THE NATURAL HISTORY AND
PHYSIOGRAPHY OF ESSEX TO ITS GEOLOGY.
Very many more illustrations than I have now been able to
mention will suggest themselves to those who have been attentive
observers of Nature in the field, but one great fact must be im-
pressed on the mind by their consideration. This is that either
directly or indirectly much of the Natural History of a county is
due to the geological structure of the district.
In this county the London Clay extends from west to east, from
north to south, but it by no means forms all the surface, for Eocene
Sands, Pliocene Crag, Post-Pliocene Gravels and Brick Earth and
Quaternary broad deposits of Alluvium constitute a very large pro-
portion of the surface area, and the Chalk forms a small portion on
the southern side. But as no hard rocks occur there are no lofty
hills or bold headlands on the coast, but instead thereof are gentle
elevations with wide valleys, a low coast-line and flat estuary marsh
land. A fault, too, extending east and west in the Thames Valley,
throws down the Chalk on the north, and so gives to Essex the
river marshes that skirt the north side of the Thames.
On this geological structure with the meteorological conditions
of the county the Flora and the Fauna depend. The great diversity
of surface formations gives a varied soil, and this produces a
diversified Flora. Within a radius of three miles from where we
now are [Wanstead], there are River Valley Alluvium, Brick Earth,
High Level Gravels, London Clay, and hill cappings of sand that
have been considered to be Bagshot Sands. This varied structure