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Transverse section along the line of fracture, b, c. In regard to their origin, Mr. Of these knives and flakes, I obtained several specimens from a pit which I caused to be dug at Abbeville, in sand in contact with the Chalk, and below certain fluvio-marine beds, which will be alluded to in the next chapter.
A valley or river channel was cut through them, probably during the gradual upheaval of the country, and the hollow became afterwards the receptacle of the comparatively modern freshwater beds A, B, C, and D. They may well represent a silted up river-channel, which remained for a time in the state of a lake or mere, and in which the black peaty mass B accumulated by a very slow growth over the gravel of the river-bed A. In B we find remains of some of the same plants which were enumerated as common in the ancient lignite in 3 prime, such as the yellow water-lily and hornwort, together with some freshwater shells which occur in the same fluvio-marine series 3 prime.
This species of Pholas still exists, and drills the rocks between high and low water on the British coast. The name of "Fluvio-marine" has often been given to this formation, as no less than twenty species of land and fresh-water shells have been found in it.
As a stratum containing exclusively land and freshwater shells usually underlies the fluvio-marine sands at Menchecourt, it seems that the river first prevailed there, after which the land subsided; and then there was an upheaval which raised the country to a greater height than that at which it now stands, after which there was a second sinking, indicated by the position of the peat, as already explained.
Forest Bed and Fluvio-marine Strata. Fossil Plants and Mammalia of the same. Overlying Boulder Clay and Contorted Drift. Newer freshwater Formation of Mundesley compared to that of Hoxne. Great Oscillations of Level implied by the Series of Strata in the Norfolk Cliffs. Earliest known Date of Man long subsequent to the existing Fauna and Flora.
The Upper division, or Weald Clay, is, in great part, of fresh-water origin, but in its highest portion contains beds of oysters and other marine shells which indicate fluvio-marine conditions. The uppermost beds are not only conformable, as Dr. Fitton observes, to the inferior strata of the overlying Neocomian, but of similar mineral composition.
As to the small number of marine shells occurring in the same fluvio-marine series, I have seen none which belonged to extinct species, although one or two have been cited by authors. I am in doubt, therefore, whether to class the forest bed and overlying strata as Pleistocene, or to consider them as beds of passage between the Pliocene and Pleistocene periods.
A. Site of Cromer Jetty. 1. Upper Chalk with flints in regular stratification. 2. Norwich Crag, rising from low water at Cromer to the top of the cliffs at Weybourn, seven miles distant. 3. "Forest Bed," with stumps of trees in situ and remains of Elephas meridionalis, E. primigenius, E. antiquus, Rhinoceros etruscus, etc. This bed increases in depth and thickness eastward. Fluvio-marine series.
BRITISH. Bembridge fluvio-marine strata. Helen's series. FOREIGN. Gypsum of Montmartre, fresh-water with Palaeotherium. BRITISH. Bracklesham beds and Bagshot sands. FOREIGN. Calcaire grossier, miliolitic limestone. Nummulitic formation of Europe, Asia, etc. BRITISH. London Clay proper. FOREIGN. Argile de Londres, near Dunkirk. BRITISH. Upper white chalk, with flints.
The materials of this uppermost deposit are arranged as if they had been the result of land floods, taking place after the formations 2 and 3 had been raised, or had become exposed to denudation. The fluvio-marine strata and overlying loam of Menchecourt recur on the opposite or left bank of the alluvial plain of the Somme, at a distance of 2 or 3 miles.
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