Excursion To Radlett. July 12th, 1884 - Geologists' Association excursion

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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)

Director: John Hopkinson, F.G.S. (Report by the Director.) (Proc. Vol. viii. p. 452.)

Starting from Radlett Station, the road leading to Shenley was taken for a short distance, when, by permission of Mr. Thomas Bagnall, Newberries Park was entered. After walking through the grounds adjoining the house a meadow near the park was crossed, and an opening in it, not easily found, revealed the only existing exposure of the Hertfordshire conglomerate, This spot had not before been visited by the Association, and on this occasion the conglomerate was examined under by no means favourable conditions.[1] The Director gave the following account of the conglomerate in its relation to the strata in which it occurs, the Woolwich and Reading series:

"We are now standing on the edge of the London Tertiary Basin. We have examined the beds forming the outcrop of the Tertiary strata to the south-west at Woodcock Hill near Rickmansworth,[2] and at Watford Heath and Bushey near Watford[3] and to the north-east at Hatfield [4] and at Hertford.[5]All along this line of outcrop we have had the Chalk on the north-west of us and the London Clay on the south-east, while immediately beneath our feet have been the sands, mottled clays, and pebble-beds of the Woolwich and Reading series, reposing on the Chalk and being overlaid by the London Clay, of which we could in some places see only the basement-bed.

" Underlying the Woolwich and Reading Beds, on the opposite side of our Chalk basin, south of London, are the Thanet Sands, but on this edge of the basin they are absent, the Reading Beds here reposing immediately on the Chalk, which appears to have suffered some amount of denudation before their deposition. They are very different in character from their representatives south of London. In the neighbourhood of Woolwich, especially, clays in thin layers predominate throughout, and fossils, which are frequent, show that the beds are of fluviatile or estuarine origin. Here there is usually but one thick bed of mottled clay, with sands above and below, and at the base a bed of flint-pebbles in sand. Fossils are seldom found, and the general character of the beds indicates a marine origin, though partly, at least, littoral.

" It is this shore-deposit that we have now before us, for the Hertfordshire conglomerate is merely a bed of the Woolwich and Reading series—the shingle-bed of flint-pebbles consolidated by the infiltration of silica. At Watford there is a shingle-bed 15 feet thick, the black flint-pebbles being in a matrix of ochreous clayey sand, but not consolidated; and in fact it is only at this spot that a shingle-bed can now be seen in situ in the form of a conglomerate, or, as it is sometimes called, ' plum-pudding-stone.' Blocks of this conglomerate occur, however, here and there, over a considerable portion of Hertfordshire, and they have also been seen in Essex. We know that the Reading Beds once extended over the greater portion of Hertfordshire, by the outliers which are seen at various places, and it seems more probable that the blocks we meet with are relics of these beds, of which the looser portions have been carried away by denudation, than that they have been transported in a northerly direction from the neighbourhood of Radlett. It is probable that the land over which the rivers flowed which deposited the clays, sands, and pebbles of the Woolwich and Reading series was to the south of the London and Hampshire Tertiary basin, where the English Channel now is, and on this supposition we should expect to see the beds to the north more marine in character than the more southerly portion of the series. From the very variable character of the whole of this series it may be inferred that in no locality where it occurs was it, or any portion of it, a deep-sea deposit. Littoral conditions may have prevailed over at least a considerable portion of Hertfordshire, so that there is no improbability in the shingle-bed having at one time been of considerable extent, and throughout this extent it may anywhere have been consolidated into a conglomerate.

" The conglomerate is here seen to be split up into blocks, and the durability of the siliceous cement is evidenced by the splitting of the pebbles. In some places the pebbles have become so soft that they can be cut with a knife, while the matrix preserves its hardness. Another point of interest is that here and there the surface of the conglomerate is seen to be smooth and rounded, as if it had been subjected to the action of ice. It is the only rock we have anywhere in this neighbourhood, and probably in our whole Tertiary basin, which is sufficiently durable to show any trace of glacial action."

A careful examination of the conglomerate was then made in different parts of the pit, and pebbles here and there were freely subjected to the action of pen-knives, a few being found which could be cut into. The splitting up of the conglomerate was observed, and its surface was seen to be in some places evidently moutonnee. The beautifully white, fine sand seen reposing upon an unconsolidated portion of the pebble-bed also attracted attention.

The road leading to Boreham Wood was then taken for a short distance, and, turning off it towards Letchmoor Heath, a stream flowing out of Elstree reservoir into the River Colne was crossed. The bed of this stream is frequently dry, and there was not much water in it, but the proximity of the underground reservoir, the plane of permanent saturation in the Chalk, was evident from a stream of water running down the road, which was not merely surface-drainage from the recent rain, its source being beneath the surface.

The route was now past Kendall Hall, and then past Kendall Pound, between which place and Battler's Green the principal "swallow-holes" in the district are to be seen, there being others near Letchmoor Heath and Bushey. On arriving at the Battler's Green swallow-holes no water was to be seen. There having been a long period of dry weather the recent rainfall had been speedily absorbed; but that sometimes a large quantity of water would be seen to disappear was evident from the amount of solid matter which had been carried away.

When swallow-holes occur in the bed of a stream the water may sometimes be seen to disappear entirely in dry seasons, while when a large amount of water flows they may take part of it only and the stream may flow on merely diminished in volume. In the carrier which conveys the water from the Elstree Reservoir to the River Brent there are some swallow-holes which have to be plugged when the water is discharged from the reservoir to prevent the waste of the greater part if not the entire volume of the water. This carrier must there be above the plane of permanent saturation of the Chalk, but most of our chalk-streams flow at the level of permanent saturation.

After walking a short distance across the fields the party separated at Battler's Green, some walking to Radlett Station, and others driving to Watford.

[An excursion to Shenley took place on July 19th, 1890, Upfield Green being Director. Shenley is near Radlett, Herts.]

Footnotes

  1. On July 19th, 1890, Mr. H. J. Lubbock, of Newberries, had this section especially opened up for the benefit of members of the Association.
  2. Proc. Geol. Assoc., vol. vii. p. 397.
  3. Proc. Geol. Assoc., , vol. ii. p. 43; vol. iii. p. 65; vol. iv, p. 284; vol. vi. p.,191.
  4. Proc. Geol. Assoc., ., vol. iii. p. 240; vol. iv. p. 518
  5. Proc. Geol. Assoc., vol. v. p. 519