« PreviousContinue »
underwent great denudation in the former part by the waters of the Middle Glacial sea, the sands of that sea, west and south of Diss, lying up to bosses of it in some parts, and overlying it in others.
The remarkable contortions of this Drift were well pictured by the late Mr. Samuel Woodward in his "Geology of Norfolk.' The contortions have been attributed partly to the thawing of masses of ice which had been fixed among the beds during their deposition, but more generally to the action of stranding icebergs.
MIDDLE GLACIAL BEDS.
The Middle Glacial beds consist of gravel and sand with occasional beds or seams of chalky Boulder Clay, as near Hertford. The gravel is composed to a great extent of Chalk ilints both subangular and rolled, and pebbles of quartz and quartzite. Its thickness is variable, being sometimes 15 or 20 feet, at others as much as 40 or even 70 feet. In some localities sand predominates, in others gravel, and occasionally a good deal of brickearth is associated with the beds. The sand sometimes contains pebbles of Chalk.
The gravel contains a great many rolled fossils derived from many different formations, but chiefly from those of Secondary age. The pits at Muswell Hill and Finchley, owing to the labours of Mr. N. T. Wetherell, have yielded a large number of specimens. (See fig. 22, p. 314.)
In some places in Essex the Middle Glacial gravel is very pebbly in nature, which in Mr. Wood's opinion was probably due to the proximity of Eocene pebble-beds, whether belonging to Reading Beds, London Clay, or Bagshot Reds,
In the neighbourhood of Hertford the Tertiary beds are capped by a pebbly gravel composed chiefly of Aint and quartz pebbles. This is very distinct from the more mixed gravel on the Chalk, and has been termed by Prof. Hughes the 'gravel of the higher plain,' to distinguish it from that in the lower grounds. He suggested that it might be PreGlacial. It occurs also at Barnet, Totteridge, &c.
Along the borders of the Thames Valley it is often a matter of great difficulty to distinguish between the Middle Glacial gravels and those formed by the river in great measure from their destruction : such is the case near Great Marlow, also at intervals along the Valley as far as Southgate.
At Danbury, in Essex, the gravel occupies a high elevation, but the hill is coated with gravel and not formed by it, as Mr. W. H. Penning has traced the London Clay exposed in gullies near the hill-top.
Deposits of loam and brickearth are associated with the gravels on the Chalk tracts of Buckinghamshire and Hertfordshire, and sometimes with those on the London Clay; but it is not possible always to fix the ages of the deposit. Thus, at Chelmsford there is a deposit of brickearth, which may be ož Glacial age or Post-Glacial.
Near Yarmouth, Messrs. S. V. Wood, jun., and Harmer have obtained from these Middle Glacial deposits shells of a southern aspect, including several extinct Crag forms, which tend to separate them from the Middle Sands of Lancashire, in which none but recent shells have been found. These include Buccinum tenerum, Fusus antiquus, F. scalariformis, Massa incrassata, Turritella incrassata, Cerithium punctatum (tricinctum), Pectunculus glycimeris, Nucula Cobboldia, Astarte borealis, Tellina obliqua, Mya arenaria, &c.
It appears, in Mr. Wood's opinion, to have been a littoral marine deposit, and to be unconformable to the Lower Glacial. This is locally the case on the Norfolk coast, where large basins or pockets filled with Middle Glacial sand and gravel have been excavated in the Contorted Drift. Some of these basins or pockets, however, are evidently due to contortion.
Mr. Wood considers that the Middle Glacial deposits are older than any of the shell-bearing glacial beds in other parts of Britain.
UPPER GLACIAL BEDS.
The Great Chalky or Upper Boulder Clay is a clayey deposit full of pellets or pebbles of Chalk, containing also Chalk-fints, blocks of various rocks, many of them scratched or grooved by ice-action.
In thickness it varies from a few feet to 150 feet. It contains numerous fossils derived from various formations, including Gryphæa incurva, G. dilatata, Belemnites, &c.
It is much used for marling fields, whence the number of marl pits. The surface is often very loamy, and in this respect hard to distinguish from the London Clay in Essex :' some of these surface beds have been used for brick-making. This surface is frequently piped’ in the same way as the Chalk, a feature evidently due to a similar cause, namely, the dissolution of the chalky matter by carbonated water. In the pit at Mackie's Nursery near Norwich, the Middle Glacial sands have by this agent been cemented into a comparatively hard rock which has, indeed, been used for building-purposes.?
This Upper Glacial clay was, in the opinion of Mr. S. V. Wood, jun., for the most part formed underneath a great
1 Sometimes called the 'Brown Clay' in the Eastern Counties.
Partly used in the construction of Norwich Castle. (S. Woodward.)
sheet of ice, and gradually extruded therefrom upon the floor of the sea, which was burdened with floe-ice.
It is well developed in Norfolk, Suffolk and Essex, in the country around Tivetshall, Eye,Framlingham, Halstead, Dunmow, Braintree, and the Rodings, but it has not been met
with south of the Thames, nor in the Thames Valley. occurs at Finchley, at Bricket Wood near Watford, and other places north of London, and around Buntingford, Biggleswade, Huntingdon, Horncastle, &c.
Overlying this Boulder Clay, there are in Yorkshire beds of a purplish-brown colour termed the purple clay' by Messrs. Wood and Rome. This clay has the most extensive development of any bed superior to the Chalk in this county, not only overlapping the basement clay in all directions, but extending far beyond the north scarp of the Wolds in an irregular belt along the coast north wards. In its lower portion it abounds with boulders of lower Secondary, Palæozoic, and Metamorphic rocks, and at Holderness it contains small quantities of Chalk. Messrs. Wood and Rome state that in the upper portion the small fragments disappear, and the large blocks also become far less frequent, so that the uppermost part (which at Dimlington can be well contrasted with the lower) is scarcely entitled to the distinction of Boulder Clay.
Bridlington Crag.-- In the lower and central portion of the purple clay are some beds of sand and gravel, one of which at Bridlington Harbour (according to Messrs. Wood and Rome) yielded the mollusca first made known by Mr. Bean as froin the Bridlington Crag. This Crag is rarely seen now save at low tide. Its thickness is estimated at not more than 10 feet, and its fossils include Saxicava rugosa, Trophon scalariformis, Asturte borealis, Nucula Cobboldia, &c. More than one-third of the species are arctic in character.
Drifts of South Wales and the South-west of England.
In South Wales there are deposits of drift gravel and boulders, apparently of local derivation, which may yet be connected with the Glacial period. Such are the scattered deposits near Cardiff, Cowbridge, Bridgend, Llantrissant, &c.
There is very little drift on the Cotteswold Hills, but in the neighbouring Liassic vales there is much gravel ; this has been divided by Mr. W. C. Lucy and Mr. E. Witchell, into the Angular Gravel of the slopes, the rolled Oolitic Gravel of the river valleys, and the Northern Drift,
Near Bath, on Hampton Down and other hills, fragments of Aint and pebbles of quartz are met with.
The Mendip Hills are remarkably free from drift, but there are some deposits of loam and clay, with here and there