Excursion to Watford. April 13th, 1872 - 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: W. Whitaker, F.R.S., and John Hopkinson, F.G.S. (Report By John Hopkinson.) (Proc. Vol. iii. p. 65.)
The members assembled at Watford Station and proceeded northwards along the main line of the London and North-Western Railway to the cutting leading to a tunnel which is now being constructed to take two additional lines of rails. The new cutting, which diverges slightly from the old one at about half a mile from the station, exposes a section of the Chalk overlaid very irregularly by a thick bed of gravel, composed of sub-angular flints derived from the Chalk, of rounded flint pebbles also originally derived from the Chalk (but owing their present form to the attrition they have undergone in the older Tertiary rivers and seas), and of pebbles of very ancient rocks, some of which were derived from the conglomerates of the New Red Sandstone. Here and there also, in the gravel, more or less regularly stratified beds of sand divided by layers of pebbles are seen.
Before leaving this section Mr. Whitaker explained some of the chief features of this gravel, which he considered to be of Glacial age, and referred to a few of the most interesting problems connected with it. He stated that although we have in the section here exposed no certain data to determine the age of the gravel, we find elsewhere lying upon it patches of Boulder-Clay, showing that, as this gravel was deposited before the Boulder-Clay, the Chalk, which here was originally some 'co feet (or more) thicker than now, was denuded to this extent before, or during, the Boulder-Clay period. The sub-angular flints were not considered to have been derived from the Chalk upon which they immediately lie, but to have been drifted, most probably partially at least by the action of ice, from other localities, being here deposited, together with the Tertiary flint pebbles, and the pebbles of quartz and other rocks ice-borne from the conglomerates of Worcestershire and Warwickshire, in shallow water where the currents were shifting., After the Chalk had been, as it were, planed down to its present thickness the gravel was deposited pretty regularly upon it; the present uneven surface being in great part due to the dissolving away of portions of the Chalk by the percolation through the overlying porous bed of water charged with carbonic acid, the carbonate of lime being carried off in solution as a bi-carbonate. Thus are formed in the Chalk what are called pot-holes or pipes. Wherever the water can get away, as through cracks in the Chalk, it is evident that this decomposition will take place, the gravel sinking into the hollows thus formed.
The party then left the cutting at the mouth of the tunnel and proceeded direct over the old tunnel and through the woods to Cassiobury Park, permission to pass through the private grounds of the Earl'of Essex having been received.
The party took the nearest path across the park and over- the fields, and on re-assembling at Watford Heath Kiln, Mr. Whitaker described the section there exposed, and a few fossils were found; Mr. W. T. Stone also distributing several fossils which he and his workmen had obtained. These principally consisted of sharks' teeth, which occur in considerable abundance in the upper of two pebble-beds forming part of the Basement-bed of the London Clay; a few only being found in the lower bed. There were also a few oyster-shells which are here only known to occur in the lower pebble-bed.
Since this section was last visited by the Association a well has been sunk through the Woolwich and Reading Beds to the Chalk, just on the edge of the mottled-clay, which here ends abruptly, its place being taken by the bed of sand on which it rests; the Basement-bed reposing in one part of the pit immediately on the mottled-clay, and in another on the sand.
After a short walk across the fields, Bushey Kiln, the last section visited, was reached. The chief point here noticed was the great difference of the section of the Woolwich and Reading Beds from that exposed at Watford Heath, showing how very irregularly these beds were deposited. A small section, exposing several layers of pebbles in sand, showed how this difference might be partly accounted for. Not only were these layers very irregularly distributed, but between each more or less horizontal, layer were to be seen thin connecting layers of pebbles passing through the sand at various angles, showing how, subsequently to the deposition of each layer of sand, strong counter currents tore up the underlying pebbles and re-deposited them at various angles. So it is, and so we should expect it to be in fresh water or estuarine strata; here sand deposited, there pebbles at one place clay, at another loam. How very different this from the uniformity over large areas frequently presented by marine strata, such as the London Clay above, and the Chalk below!