Half way down the sides of the hills, in the district last named, the Chalk is more or less completely exposed at the surface along a narrow zone, below which we come upon deposits of gravel, sand, and clay filling the bottoms of the valleys. At Bowling Alley, Harpenden, these deposits are fully forty feet in thickness. Although they have been supposed to be the result of river action, it is more probable that they are due to rain-wash. Indeed this is practically proved in the case of the valley leading from Harpenden towards No-Man’s-Land, where the lower end is blocked by a ridge of gravel, which could not possibly have been formed by river action. The stones in these valley-gravels are of irregular shape, and thus quite different from the rounded pebbles of the gravels of the Woolwich and Reading beds, as seen at St Peter’s, St Albans. At Harpenden the uppermost layer of valley-gravel is extremely clean and sharp, generally of a golden yellow colour with blackish veins. Deposits of brick-earth occur locally throughout the Chalk area. Over the greater part of the county the soil is the result of the decomposition of the foregoing superficial formations; and is consequently in most cases of a stiffer and more clayey character on the hill-tops than in the valleys, where it frequently forms only a bed of a foot, or even less, in thickness above the sharp, running gravel. Everywhere in the Chalk districts the soil contains a vast number of flints; but it is, nevertheless, admirably adapted for corn-growing, and especially for malting-grain; Hertford being one of the four English counties best suited to crops of the latter nature. On many of the unenclosed commons the soil is, however, of a poor and hungry nature, producing various kinds of inferior grass, together with spring-flowering gorse, as on Harpenden Common, or heather, as at Kingsbourn Green, between Harpenden and Luton, and at Gustard Wood, near Wheathampstead. On the higher Chalk Downs near Dunstable and Royston there is little or no soil properly so-called; the short, but sweet and nourishing grass growing on the chalk itself. A very different type of soil obtains in the London Clay area in the south and east of the county; this being heavy and clayey, and thus better suited for grass than for corn; in fact in the old days the Middlesex portion of this district was known to the country people as the “Hay-country.” Along many of the river-valleys peaty soils of a marshy and swampy nature prevail. 7. Natural History.