The contours of a district depend almost entirely upon the nature of its geological formations, and the action of rain, rivers, and frost upon the rocks of which they are composed. These formations in the case of this county are briefly described in a later section. Here it must suffice to state that hard slaty rocks form jagged mountain ranges, while soft limestones like our Hertfordshire chalk weather into rounded dome-like hills and ridges, and heavy clays form flat plains. As the bed-rock, or, as we may say, foundation, of Hertfordshire, is constituted either by chalk or clay, it is the two latter types of scenery that mainly characterise this county. It is only, however, on the north-western and northern borders of the county, as at Ivinghoe in Bucks, or between Sandon and Pirton, that we find typical chalk scenery, where the downs forming the north-easterly continuation of the Chiltern Hills of Buckinghamshire enter our own area. Here we find the rounded downs and hollow combes characteristic of the South and North Downs of the south-eastern counties; and the same absence of woods, except where artificial foresting has been attempted. Some approach to true chalk scenery is likewise shown on the line of hills on the London side of Elstree, where they overlook the clay plain forming this part of northern Middlesex. There is, however, a difference in the scenery of this part as compared with that of the downs to the north in that these hills are partially covered with a capping of clay or gravel, while the beds or layers of which the chalk is composed slope towards the plains at the base of the ridge instead of in the opposite direction (see section on Geology).

Elsewhere the chalk is covered over to a greater or less degree, alike on the hills and in the valleys, with thick deposits of clay, sand, and gravel. These communicate to the hills and valleys a contour quite different from that of chalk downs; and in many parts of the county we have a series of more or less nearly parallel lines of low undulating hills separated by wide, open valleys. Examples of this type of scenery are conspicuous in walking from the valley of the Ver at Redbourn across the hill on which Rothamsted stands, then descending into the riverless valley of Harpenden, again crossing the hill to the east of the latter, and then descending into the valley of the Lea near the Great Northern railway station.

This capping of clay permits the growth of forest on the hills, as do the alluvial deposits in the valleys; so that in its well-wooded character the scenery of the greater part of the county is altogether different from that characterising the bare North and South Downs and the Chiltern Hills. Where the chalk comes near the surface there is a marked tendency to the growth of beech-trees, splendid examples of which may be seen in Beechwood Park. Elsewhere in the chalk districts the elm is the commonest timber-tree; although it should be said that this species of tree was originally introduced into England from abroad.

Reference has already been made to the numerous open commons dotted over the chalk area of Hertfordshire. These appear to have been left as open spaces at the time the country was enclosed, owing to the sterility of their soil, which is unsuited for growing good crops of either corn or grass. Many of these commons, as in the neighbourhood of Harpenden, were enclosed some time previous to the battle of Waterloo, when corn in this country was so dear, and every available piece of land capable of growing wheat consequently of great value. It may be presumed that the commons with the best soil were selected for enclosure; but most of such enclosed commons produce inferior crops, partly, it may be, owing to the plan on which they are cultivated. Till twenty years ago or thereabouts all such commons in the writer’s own neighbourhood were divided into a number of parallel strips, separated by grass “baulks”; these strips representing the respective shares of the copyholders of the district, who had the right of grazing on the original common. The absence of hedges rendered it necessary that the same kind of crop should be grown each year on every plot. This made it not worth the while of the occupiers to spend money on high cultivation. Of late years many of these enclosed commons have either been built over, or come under a single ownership, thereby obliterating one more interesting page in the history of the county.

To the south of the Elstree range of chalk hills the scenery suddenly changes, and on emerging into Middlesex from the tunnel through these hills on the Midland Railway we enter an extensive grassy plain, characterised by its abundance of oak trees, the scarcity of elms, and the total absence of ash. Parts of this plain, which is continued through Middlesex to London, form the great grass-growing and hay-producing district of Eastern Hertfordshire.

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.

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.