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. [Illustration: A Typical Hertfordshire Village: Much Hadham]