A plain very similar in character to that south of Elstree is entered upon to the northward of Baldock, just after leaving the line of chalk hills, although the greater portion of this northern plain is situated in Cambridgeshire and Bedfordshire. When travelling northward from London by the Great Northern Railway, observant persons, after passing through the tunnels traversing the chalk hills north of Hitchin, cannot fail to notice the general flatness of the country as this great plain is entered. Although so like the southern plain in general appearance, this northern plain, as is noticed in the section on Geology, is composed of much older rocks. [Illustration: Bancroft, Hitchin] The heights of some portions of the chalk area above mean sea-level are, for this part of England, considerable. Thus Great Offley Church is 554 feet, St Peter’s Church, St Albans, 402 feet, Stevenage 306 feet, Hitchin Church 216 feet, while a hill near Therfield attains the height of 525 feet. Hastoe Hill near Tring is 709 feet. 5. Watershed. Rivers. In commencing this section it will be well to devote a few lines to the proper meaning of the frequently misunderstood term “watershed.” It means the line of water-division, or water-parting; that is to say the line along a range of hills from which the streams flow in one direction on one side, and in the opposite direction on the other. The ridge-tiles on a roof form an excellent illustration of a watershed. Every local stream is separated from the one nearest to it by a watershed; while a river-system, such as that of the Thames, is separated by a much more important watershed from the other river-systems which take their origin near its source. Such river-systems, each enclosed by a single watershed which it shares with its neighbours, are known as drainage-areas, or catchment-basins. Practically the whole of Hertfordshire lies in the Thames drainage-area. At first sight it would be natural to suppose that the summit of the line of chalk downs forming the continuation of the Chiltern Hills in the neighbourhood of Tring and continuing thence to Dunstable would constitute the watershed between two river-systems. But this is not the case, for the Thames cuts through the Chiltern Hills between Wallingford and Pangbourne, and thus receives the drainage of both the northern and the southern flanks of that range. The watershed formed by the continuation of the Chilterns in the Tring district is therefore one of second-rate importance.