eds_ we find that well-known stratum called
the _London Clay_, which is of bluish hue when dug at any considerable
depth. It is found in some of the same districts as the _Woolwich_ and
_Reading Beds_, and from Hertford and Watford it extends to N.E. and
S.W. respectively until it leaves Hertfordshire. Its direction may be
approximately traced by a series of hills, none of which are of any
great height.

_The Drift._--In Hertfordshire, as elsewhere, the strata whose names are
so familiar to geologists do not form the existing _surface_ of the
ground. For the origin of this we go back to a comparatively recent
period, when disintegration was busily working upon the solid rocks, and
glaciers were moving southwards, leaving stones and much loose _debris_
in their wake. Rivers, some of which, as in the Harpenden valley, have
long ceased to run, separated the flints from the chalk, forming a
gravel which is found in quantities at Harpenden, Wheathampstead and St.
Albans, and is, indeed, present in all valley-bottoms, even where no
river now runs. Gravel, together with clays, sand, and alluvial loams,
forms, for the most part, the actual surface of the county.

_The Rivers_ of Hertfordshire are many, if we include several so small
as hardly to deserve the name. They are the Ash, Beane, Bulbourne,
Chess, Colne, Gade, Hiz, Ivel, Lea, Maran, Purwell, Quin, Rhee, Rib,
Stort and Ver.

1. _The Ash_ rises near Little Hadham, and, passing the village of
Widford, joins the Lea at Stanstead.

2. _The Beane_, rising in the parish of Cottered, runs to Walkern, where
it passes close to the church, and flows from thence past Aston and
Watton, and into the Lea at Hertford.

3. _The Bulbourne_ rises in the parish of Tring, passes N.E. of
Berkhampstead and S.W. of Hemel Hempstead and unites with the Gade at
Two Waters.

4. _The Chess_ enters the county from Buckinghamshire at Sarratt Mill,
and flowing past Loudwater joins the Gade at Rickmansworth. The Valley
of the Chess is one of the prettiest districts in the shire.

[Illustration: ON THE RIVER COLNE]

5. _The Colne_ rises near Sleap's Hyde, is crossed by the main road from
Barnet to St. Albans at London Colney, and by the main road from Edgware
to St. Albans at Colney Street. Thence it passes between Bushey Hall and
Bushey Lodge, flows through Watford to Rickmansworth where, uniting with
the Gade and Chess, it enters Middlesex near Stocker's Farm.

6. _The Gade_ rises near Litt