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shelter for animal life, and bare of herb and tree. It was a country of flood and marsh, and of rivers wildly wandering at will; but it was also a country of forest growths unchecked by man, and of vegetable food for the hugest of land-animals. It was a place where herds of gigantic herbivora sought and found a home. It is, however, to a land-surface more scantily clad than that of the mammoth-haunted forest that the geologist aspires to take us back. The memoirs of the Thames transport him to the time when the land was as yet unclad with grass and shrub and sheltering forest tree. It was but slowly emerging from the ocean, as other islands are gradually rising from the waters to-day, to become the home of terrestrial life. It was rising from the waves to become the successor of an older world-surface, once as beautiful and vivid as this on which we live, but which reached its term of existence under the sun, and then was slowly submerged beneath the sea, from which it had as slowly arisen.
Let us then share with the geologist the vast perspective which Nature opens to us in familiar London landscapes. Let us ascend beyond the mammoth-period in the history of the Thames, and reach the beginning and rudiments of terrestrial history around London. The hills and valleys of our environs will enable us to become as it were contemporary with the origin of the Thames and the excavation of its valley, and the different climate of the England of the period.
The Glacial Clay of Muswell Hill and the Essex Heights.
On the summits of some of the hills on the northern side of the Thames, especially on the Essex Heights, we find a remarkable clay. This clay is the earliest record of the origin of the Thames, and the contemporary physical conditions of the Thames-country. Its composition and contents declare it to be of an icy or "glacial" origin. It contains a débris of gravel, and chalk, and rocks of older date, transported by the action of ice from distant areas. It is the last deposit that submarine England received on its surface before it arose from the sea. It is the last floor and layer which Nature added to the ground beneath us, before she raised it from the waves to
become a terrestrial dwelling-place. We find it to-day as near London as Muswell Hill and Finchley.* It tells us of the Arctic character of the climate into which the rudimentary land-surface of England arose from the sea. It takes us back to the time
When Britain last at Heaven's command
Arose from out the icy main,
for Britain had been alternately clad with an Arctic coating of ice, and had formed the floor of the submerging Glacial sea. As a seabed it had received on its bosom the freight of stones and sand and mud that melted out from the floating ice above, as the Atlantic floor is now receiving the droppings of floating icebergs. This Glacial Clay of the Essex Heights and Muswell Hill belongs to the last layers of sediment that our land received from the sea before it was raised to the sunlight to form a new terrestrial surface. We find it to-day in the eastern and midland counties of England, stretching at least 140 miles from London, and forming an agricultural soil. In Cambridgeshire and Huntingdonshire it still retains a thickness of 160 feet. So huge a deposit tells us something of the duration of the marine submergence of our island. It enables us to picture to ourselves the extent to which the drowned and buried world below, with its forests, watersheds, river-basins, and valleys, was overlaid with the Glacial débris.
The time at length arrived for the re-emergence of the land above
*Muswell Hill is celebrated for its Glacial drift. The gravel-pits here were found to be exceptionally rich in Glacial deposits. Just beneath the vegetable soil were blocks of granite, syenite, greenstone, coal measure sandstone, with its fossils, and other palæozoic rocks, and the wreck of chalk and oolite, confusedly mixed together.-Antiquity of Man, chap. ix. For the section, see Mr. WHITAKER, Memoirs to Sheet of Geological Survey. The gravel-pits are now within the grounds of the Alexandra Palace and are filled up. The Highgate and Edgware Railway runs through the Boulder Clay at Finchley.
The upper Glacial Clay, where it has most escaped denudation, still retains a thickness of 160 feet. It presents so uniform a character of slow and steady accumulation by the outspread of water-borne clay sediment, accompanied by the dropping of chalk-débris from ice, that there is but little room to doubt its having spread over the counties of Surrey and Kent as well as Essex, in great thickness. From its attenuation by denudation towards the north brow of the Thames Valley, a much less thickness than 160 feet remains in
that part. Quar. Jour. Geol. Soc., SEARLES V. WOOD, jun., F.G.S.
the Glacial sea. It was in this stage of Britain's terraqueous history that the land around London began to acquire its drainage system and landscape contours. The Glacial Clay was the new landsurface, out of which the contours of the landscape had to be shaped, and through which the rivers had to cut their channels. But during the long years of slow re-emergence, the rising land was wasted by the planing and scouring action of the waves, and
Swill'd with the wild and wasteful ocean.
Hence the young and oozy deposit of Glacial Clay, the latest floor which Nature had laid down as if for the new land-surface, was largely removed, and the lower floors of Bagshot Sand and London Clay were again laid bare, as we find them to-day. Thus the Glacial Clay, which once spread over the country in a vast continuous sheet, is now found only in remnants and patches yet enough remains to enable the geographer to compile an approximate map of the once persistent area.
