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INTRODUCTION.

PHYSICAL GEOGRAPHY, AND BOTANICAL DIVISIONS, OF

THE COUNTY.

THE County of HERTFORD lies between the parallels of 51° 36′ and 52° 4′ North latitude; and between the meridians of 0° 13′ E. and 0° 44′ W. longitude. The County Town itself is situated, on the meridian of St. Paul's, in latitude 51。 47′ 45′′ N.

Neglecting the minor irregularities of its shape, the whole County may be considered as forming a rectangular parallelogram, having its shorter sides about 18m. in length, and inclined about 30° to the meridian, so that the longer sides-which are about 32m. in length-run nearly from N.E. by E. to S.W. by W. The greatest diagonal extent from the neighbourhood of Stortford to that of Tring is about 40m.; rather exceeding the distance from near Royston to Rickmansworth.

The surface of our County is agreeably diversified by numerous ridges of moderate elevation, and intersected by the valleys of numerous small rivers. The greater portion of it, however, may be considered as forming an inclined plain, sloping upwards from S.E. to N.W., from an elevation of some 200ft. above the sea at its lower edge to one of 500ft. at the upper.

Geologically considered, Hertfordshire is for the most part a portion of the circumference of the London Basin. A few small portions of its N. and N.W. borders are occupied by the gault clay: for example, at the N. of Tring, Hitchin, and Baldock. Over this, the green sand being either wholly wanting or extremely thin, lies the chalk marl or clunch, forming with the gault a plain at

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the foot of the escarpment of the chalk hills from Tring along the N.W. border to Royston. Next in order comes the chalk, which either forms the surface of all the rest of the County, or underlies it at no great depth. This stratum rises rather abruptly from the inferior ones just mentioned, attaining an elevation of 300 or 400ft. above them in the space of half as many miles, and presents from the level country an almost cliff-like escarpment in many places. The greatest elevations are at Tring and Aldbury Commons, which are probably upwards of 700ft. above the sea: Kensworth, near Dunstable, stated at 904ft. and Lilley Hoo, near Hitchin, 664ft. The gradual descent of the surface of the chalk towards the S.E. forms the inclined plain already spoken of as including the chief part of the County. Throughout this space the chalk appears almost everywhere in the bottoms, unless where concealed by débris and alluvial deposits; but on the uplands it is generally covered by tertiary or diluvial strata. South of Royston the chalk is capped by a deposit of clay, which we judge to be of diluvial origin; but we have not sufficient acquaintance with fossils to attempt a determination of its age. This deposit extends westward nearly to Stevenage, changing to a loamy gravel, and ultimately to sand at Hitchin. The uplands S. of Tring and Dunstable are also capped with a thin stratum of clay, abounding with angular flints. Farther to the S.E. the chalk is immediately overlain by beds of gravel, the lowest member of the tertiary formation in this quarter. These generally crop out on the slopes of our valleys, while the ridges are occupied by a capping of clay increasing in breadth and thickness as we approach the E. and S.E. borders of the County. This is the plastic clay of geologists: its surface forms on the whole. an inclined plain ascending from the valley of the Thames to the southern borders of Hertfordshire, along which it attains an elevation averaging 400ft. above the sea. This stratum ceases rather abruptly, and with a steep escarpment,

towards the N.W., in the neighbourhood of Hertford on the E., and Watford on the W.; but in other places it passes more gradually into the general level. The highest portions of this stratum, as at Hertford Heath, Brickendon, Northaw, Totteridge, and Bushey, are covered by beds of a blue pebbly gravel, resembling the shingle of the sea beach, and sometimes aggregated into a coarse conglomerate or pudding stone. This, agriculturally considered, is the worst land in the County.

The principal ranges of hills have already been indicated in our sketch of the Geology of the County, being formed by the outcropping of the plastic clay and chalk respectively. There is another tolerably well-marked ridge stretching from Watton S. Westward by Welwyn and Sandridge towards Hemel Hempstead. But in general the apparent hills of Hertfordshire are not ridges elevated above the general level of the surface; but appear to be such only when viewed from the valleys of the rivers, whose waters have cut and furrowed deeply below the general level. A consideration of the rivers is therefore. more important than that of the hills in a view of the Physical Geography of the County.

