Excursion to Berkhampstead and Bourne End. Saturday, 21st April, 1883 - Geologists' Association excursion
|From: A record of excursions made between 1860 and 1890. Edited by Thomas Vincent Holmes , F.G.S. and C. Davies Sherborn, F.G.S. London: Edward Stanford [For the Geologists’ Association], 1891. Source: Cornell University copy on the Internet Archive (Public domain work)|
Directors: Dr. John Evans, F.R.S., and John Hopkinson, F.G.S. (Report By John Hopkinson.) (Proc. Vol. viii. p 117.)
In Wiltshire there is a River Bourne, which is said to flow in the winter and spring, usually ceasing in the summer and autumn; but the river now under consideration is of a more erratic nature than this, running on the average only about once in from three to seven years. It is known as the. Hertfordshire Bourne, and has a parallel in a stream on the opposite side of the Thames valley, known as the Surrey Bourne, which usually flows in the same years as the Hertfordshire stream. To see this Hertfordshire Bourne flowing, and to trace it from its source to its outflow into the River Bulbourne at the hamlet to which it gives the name of Bourne End, was the object of this excursion.
Before, however, proceeding to the valley of the Bourne, the members first visited the ruins of Berkhampstead Castle, close to the station.
The church, dedicated to St. Peter, was next visited under the guidance of the Rev. J. W. Cobb, its principal interest centring in its connection with the poet Cowper, whose father was rector of the parish. From the church Mr. Cobb conducted the party to the rectory, Cowper's birthplace. In the rectory grounds " Cowper's Well " was pointed out, and a large piece of the Hertfordshire conglomerate was inspected.
Continuing to ascend the hill on the south-west of Berkhampstead, Haresfoot Park, the seat of Mr. F. A. Dorrien Smith, was soon reached; and, after walking through the park, a descent was made into the valley of the Bourne, permission having been obtained from Mr. Dorrien Smith to follow the course of the stream, which flows over his property. The Bourne was encountered where it is crossed by Harratt's End Lane, and where its source was found when it was visited by the Hertfordshire Natural History Society, in 188r. Now, however, it was flowing here in a considerable stream, the field above the lane was in great part submerged, and to find the present source of the river it was necessary to walk about half a mile up the valley by the side of the lake thus temporarily formed, and through some muddy fields.
There is no historical record of the flowing of the Bourne in olden times. Indeed, the existence of this Bourne does not appear to have been placed on record until 1876, when Dr. Evans gave an account of it, recording its having flowed in that year, in 1873, and in 1853, and probably also in 1860 and 1866. The valley of the Bourne, however, in itself furnishes evidence, in the extensive denudation by which it has been formed, of the existence of a stream, which may or may not have been intermittent, long before historic times.
It was in a field in Buckinghamshire that the party had now assembled, being on the right bank of the stream, in view of its present source and of its course for some distance. Here Dr. Evans explained the geological and meteorological conditions to which the existence of this and other intermittent streams is due, his remarks being as follows :
" All streams are dependent upon the rainfall; if on hard or impervious soils, directly, for the water in that case at once runs off on the surface; but if on absorbent soils or subsoils, such as chalk, the result is different, for much of the water percolates downwards until it meets with some obstruction. It arrives sooner or later at the surface of a natural subterranean reservoir, for at a certain depth the chalk becomes thoroughly saturated with water.' There is a vast amount of water continually in the chalk, and even in the dryest weather there is always water in most chalk-streams, as they are not immediately dependent upon rain. The amount of water flowing from one of these areas is merely a fraction of the amount of rain falling upon it. The yearly rainfall over a period of 50 years in this neighbourhood is 25 or 26 inches. For the 12 years ending 1872 an average of 251 inches per annum fell, of which about half fell during the summer months (April to September), and the other half during the winter months (October to March). In order to ascertain the amount of water which finds its way more than one or two feet below the surface, Messrs. Dickinson & Co. had a series of gauges constructed, consisting of cast-iron cylinders, 18 inches in diameter, and 3 to 6 feet in length, turned to a knife-edge at the top and sunk to their full depth below the level of the ground in which they are placed, but so that the edge just projects above the ground. Some of these cylinders are filled with the ordinary surface-soil of the neighbourhood, and others with chalk (from the Upper Chalk) in a fragmentary condition, and with sand; grass is growing on the upper surface of the contents of the cylinders, and also on the ground surrounding them; and there are means provided of collecting and measuring the amount of water which finds its way through the cylinders, that is, through three or six feet of soil, chalk, or sand. It is a remarkable