GEOLOGY with London Clay also ; then, at a little greater distance from the main mass, there is an outlier between Bennington and Watton, followed by a larger outlier on which Datchworth is situated, and a smaller at Ayot, these two being in the general direction, and the three having London Clay over the Reading Beds. All these are in the river-basin of the Lea, and their united area is about twelve square miles. In the Colne river-basin there is first an outlier at St. Peter's, St. Albans; there are three small outliers near together at Leverstock Green, Bedmond, and Abbot's Langley, the first of these being beyond the general line ; three small outliers near Sarratt follow ; and there is a small one near Chorley Wood, Rickmansworth. Most of the outliers in the Colne river-basin are of the Reading Beds only, and their united area is about three square miles. For some distance this string of outliers roughly coincides with a tolerably well-marked ridge of hills stretching from Watton south-west- ward by Welwyn, Sandridge, and St. Albans, where it exceeds 400 feet in height, to Hemel Hempstead. This ridge probably indicates a line of flexure in the Chalk, which, while dipping elsewhere in a regular manner from the Chiltern Hills towards London, is slightly depressed along this line. The Eocene beds upon it may have thus been let down below the plane of denudation, allowing patches of them to be preserved. Their clays being better able to resist subsequent sub-aerial denudation than the surrounding chalk, which also is constantly being chemically dissolved, by the gradual wearing down of the surface of the Chalk they would in course of time be left as hills. These outliers completely change the character of the soil overlying the Chalk district. Some appear as well-wooded eminences on which the oak and elm flourish best ; others, chiefly where the sands of the Reading Beds are more developed than their clays, or where the London Clay upon them is capped by pebble-gravel, are sandy, gorse-covered com- mons. Nearly all are worked for brick-making. Far away to the north-west there are three very small outliers of the Reading Beds, of three or four acres each, following each other in a line from near Kensworth to Berkhamsted Common, the last of these only being in Hertfordshire. The presence of these outliers is important as showing the former great extent of the Eocene Beds, of which they furnish more conclusive evidence than do the boulders of Hertfordshire conglomerate which are found at even a greater distance from their parent bed. To another line of flexure an uprise of the Chalk the existence of a series of inliers in the London-Clay area is probably due. Inliers are patches of lower beds exposed by the removal of the higher strata which once covered them. In Hertfordshire the Reading Beds are thus exposed beneath the London Clay in two inliers between Cough's Oak and Northaw, and if this presumed line of flexure be continued parallel with the outcrop of the Reading Beds into Middlesex, an inlier will be met with extending from Pinner, past Ruislip, to just beyond Ickenham. 15
