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Rh between these two districts, though they do not strictly determine the limits on either side.

In the southern district, the London Clay, is mostly marked by low, rounded undulations, broken by tortuous watercourses, which provide the natural escape for surface-water, and a ready outfall for artificial drainage.

The upper levels are very frequently covered with beds of gravel, which retain a certain quantity of water for the supply of shallow wells, which, as on Bushey Heath, attract a considerable population. The water also finds vent in land-springs at the junction of the gravel with the clay, the feeders of the brooks which run into the rivers Coln and Lea. Occasionally these waters pass, in their course towards the valley, the outcrop of the sand which underlies the clay beds of the Plastic clay formation; it then sinks by swallow or “swilly holes” into the subjacent chalk, and goes directly to augment the springs whence the rivers derive their perennial sources. Very large volumes of water so sink into the earth, and the mischief which would arise from the flooding of these brooks in winter is thus much abated. It has been suggested by very high authority that the perennial supply of water to rivers might be materially augmented if artificial means were used to facilitate the absorption of these waters. Very remarkable instances of this natural drainage may be seen more or less along the outcrop of the sand beds of the Plastic clay formation in the parishes of Bushey and Aldenham. In the watercourse which leads from the reservoir at Elstree, it has been found necessary to stop these swallow-holes to prevent waste.

The construction of artificial swallow-holes deserves our consideration as a means both of maintaining a perennial supply of water to our rivers, and also of facilitating drainage operations on a system suggested more than a century ago by Elkington.

The soil of the upper levels of the clay district marked by the rounded flint-pebbles embedded in the sand is wet and unkindly, not capable of bearing grass of any value, and ungrateful under the most liberal treatment as arable land. This gravel, with its characteristic blue-pebble, is transported in many cases below the higher levels, where the sterility of the soil is in proportion to the thickness of the bed. Where the London clay comes to the surface it forms a stubborn soil, which, however, by draining and a liberal treatment is made to grow abundant crops of grass. It also favours the luxuriant growth of oak, elm, and ash timber.