thing that, although there may have been a rainfall of 15 or 16 inches in the summer months, not more than one inch finds its way through the ordinary soil, and 1¾ through the chalk; while, with an average rainfall for 23 years of 14.17 inches in the six winter months, 6.03 inches went through the three feet of soil, and 8.8 inches through the three feet of chalk. For the same (winter) period for the 12 years ending 1872, the average rainfall was 12.8 inches, of which 5.1 percolated through the soil, and 7.3 through the chalk. With an increase of about two inches in the rainfall in the winter period there was thus an increase of one inch in the quantity which went through three feet of soil, and 1½ that which went through three feet of chalk. In this way it was found out what is about the proportion of the water which, percolates into the soil as compared with that which is carried off by evaporation and vegetation. Some idea of the vast amount of water absorbed from the ground by trees may be gathered from Professor Attfield's paper ' On the Physics and Chemistry of the Sap of Plants.' "
"After a long period of drought the surface of the underground reservoir falls, becoming very nearly level, in consequence of the water leaving it as springs not being replaced, but after heavy rains the inclination of the surface of the water is again raised. In a well at Studham, miles away from any stream, the level of the water in two successive years has varied as much as 70 feet. When the subterranean reservoirs receive a great accession of water by a long-continued rainfall, these bournes, or intermittent streams, make their appearance; but it might happen that the rain which fell in the course of one winter would not be sufficient to raise the surface of the reservoir sufficiently high for a particular stream to flow, and on the other hand it might flow when there had not been any particular heavy rainfall, in consequence of the accumulation of the rain of previous years."
"The Bourne is flowing strongly this year, it flowed last year, and also the year before, thus disproving the old idea that it only flows once in seven years. The plane of permanent saturation is not level, but is inclined towards those places where the water finds its way to the surface of the earth in the form of springs. This plane varies at different times in the year, sometimes being inclined at an angle of 20 feet, and sometimes even at one of 25 feet to the mile, while sometimes the slope is not more than 12 feet to the mile. ' The rivers flowing past Watford and St. Albans will be found to have an inclination of about 12 feet 6 inches to the mile in the lower portion of their course, and 18 feet 6 inches in the upper portion. This shows what is the amount of impediment to water passing through the chalk in a lateral direction; for, assuming that water could find its way through the chalk at a less inclination than 12 feet 6 inches to the mile, these rivers would cease to flow, or, at all events, the streams would never be seen upon the surface. The whole of the chalk being porous or pervious, unless there were a sufficiently saturated bed beneath the river to hold the water up at an inclination at least equal to that of the river, it would sink into its bed and disappear. The inclination of the subterranean water passing through the chalk must therefore be at least 12 feet 6 inches to the mile in this district."
"In certain years the subterranean water is raised in the body of the hills, and the valley of the Bourne, being cut at right angles or nearly so to the main stream, intersects the general surface of the plane of saturation, and the water appears on the surface of the ground at the bottom of the valley. This theory of intermittent streams being due to the intersection of the plane of saturation, is well illustrated by the fact of the Bourne being sometimes found running in one place, then disappearing, and further on running again, and again disappearing. The plane of saturation being more even than the surface of the ground, where there is an elevation of the surface the water finds its way underground instead of running on the surface. Owing to the large amount of rain which has fallen in recent years, the Bourne has flowed very frequently during this period, viz., in 1876, 1877, 1881, 1882, and 1883. On some occasions it commences flowing in summer; this year the flow of the stream will probably cease in June or July."
The course of the Bourne was then followed to Bourne End, where the stream flows into the Bulbourne. In a portion of its course it was found to have taken temporary possession of the road, a detour having to be made into the adjoining fields to avoid it, and at Bourne End it was observed to be flowing through the usually dry culvert under the high road in a powerful stream.
The party then proceeded to Boxmoor Station.
Ordnance Survey. Geological. Sheet 7. 8s. 6d.
New Ordnance Survey. Sheets 238, 255, 256. 1s. each.
W. Whitaker, Geology of London, 2 vols., 8vo, London (Geol. Surv.). 1889. 11s. [For literature.]
W. Whitaker, Guide to the Geology of London, ed. 5. 1889. 1s.
J. Hopkinson, The Bourne Valley, Boxmoor, Trans. Herts. Nat. Hist. Soc., vol. i., part 8, p. lviii. 1882.
- "The Hertfordshire Bourne," "Trans. Watford Nat. Hist. Soc.," vol. i. p. 137.
- " Trans. Herts. Nat. Hist. Soc. vol. ii. p. 229.