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:1 Sub-aërial.-Soils, Peat, Submarine Forests, Caverns and Caverndeposits, Springs and Tufa, Blown Sand, &c. Fluviatile and Lacustrine.-Valley Gravels and Brickearth. Marine. Raised Beaches, Burtle Beds, Shingle-beaches, &c. 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 fluviatile 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.) GLACIAL PERIOD. 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 glaciers may be traced; Blocs perchés are found in most valleys in the Lake District and North Wales, while grooved and polished rock-surfaces and striated stones have been detected in many places. In some of the low-lands are deposits. of distant origin which cannot have been accumulated by rain and rivers, nor have been formed unaided by marine action. In 1831 Mr. J. Trimmer detected traces of stratified drift containing marine shells at a height of 1,300 feet on Moel Tryfaen, which indicated a submergence to this extent in Post-Pliocene times; and in 1842 Dr. Buckland recognized the former existence of glaciers in many parts of North Wales. Since this period the subject has been very extensively studied, and the general conclusions may be stated to be, that during what is called the Glacial period the northern parts of England and Wales were covered with ice;-(to quote Prof. Ramsay) the average level of the land may then have been higher than at present, by elevation of the whole, and a little because it had not suffered so much degradation: but whether this was so or not, the mountains and much of the low-lands were covered with a universal coating of ice, probably as thick as that in the north of Greenland in the present day.' Small glaciers were locally formed on the Snowdonian and Cumbrian hills. During the time that these great results were being produced by glacial action, there were occasional important oscillations in temperature, so that the ice sometimes increased and sometimes diminished, and land animals that lived habitually in more temperate regions, at intervals advanced north or retreated south with the retreating or advancing ice. At length, however, a slow submersion of the land took place.' During the time when the lands in North Wales and the north of England were covered with ice, it has been conjectured that the southern tracts of Wales and England were submerged, and then material extruded by the glaciers was dispersed over the submerged country, while ice-bergs may have given their contributions to the drifts that are scattered over the country. The subsequent period of greater submergence, which in Wales was more than 1,300 feet, was attended by the dispersion and deposition of more material. Again the land rose, and glaciers were formed in North Wales and in the Lake District. Those who visit North Wales should carry Prof. Ramsay's 'Old Glaciers of Switzerland and North Wales' in their pockets. With this guide the many obvious facts will be understood even by those who have never seen a glacier-the old records of glacial action are as certainly marked on the rocks and in the valleys, as is the hand of man in the hieroglyphics on ancient monuments. This is but a feeble sketch of the great changes indicated during the Glacial period, but it would be impossible here to do justice in so small a space as must be allotted to the subject, and those who would pursue the subject further should consult Mr. James Geikie's Great Ice Age.' It may, however, be useful to give the following summary by Mr. Geikie of the great changes that occurred over the British Islands in the Quaternary period :1- 1. Indications of approaching cold are met with in the Newer Pliocene deposits. 2. Intense glacial conditions, with great confluent glaciers; intermediate mild, and warm periods; arctic and southern mammalia visit Britain alternately, according as climatal conditions become suited to their needs. 3. Disappearance of arctic conditions. Britain continental, with climate changing from cold to temperate and genial, and again to temperate. In early stages of continental conditions the arctic mammalia invade Britain; subsequently these disappear, and are succeeded by the hippopotamus, and other southern forms. 1 This Summary, which has been kindly furnished to me by Mr. Geikie, represents the views which are set forth in the New Edition of his work. |