ORIGIN OF THE LEA VALLEY. 149
Ordnance Datum in a rounded, gently undulating chalk surface,
which has lain open to sub-aerial denudation for a very long
time.
In the Chalk of Dunstable Downs many springs arise on
the Totternhoe stone—a compact grey limestone. A number
of springs which once rose at the junction of the Lower and
Middle Chalk at Houghton Regis, Leagrave, Limbury, etc.,
have now ceased to flow and have left behind them the dry
valleys or coombes typical of the Bedford—Hertfordshire
border.
In cuttings near Luton, several small faults in the Chalk
have been recorded. Such minor fractures serve to make
valley deepening and lengthening proceed more rapidly and
so facilitate the extensive removal of the soft Upper Chalk
and the Reading Beds.
The drainage system so far described probably originated
towards the close of Pliocene times. There is little reason
to suppose that any great change has occurred in the Lower
Thames Valley since those times, other than a general deepening
and widening on the north as its streams gradually encroached
upon the northerly river-systems represented to-day by the
Ouse and the Cam.
Two highly important factors have been at work in the
Lea Valley since its initiation, viz., the formation of Tertiary
folds and the extension of the great Pleistocene Ice Sheet. As
regards the former factor, there are several well marked lines
of disturbance traceable either on the surface or in borings.
These can be summarised as follows :
I. A depression marked by a line of inliers in the Eocene,
giving a ridge of hills—Watton, Welwyn, St. Albans, Hemel
Hempstead.
II. A similar line extending from Kemsworth to Berk-
hampstead Common,
III. A line of London Clay inliers—Gough's Oak, Northaw,
Pinner and on to Windsor Castle Hill, marking an upfold which
probably continues north-eastward along the Cobbin's Brook
valley.
IV. A downfold which extends across the Lea from S.W. to
N.E., and includes the Epping Forest Ridge.
V. An unsymmetrical fold (a monocline) which lies north