In fact, at the time of the Cromer and Kessingland. "rootlet bed," it is probable that much of the southern part of the North Sea area passed into the condition of a broad plain of dry land studded with large shallow lakes, like the "broads" of modern Norfolk. Such is the opinion which Mr. C. Reid was led to form from a study of the Forest Bed and its associated deposits; and the following remarks are quoted from his memoir:1-"The large number of mammals already known from the Forest Bed seems clearly to point to a connection with the continent; " but "both the fauna and flora, leaving out the large mammals and other extinct forms, are curiously like that of the broad' district of Norfolk at the present day; and this, like the rest of the evidence, points to a wide alluvial plain with lakes and sluggish streams, bounded on the west by a slightly higher sandy country covered with fir-forests and distant from any hills."

It is supposed that this plain was traversed by a large river coming from the south-east, as stated on p. 246, and that this river was no other than a continuation of the Rhine; a view first suggested by Mr. Gunn in 1867, but adopted and strengthened by subsequent writers. Thus Professor Prestwich, writing in 1871, remarks that on the table-land above the Meuse in Belgium there is a gravel of very similar character, and though, according to Mr. C. Reid, this contains veined quartzites of a character unknown in the Forest Bed gravels, yet the general similarity of the gravels suggests that they belong to one and the same system of drainage. Mr. Reid himself suggests that the fragments of Carboniferous slate and chert may have been derived from rocks that came to the surface as part of the old ridge which Mr. Godwin-Austen has described," and as the Ardennes are part of this ridge, it is difficult to see why he should have any hesitation in assenting to Professor Prest1 "Geology of the Neighbourhood of Cromer," pp. 60, 61.

66

wich's view: the Meuse was then in all probability a tributary of the Rhine, and it is to the Rhine itself that Mr. Reid refers the transport of the pebbles in question. In his memoir on the neighbourhood of Cromer Mr. Reid has given a sketch-map representing the probable position of the estuary of the Rhine during the formation of the Forest Bed. This is reproduced with slight alterations in Plate XIII., the limits of the sea at the close of the Pliocene epoch being shown by the boundary of the darker tint. It is assumed that the whole area of the North Sea was greatly contracted by upheaval from its previous extent, though whether this elevation included the Scoto-Icelandic area, and was sufficient to raise the connecting isthmus once more above the sea, is doubtful, but such a connection is shown on Plate XIII. because it is suggested in the sequel that a further elevation to this extent did supervene in the interval between the Pliocene and Pleistocene epochs.

OR a detailed account of the deposits of this period I

FOR

66

must refer the reader to my previous volume on Historical Geology," and to Mr. H. B. Woodward's "Geology of England and Wales." For my present purpose it is only necessary to give such a summary of what is known respecting the distribution and succession of these deposits in England, Ireland, and Scotland as may afford a basis for considering the physical and geographical changes which took place during the period.

The Pleistocene deposits are generally treated under the heads of Glacial and Post-glacial Beds by British writers, but, though this is a convenient division in dealing with the deposits of a limited area, it becomes misleading when those of a larger region are compared with one another, as for instance those of southern and northern England. The so-called Ice Age or Glacial Period must be regarded as a special episode or phase of Pleistocene time, and though its influence may be traceable over a large area of the earth's surface, yet the deposits which owe their origin to the direct action of ice are limited to certain regions within the 40th parallels of latitude in both hemispheres.

There is no doubt that what took place in the Glacial period was simply an extension of the glacial conditions which now exist in the polar regions. The fauna of the Pliocene Beds affords evidence of the gradual refrigeration

of the climate in that period, and there can be little doubt that glacial conditions prevailed in high northern regions while the Crags were being formed in the east of England. The ice must have been creeping southward for a long time before it reached the latitude of Norfolk and led to the formation of the remarkable accumulations which succeed the Pliocene beds of that district. Again, it is certain that ice lingered in Scotland, and in the mountain districts of England and Ireland, long after it had disappeared from the southern part of our country.

§ 1. Stratigraphical Evidence.

In our review of the Pleistocene period it will therefore be best to consider the whole succession of deposits which occur in different parts of our islands consecutively, so far as that succession has been ascertained.

Scotland.

Glacial Deposits.-Many attempts have been made to classify the Boulder-clays and gravels of Scotland, but no two writers seem to be quite agreed as to the precise sequence; some still speak of the Boulder-clay of Scotland, as if there were only one, and others are willing to believe in any number of such clays if only they are separated by fossiliferous sands or gravels. Mr. Jamieson, however, has shown that there are at least two distinct Boulder-clays,' and from Professor James Geikie's descriptions 2 we may infer that there are at least three sets of Glacial deposits. The oldest are Boulder-clays of the hard and compact kind which is locally known as Till; the stones which they 1 "Quart. Journ. Geol. Soc.," vol. xxi. p. 162, and vol. xxxviii. p. 160.

2 "Prehistoric Europe,” pp. 261 and 386; “Great Ice Age,” second edition, p. 216.

contain show that the ice under which they were formed radiated outwards from the main watersheds of the country, and they do not contain any intercalated marine deposits. Elsewhere, as in Lewis, Caithness, Aberdeen, Arran, and the central Lowlands, there are Boulder-clays which contain broken sea-shells and include sands and gravels of marine origin; these deposits have not yet been found for certain much above 500 feet, but Mr. Jamieson has described stratified drifts occurring at a much greater height. Lastly, there is a set of stratified deposits (soft sands, gravels, and clays) which contain some Arctic species of shells, but do not occur above the 100-feet contour line, and pass into ancient river-gravels above that level. These last appear to have been contemporaneous with the kames or eskers, and with the moraines of the later glaciers in the Highland valleys.

This succession of deposits is particularly clear in Aberdeenshire, where, resting on a rock-surface that is glaciated from west to east, lies a tough grey Boulder-clay containing stones which have clearly been brought from western localities, or derived from the rocks in the neighbourhood. This clay varies greatly in thickness, as if it had suffered from subsequent erosion, and is sometimes reduced to a layer of grey rubbish and boulders. Above this, or in its absence resting on a rock-surface that is glaciated from the south and S.S.W., is a red Boulder-clay containing stones which have come from localities to the southward. This clay is sometimes as much as 100 feet thick, and it is associated with sands and gravels which contain broken marine shells; it ascends to a level of 300 feet above the sea, and gravelly deposits containing similar stones occur up to 500 feet. Lastly, by the estuaries of the Tay and the Earn, there is a mass of stratified drift, the upper surface of which is almost flat and is about 100 feet above the sea; it maintains this level all the way from the coast to Dalreoch in