Resting upon the Reading Beds 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 Little Gaddesden, skirts Hemel Hempstead Church on the W. side, and passing King’s Langley and Hunton Bridge, flows through Cassiobury Park and joins the Chess and Colne at Rickmansworth.

7. The Hiz, rising at Well Head, S.W. of Hitchin, crosses that town, joins the Purwell at Grove Mill and leaves the county at Cadwell.