The Glacial or Boulder Clay of Muswell Hill and the Essex Heights is thus the earliest record we possess of the origin of the Thames and the climate and physical conditions of the period. Through this icy sediment, the uppermost floor of the newly-raised land-surface, the Thames had first to wear a channel. Let us learn to look at it in our future rambles round London as a wonderful relic of the earliest ages of the landscapes around us, and as a clue to the beginning of terrestrial history in the country of the Thames. It should share with the mammoth pits at Ilford and Erith the keener interest with which we shall henceforth regard familiar scenes and resorts in our environs, which geology irradiates with so wonderful yet sober and truthful a light.
Map of the Early Thames-Country.
The valley of the Thames was begun to be formed while the emerging land was as yet an archipelago. In this early period, familiar hills around London were then islands. Harrow, Hampstead, and Highgate, and the Essex Heights stood up in isolation. above the surrounding waters. The past terraqueous conditions of
the environs of London have mapped themselves in the ground beneath us. They form a natural plan, figured and embedded in the land around London, and it is thus we may learn the different aspect of our environs in these early and genetic ages of the landscape's history.
Let us now look at a map on a larger scale, which we find figured to-day in Nature's own hand, and which gives us a wider view of the aspect of the early Thames-country. It will enable us to know for ourselves the physical conditions which would restore, even in these modern days, the marine and archipelagic condition of the basin of the Thames.
The picture shows the land again sunk to a lower horizon. sea invades the Thames Valley, and rises for hundreds of feet up its sides. The hills of Middlesex and Essex again appear as islands among the surrounding waters. An estuary miles in width prevails between London and Maidenhead. It narrows at Maidenhead into a tortuous course until it opens into the sea west of Reading (an arm of which in Glacial times is shown to have stretched northeast, and to have excavated the great Vale of Aylesbury as far as the isthmus of the period, which may still be traced at Hitchin).* Such has been the aspect of the basin of the Thames more than once since the great Glacial submergence of England. Such is the map which Nature has embedded in the ground beneath us.
Our first vision of the ancient Thames is that of a marine valley, up which an arm of the sea stretches to the narrow strait at Maidenhead, where it communicates with the sea which stretches away still further inland. The land was as yet only rising above the waters. So little had it emerged that the sea still held possession of the low-lying grounds in the heart of England. The present configuration of the country had but just began to appear
*In the library of the Geological Society of London are maps of the surveys of the post-Glacial beds of the south-east of England, by Mr. Searles V. Wood, jun., a gentleman who bears an honoured name in the
annals of English geologists. They give the successive aspects of the terraqueous surface as the Thamescountry arose from the waters.-See Quar. Jour. Geol. Soc. vol. xxiii. and Geol. Mag. vol. iii.
above the waters. Between islands and promontories, and up straits and inlets, the Glacial seas eroded their way, widening and deepening their course, and cutting their channels through the Glacial Clay. But the land was still rising. The inland waters were gradually retreating to the outlying depths. The channels by which the currents receded gave contours to the emerging land, and determined the seaward course of the future rivers.* The ebbing waters
With serpent error wandering found their way,
They wore depressions in the floor beneath, which in time became basins and valleys, and river-channels, and hydrographical areas. A larger tract of terra firma was gradually gained from the sea and exposed to the influence of rains and streams, of snows and ice, and all the agencies of a terrestrial scene. Thus, with the continuous elevation of the land, the Thames, from being a marine valley, became in course of time a freshwater river.
The complete excavation of that great valley of the Thames which we see in its latest form in the landscapes of to-day has a prolonged and eventful history of its own. The process begins with the date of the emergence of the land from the Glacial sea, and
*Did the course of the Thames originate with the emergence of the land from the Glacial sea? Or did it in post-Glacial times, simply re-excavate a valley which belonged to the surface of the former world? It might be thought that ""Twere to inquire too curiously to consider so!" did we not know that such questions are common in physical geology, and that there are some data for determining how far the re-emerged land reassumed its older contours and drainage-areas. Notwithstanding the countless thousands of years during which our land was moulded by the ice of a Greenland climate, or loaded as a sea-floor with the detritus of the
ice-wasted rocks, the principal physical contours of our country have suffered but little change by the experience of that vast period.-Lectures on the Physical Geology and Geography of Great Britain: Professor RAMSAY. During the long years of re-emergence the rising land would indeed be wasted by the planing and scouring action of the waves--but the newer and softer deposits, the sedimentary beds of the glacial sea, would chiefly suffer; the ancient and harder contours of the land being simply uncovered by the process. It is found, with rare exceptions, that the strata of England, including those harder formations through which the Thames