It is important to add that, at a gradually increasing depth as we proceed south, the Gault underlies the whole of the Hertfordshire Chalk, and renders the latter such an excellent water-bearing formation. If the Gault be perforated we come upon the Lower Greensand, another excellent water-bearing stratum, which comes to the surface in the neighbourhood of Silsoe, in Bedfordshire. We may now turn to the formations overlying the Chalk in the southern half of the county. Here it should be mentioned that all the formations hitherto described overlie (or underlie) one another in what is termed conformable sequence; that is to say, there is no break between them, but a more or less nearly complete passage from one to another. Between the Chalk and the overlying Tertiary formations, there is, on the other hand, a great break or “unconformity”; the surface of the Chalk having been worn into a very irregular contour, above which we pass suddenly to the Tertiary beds, generally containing at their base a number of rolled chalk flints. This indicates that before the Tertiary beds were laid down, the Chalk had become dry land; after which a portion of it once more subsided beneath the ocean. The Tertiary beds are in fact formed for the most part from the _débris_, or wearing away of the old Chalk land. The lowest Tertiary stratum of eastern and southern Hertfordshire is known as the Woolwich and Reading beds. These consist of alternations of bright-coloured plastic clays and sandy or pebble-beds; their maximum thickness in the county being about 35 feet. They form a band extending from Harefield Park to Watford, and thence to Hatfield and Hertford. Below the Woolwich and Reading beds we come on the London Clay, of which the basement bed contains a layer of flint pebbles, although the remainder of this thick formation is a stiff blue clay, turning brown when exposed to the action of the weather. Originally these Tertiary formations must have extended all over the Chalk of central Hertfordshire, as is demonstrated by the occurrence of patches, or “outliers,” of them over a zone of considerable width. Such Tertiary outliers occur at Micklefield Hall, Micklefield Green, Sarratt, Abbot’s Langley, Bedmond, Bennet’s End, and Leverstock Green, and in the northern, or St Peter’s portion of St Albans.