Excursion to Watford. May 17th, 1879 - 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)

Directors: W. Whitaker, F.R.S., and John Hopkinson, F.G.S. (Report By The Directors.) (Proc. Vol. vi. p. 191.) A large party assembled at Bushey Station and proceeded to the Colne Valley Water-works, where Mr. Verini, the Secretary of the Water Company, explained the method of pumping and softening the water. The members left the water-works for the adjoining chalk-pit, where, after a few fossils had been found, Mr. Whitaker gave an explanation of the section, of which he had published a short description is years ago (when probably it was not in such good condition as now), somewhat as follows:

Ochreous sandy gravel, partly in pipes.
Clay, bluish-grey at top, the rest light-grey, with a layer of a claret-colour at the base.
Chalk with flints.

A bed of clay, the lines of bedding of which were waved, was, he said, deposited very irregularly on the Chalk. The chalk was evidently cut out in a hollow before the clay was deposited, showing a very great interval of time between the deposition of the chalk and of the clay above. Elsewhere such hollows were nearly always caused by the sinking of the overlying beds through the dissolving away of the chalk. The clay could not be older than the Glacial Drift, and he was inclined to class it as Glacial, though Dr. Evans and other geologists believed it to be Post Glacial. The mistake was often made of supposing that all beds termed Glacial were considered by geologists as having been deposited by ice or in an arctic climate, but all that was meant by the term (as expressive of geological age) was that such beds were deposited during the Glacial epoch, in which were intervals of warmer climate as well as cold periods. The bed now seen looked like some beds of brick-earth which elsewhere occurred under Boulder Clay, and it might have been formed in some lake of no very great extent. On the top of this clay might be seen a bed of gravel, but he could not say whether it was a river-gravel or a glacial gravel. If it belonged to the Glacial period, the beds of clay below must also be glacial. Glacial and Post Glacial were, however, only relative terms, for glacial conditions lasted longer in the North of England than here.

On leaving the chalk-pit the road past Bushey Station was taken, and from an elevated' position above Wiggenhall, affording a good view of the valley of the Colne and the hills on either side, Mr. Whitaker pointed out the connection of the features of the country with its geological structure. The range of hills on the edge of which we then stood was, he said, known as the Tertiary escarpment; the term escarpment meaning a ridge along which the beds were cut off. These Tertiary beds had once extended much further over the county, and the escarpment had been at one time beyond its present position, as shown by outliers of the London Clay and of the Reading Beds. The Colne had most probably determined the present line of the escarpment by cutting its way back, but further down-stream it had turned southward, and cut across the beds. The slope on the opposite side of the valley rose gently to a corresponding elevation to that on which we were, and we should find that this ground consisted of gravel-flats of the same character as the one we had just walked over, the river having cut away the beds between.

The valley of the Colne was then crossed, and at the Colney Butts gravel-pits Mr. Whitaker stated that the gravels seen belonged to the Glacial Drift and were probably of marine formation, for in some places, as in Suffolk, marine fossils were found in sandy beds of similar age (older than the Boulder-Clay). The larger stones, perfectly rounded, must have come from the north; the pink quartzites were supposed to have come from the Lickey Hills; and the flint pebbles had come from Tertiary beds, in which they had originally been deposited after the denudation of the Chalk, in which the flints were first formed—a vast quantity of chalk having been denuded to form such extensive gravel-beds.

The Hagden Lane gravel-pits were next visited, and here the irregular surface of the Chalk under the gravel was well seen. Mr. Whitaker remarked that all gravels tended to form nearly level flats wherever there was a large extent of gravel. These gravels were noteworthy as showing no layers of bedding. The very uneven surface of the Chalk seemed to be due to its dissolution by water holding carbonic acid in solution, which percolated through the gravel above. The gravel last seen, where not let down by this dissolution of the chalk, was not more than zo feet thick, though it occupied large areas.

Map[edit]

Ordnance Survey. Geological. Sheet 7. 8s. 6d.

New Ordnance Survey. Sheets 238, 255, 256. 1s. each.

Books[edit]

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.