Abstract
THE party first proceeded to examine the section of the “Woolwich and Reading Beds,” just north of the station. This section was described by Mr. Prestwich in 1850 (see Quarterly Journal Geological Society, vol. vi. p. 260, fig. 6) not long after it had been exposed by the railway-cutting. A year ago it was laid bare afresh when widening the railroad; but already the slipping of the clays has obliterated some points of interest. Traces of the shell beds, with Cyrena and Ostrea, below the representatives of the “Oldhaven beds,” are to be found at the base of a telegraph post, 104 yards south of the road bridge; and the underlying mottled clays, with a dip of 4° to the north, are easily recognised for about 190 yards to the south, where a small valley (about 50 yards across) has been formed by denudation out of the sand and lowest green sandy clays resting on the Chalk, which forms the northern foot of the Hogsback or Surrey range. Here the Chalk is seen to be traversed in every direction with fissures, often “slickensided,” horizontally or nearly so, some empty, some filled with vein flint, and some with loamy stuff. Nodules and occasional thin laminæ of flint follow the dip of about 6° to the north, and many are in a crushed condition. Bands of marly chalk also lie on the same plane. Some Echinoderms were met with. The party then proceeded to visit the much larger excavation in the Chalk at the entrance of the railway tunnel. Here the dip, well marked by flints and marly bands, is about 12° to the north. Fossils (Sponges, Echinoderms, Inoceramus, & c.) abound in this pit. The usual chalcedonic and quartzy interiors of hollow flints attracted notice, and Prof. R. Jones drew attention to facts that seemed to him to bear evidence of flint being a pseudomorph after chalk. They next visited a quarry in the Lower Greensand, on the escarpment overlooking the pathway to Losely. In this section of those Neocomian beds lvnown as the Bargate Stone, the waterworn sand of quartz, ironstone, lydite, and hard green silicates, is so largely mixed with calcareous fragments (the débris of shell beds, polyzoan reefs, &c.) that it is here and there cemented together hard and compact enough to serve as a building stone and road-metal. Mr. Meyer here directed attention to the horizon at which he obtained an unrolled tooth of Iguanodon, indicating the existence of this great Dinosaur at, perhaps, the latest period to which any of its remains as yet known belong. The “false-bedding” of the sands—due to the southward set of prevalent tides and currents, and the probable origin of their materials from the “old palæozoic ridge or shoal,” were also studied, and the formation of the escarpment, with the correlative parallel cracks and fissures of the strata. The party then crossed the Ferry, where St. Catherine's Spring issues, beneath the hill, from a little cave in the red-orange-tinted sand. Here for thirty feet at least the Guildford gap has been found by boring to be occupied by bouldered chalk and other detritus due to the destructive, and yet conservative, agencies of nature. The soft irony beds of the Lower Greensand were next met with, and followed followed for about a mile, until a short field-lane, crossing the Gault and Upper Greensand, led into the Chalk-marl quarry below Warren Farm. Here the loss of the clay beds (Gault) from below, by their having been squeezed out along the southern side, had allowed the hard marl-rock to subside inwards and suddenly at the escarpment, and to rest at high angle (70° and more), whilst the Chalk of the hill range above dips only 5° or 6°. As the hard rock bands, here quarried for lime, are followed end-on along the strike (open to-day), the backs of lower beds form one side of this deep narrow pit; and the truncated edges of these somewhat bent and much fissured strata warn the instructed eye of the danger of standing either below them or above them, lest either rain or drought should detach their clinging surfaces from the sloping bed-plane. Large Ammonites and Nautili are the chief fossils met with here; but Pecten Beaveri and Terebratula are also found. In an old excavation in the lane Siphonia has been found in the representative of the Upper Greensand which is overlain by dark-green sandy clay and Gault, turned up at a high angle (and probably squeezed out) in the breadth of a few yards, before the iron-sands are reached on returning to the hill-side. The party next came to the foot of St. Martha's Hill, or Martyrs' Hill. Before mounting this hill of sand, seamed irregularly with ironstone, some of the geologists descended the Halfpenny Hatch lane, leading down towards the East Shalford bottom, and saw a section of sand and calcareous sandstone, with a fuller's earth band and pebbly beds, similar to those in the quarry on the other (western) side of Guildford. The underground structure of South-Eastern England is connected with that of the Boulonnais, of Belgium, the Ardennes, and Westphalia; and the folds and ridges of palæozoic rocks, that in those countries bear up, either at the surface or just beneath the Chalk, or the attenuated Oolites, valuable coal-beds, are continued through, in a broad sweeping line, underneath parts of Surrey, Kent, and Sussex, until visible again near Frome, in the Bristol coal-area, in North Devon, South Wales, and the South of Ireland. The old faults and fissures affecting this linear tract of old strata had long before the Coal-period raised and depressed the lands and sea-beds; and, as a great spur of the old Scandinavian lands, this tract afforded ground for the littoral growth of the jungles that formed the coal on its oscillating borders and in its lagoons, now shut up by bars, and now losing their marsh features by influx of the sea. Succeeding ages still brought oscillations and changes, until the Jurassic seas crept over this old ridge or shoal, and the Cretaceous seas quite buried it, at first in sands and ultimately by the calcareous ooze of oceanic depths. But again another contracting crush of the earth's crust operated on the old weak lines, and the buried ridge slowly uprose, and its coating of thick strata were worn off by sea and rain, making pebbles and sand for the Lower Tertiaries; and still rising, it was at length laid bare in the Franco-Belgian and the Bristol areas; whilst our Wealden valleys of elevation, and those of Kingsclere, Shalbourn, and Pewsey, show where its uneven back approaches near the soil.
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Excursion of the Geologists' Association to Guildford and Chilworth, June 1 . Nature 6, 235–236 (1872). https://doi.org/10.1038/006235a0
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DOI: https://doi.org/10.1038/006235a0