The Rivers of Herts. belong to two principal classes, namely, the feeders of the great Ouse, and the tributaries of the Thames. Of the former class are:-1, The Ivel, and 2, The Rhee; of the latter are:-1, The Thame; 2, The Colne, with its tributaries, the Ver, the Gade, and the Chess; and 3, The Lea, with its feeders, the Maran, the Beane, the Rib, the Ash, and the Stort.

I.—1. The Ivel rises at Clerkenwell, on the N. side of Baldock, from the lower chalk stratum, and leaves the County at Radwell, after a course of two miles only. Two miles further, it receives a Streamlet originating in strata above the chalk at Wallington, and a mile N. of Arlesey (Beds.) it receives the Hiz. This latter rises at Well Head, one mile S. of Hitchin, and passing through that town, gives it its name (Hiz-chine). At Grove Mill

it receives the Pirre, a stream originating partly in springs from the chalk, and partly in surface drainage from Weston and Graveley. A mile further to the N. the Hiz is augmented by the Oughton, rising at Oughton Head, one of the numerous strong springs from the bottom of the chalk which abound in this neighbourhood. St. Faith's Well, and other springs, at Hexton, give rise to a stream running N. to Shefford (Beds.), and thence to the Ivel at Langford.

2. The Rhee, or W. branch of the Cam, rises at Ashwell, at the E. end of the town, and receives the Ruddry Brook at the most Northern point of the County, where these streams form the boundaries of Hertfordshire against Cambridge and Bedfordshire.

II.-1. The drainage of that part of the County which lies to the N. of the chalk hills at Tring, is effected by various small streams, whose union forms the Thame. Most of the water is, in the first instance, collected in the reservoirs which supply the Grand Junction Canal and its branches to Aylesbury and Wendover.

2. The Colne, at least that branch of it which retains the name farthest towards its source, originates in surface drainage about two miles W. of Barnet, and by the time it reaches North Mimms, it is augmented to a large brook by the drainage of a considerable tract of country W. of the road from Barnet to Hatfield. At this point, however, the whole of its waters is, in ordinary weather, absorbed in the earth, in consequence of the porous chalk stratum rising nearly to the surface with nothing but sandy gravel above it. The principal " Swallow-hole" is in Potterell's Park, where a large piece of ground is covered with meandering channels, terminating in numerous funnel-shaped basins. In very wet weather, and in winter, the Swallow-hole is over-charged, and the unabsorbed water overflows through Mimms' Park and over Colney Heath to Smallford, where it receives several small streams from the North. Thence it flows S.W. to

London Colney, and crossing the Watling Street at Colney Street, it falls into a far larger stream, called the Ver or Mure, whose name it supersedes. This latter rises at Mere Gate (ie. Marsh Gate), by corruption (Markyate) Market Street, and following nearly the course of the Watling Street through Redbourn to St. Alban's, is continually augmented by springs issuing from the chalk in or near its bed. After receiving the Colne Proper a little below Colney Street, the united stream, under the name of Colne, proceeds S.W. to Watford, and between that place and Rickmansworth receives the Gade, a stream which rises near Great Gaddesden (i.e. the Gade's Dene), and receiving the Bulbourne at Two Waters, runs nearly S. to Rickmansworth. The lower course of the Gade to Two Waters, and that of the Bulbourne to its source at Dudswell, are rendered navigable by art, and form part of the Grand Junction Canal. Like the Ver, these streams consist wholly of spring water from the chalk. At Rickmansworth the Colne receives the Chess, a small stream of clear water from Chesham (Bucks.), and turning to the S. it separates Herts. from Middlesex for several miles in the direction of Uxbridge, past which it flows to the Thames between Egham and Staines.

3.-The Lea rises at Lewsey (i.e. Lea's Water), near Dunstable, and passing Luton (i. e. Lea Town), enters Herts. at Hyde Mill. Hence it runs past Wheathampstead and Hatfield, but remains a very trifling stream till augmented by a great spring rising from the chalk on its left bank at Woolmers (i.e. Well Marsh) below Essendon, which in summer time fully doubles the previous volume of the Lea. From Hatfield downwards the Lea receives numerous torrents on its right bank, which renders its stream turbid after heavy rains; but in drier weather these lose their waters in the chalk before reaching the river. A little above Hertford the Lea is again doubled by the clear waters of the Maran, a stream rising in Lilley Bottom, and collecting the waters of many springs at

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