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QUATERNARY OR POST-TERTIARY.
POST-PLIOCENE OR PLEISTOCENE.
In considering the deposits of Post-Pliocene date that occur in different parts of England and Wales, one is immediately struck with their wide range and extensive distribution as compared with the other and older strata.
These deposits, known under the familiar name of Drift, have but lately received their due acknowledgment and rank as geological formations, being in many publications treated as superficial deposits, and as apparently belonging to quite another order of creation to the more ‘solid' rocks that underlie them. As these Drift deposits rival and even exceed in thickness some formations in our geological tables, and as, taken collectively, they have a more direct influence upon Agriculture than any other formation, their importance may be readily conceded. But I am well aware that in most cases it would be exceedingly difficult, if not impossible, to show these Drifts with any approach to accuracy on a map smaller than that of the oneinch Ordnance Survey, and upon that they are now represented by the Geological Survey of England and Wales. Moreover, the Drifts themselves play a comparatively insignificant part in the formation of our scenery, for the great features that our country now possesses were nearly all marked out before the Drifts were deposited.
The word • Diluvium' was originally applied to deposits supposed to have been formed or deposited during violent water action, such as would take place in periods of excessive rain-fall, causing torrents of water in the rivers, floods or deluges, or by the sea or rivers bursting through some bank or sea-wall. It is a term now little used, owing to the deposits termed Diluvial being frequently misunderstood to indicate a Universal Deluge. The term Drift' is generally synonymous, and would naturally refer to beds formed from rocks at a distance from the localities where they have been deposited. For instance '—to quote Conybeare (1822)
—when we find rounded pebbles derived from rocks which exist in situ only in the mountains of the north and west, scattered over the plains of the midland counties, we may be sure that the currents drifted from the former point to the latter; and it often affords a curious and interesting problem to the geologist to trace these travelled fragments to their native masses, often hundreds of miles distant. The accumulations of this gravel above referred to, in the midland counties, especially along the plains at the foot of the escarpment of the chain of the Inferior Oolite on the borders of Gloucestershire, Northamptonshire, and Warwickshire, are of surprising extent, and the materials brought together are from so many quarters, that it would not be difficult to form a nearly complete suite of the geological formations of England from their fragments here deposited. Portions of the same gravel have been swept onwards through transverse valleys affording openings across the chains of Oolite and Chalk hills, as far as the plains surrounding the metropolis ; but the principal mass of the diluvial gravel in this latter quarter is derived from the partial destruction of the neighbouring Chalk hills, consisting of flints washed out from thence, and subsequently rounded by attrition.'
In considering the relations of the different Drift deposits we enter upon a subject of great difficulty, and yet, perhaps, during the past ten or fifteen years no subject has received greater attention than the elucidation of their method of formation and respective ages.
The Drifts include deposits of loam, sand, gravel, and clay; they occur at all levels; some clearly owe their origin to the action of our present rivers, some to the agency of the sea, and some to glaciers, ice-sheets, and icebergs: some also indicate a combination of two or more of these agencies in their formation, dispersion, and accumulation.
That the Pliocene Period was succeeded by one of intense cold has been fully proved; the changes of temperature were gradual, but at the same time, looked at in a large way, this cold period can be well defined, and the deposits belonging to it are considered as of Glacial age. Thus, in speaking of Glacial deposits, it must be remembered that the term refers rather to age and climate than to method of formation.
It must be borne in mind, however, that this Glacial Period, or “Great Ice Age,' was not necessarily the most important Glacial period that has occurred during geological times. Mr. Croll has stated his opinion that these periods of intense cold were periodical: we have indications of transport by ice agency in the Chalk period ; and in Permian times, as Prof. Ramsay has shown, we have true Glacial deposits. The deposits, however, that evidence the Post-Pliocene Glacial period are the most extensive of the kind, and their preservation is due in a great measure to the fact that Post-Glacial changes have not been of sufficient magnitude to allow denudation to obliterate them. The same may be said of the extensive river-gravels and brickearths.
Mr. James Geikie has well observed that, even now, the action of the weather, of frost, rain, and rivers, is slowly but surely effacing the marks left by the old glaciers. And should our islands eventually become submerged, it might well be that, as the land sank down, what the atmospheric forces had failed to obliterate would succumb to the action of the sea; and should the land be afterwards re-elevated, it is very doubtful indeed whether a single recognizable trace of former Glacial work would remain.
In the following pages I shall, after describing the nature of the deposits, only indicate their distribution in a very general manner.
Some of the Drift deposits may indeed be of earlier age than the Glacial period, for if we believe that a considerable mass of the Chalk, which must have covered the greater part of England and probably the whole of Wales, was removed by marine denudation, and some of the Tertiary beds are made up entirely of flint pebbles, it is not impossible that some of the plateau-gravels whose age is as yet undetermined, may be due in part to marine denudation of the Chalk in Tertiary times. Moreover, the deposit called Clay-with-flints may be of all ages subsequent to the consolidation of the Chalk and its elevation to form a land area.
The Quaternary phenomena and deposits may be roughly classed as follows:
Sub-aërial.-Soils, Peat, Submarine Forests, Caverns and Caverndeposits, Springs and Tufa, Blown Sand, &c.
Fluviatile and Lacustrine.- Valley Gravels and Brickearth.
In the present state of our knowledge, it is quite impossible to classify these various deposits exactly according to their relative ages. Some Caverns may be exceedingly old; in fact, their formation might commence soon after the consolidation of the rocks in which they have been excavated. The deposits met with in them may, so far as the physical evidence is concerned, be of many very different ages.
1 This is only given as a very general classification. ·
Some of our valley-deposits may (it has been conjectured) be Pre-Glacial, but it will be sufficient to indicate this opinion without attempting to classify the gravels and brickearths formed by different rivers.
There is no question, however, about the antiquity and relative position of most of the deposits of Glacial age, and of these we may first treat.
In places it is difficult to distinguish between glacial gravels and river gravels.
• Considerable accumulations of Auviatile gravel occur in the valleys, often at great heights above the present rivers. This gravel, when traced up stream, becomes coarser and earthier, and not a few of the stones even show faint traces of striæ. As we follow it still farther into the mountains, it appears to pass into, or at least it cannot be distinguished from, moraine débris. Opposite the mouths of some of the mountain valleys, great deposits of hummocky angular and sub-angular gravel make their appearance. (J. Geikie.)
The Glacial period, so called, has itself been marked by many changes, the duration of which geologists can only speculate upon.
Upwards of 200,000 years ago the earth (says Mr. James Geikie), as we know from the calculations of astronomers, was so placed in regard to the sun that a series of physical changes was induced, which eventually resulted in conferring upon our hemisphere a most intensely severe climate. But the observations of geologists, comparing the past with the present, had many years ago led them to infer such a period of cold.
In the mountainous districts the direct action of former