ABOUT the year 1854-5 extensive borings of the Trou-rocks in the neighbourhood of South Shields, previous to quarrying the Magnesian limestone of that locality for the new pier works, afforded me an opportunity long-desired of examining extensively the character and appearance of the rock-surface under the superficial covering of clay and sand which, at this place, covered the royalty to the depth of five or six feet or more. For a long time my searches for indications of the true rock-surface were unsuccessful, owing to the circumstance of the denudation of the clay covering in that portion of the quarry first worked, and the consequent defacement and obliteration of the original surface. But as the works proceeded, and the thickness of the clay-covering increased, I was soon gratified with a proof of the former glaciation of nearly the whole of the rock-surface, and the existence upon it, in many places, of a rough angular gravel, and also of large rounded blocks of polished, striated, and scored Mountain-limestone, Millstone-grit, Sandstone, and Basalt; the latter all derived from the higher lands of the county; from the district between the Pennine Range and the East-coast.
On observing the direction of the striæ, and the inclination or slope of this glaciated surface within short distances, it was then for the first time suggested to me that these appearances could not be explained by the opinion generally entertained by English geologists of the stranding of icebergs, and the abrasion caused by large masses of floating ice charged with boulders slowly moving over and grazing the rock-surface in the orthodox direction of N.W. and S.E., for here, within a few hundred yards, were glaciated rocks with the striæ, indicating the movement of the glaciating-agent to the N., to the N.E., and the E.; and not only the most elevated points had been ground down, but the slopes and sides of hollows; and masses of rock projecting above the ordinary level also showed the glacial markings to greater perfection on their sides than on some of the bigger points. There was also added, to strengthen this conviction, the fact that the materials accumulated in the extensive deposits of boulder-clay at the mouth of the Tyne could be traced to the higher parts of that river, and that all the extensive accumulations of this material, further to the South, contained specimens of Magnesian_ limestone, polished and glaciated, derived from the Magnesian-limestone plateau; such limestone never occurring in the boulder-clay. at the mouth of the Tyne, or on the Northumberland coast, except near the outlier of the Magnesian-limestone at Whitley.
It therefore appeared certain, from these facts, that the moving ice, bearing these special boulders which can be traced to their original beds up the Tyne, must have travelled from the West; from the higher ground towards the coast; and that the hypothesis of continental- or land-ice, involving, as it did, the subsidence of nearly the whole island, and afterwards an immense change of climatal conditions to those at present existing, seemed more in accordance with the facts registered on these rocks, and absolutely necessary to explain the appearances of the glaciated surface, and the formation and deposition of the boulder-clay, than the vague and merely-conjectural theory of the stranding of icebergs drifting southwards from some unknown northern locality, and bearing rocky materials not belonging to the countries whence they came, but identical with those to which these hypothetical icebergs had drifted.
The superficial deposits resting on the rock-surface of these counties are so closely connected with the theory of glaciation by continental- or land-ice, that a short account of them is necessary to elucidate this theory.
1. Beds of peat, and submarine forests with fossil remains of oak, alder, mountain-birch, and hazel; horns of Cervus Alces and Cervus Elaphus, Bos primigenius,
2. Rubble transported from moraine heaps of upper valleys.
3. Gravel beds, forming remains of ancient, raised-beaches.
4. Sand, forming elevated mounds along the courses of valleys.
5. Brick clay, with intercalations of laminated-clay, sand, and peat-bed containing skeleton of Megaceros Hibernicus, and stems of Calluna vulgaris.
6. ?Scandinavian drift, containing angular flints, and small fragments of rock, probably derived from boulder-clay.
7. Boulder-clay or drifted, glacier-moraine containing fragments of Cyprina Islandica.
8. Ancient gravel bed resting on rock-surface.
The most important of these, and that which generally rests on the rock-surface, is the covering of clay, of variable thickness, filled with fragments of rock, from the smallest grain to blocks and masses of five or six tons weight. This deposit is also most extensively distributed, having been spread not only over all the lower portions of Scotland and the North of England, but as far South as the chalk range. At the present time its presence in these counties is limited chiefly, though not entirely, to the old valley channels, depressions, and denudations of the rock-surface. Traces of it, showing its former deposition, are visible on all the higher elevations that occur on the coast line, and on the tops of the highest hills of the Magnesian-limestone terrace, but on these heights it is more generally represented by large Mountain-limestone or whinstone boulders than by a regular bed. On the coast line of this immediate neighbourhood, it forms the cliff from St. Mary's Island to Whitley Terrace. The thickness of the deposit is here unknown. In one place near Whitley, the hard sandstone rock on which it rests was ground flat, and strongly grooved and striated with fine parallel striæ. The most conspicuous section, and the one most worthy of notice, occurs between the Low Lights and the Spanish Battery. It rests here on the northern side of the great, Tyne valley, which extended for two miles from the latter locality to the Trou-rocks : it is of an unknown depth. Very thick deposits of it form the sites of the towns of North and South Shields, and the district of Jarrow Slake; and, not to multiply instances, the greater portion of the town of Newcastle is built upon it, of which we have full proof by the number of boulders thrown out in the clay when a sewer, a well, or a railway cutting has to be made. This Tyne valley boulder-clay deserves special mention, because not only is it thickest in this the deepest valley of the district, but the fragments deposited at the present mouth of the river are so large as to have attracted the attention, not of geologists only, but of ordinary observers; and so numerous were the blocks formerly washed out of this deposit, and accumulated on the Black Middens, that it was necessary for the safety of shipping entering the river to have them removed. In one part of this section, near the Spanish Battery, the sandstone underneath the clay is distinctly ground clown, but the soft nature of the rock-surface exposed under the other portion of the cliff has not registered any traces of glacial markings.
In one portion of this section, under the Old Barracks, may be distinctly seen an old deposit of gravel, formed chiefly of annular Magnesian limestone, quartz, granite, and porphyry, very little water-worn. The boulder-clay is here from forty to fifty feet in thickness, and is charged with fragments of rocks of all sizes up to several tons weight. The matrix of this deposit is generally a tenacious, stiff clay, through which the smaller boulders, pebbles, and sand, are indiscriminately distributed; but the larger blocks are arranged in more regular order, as if deposited periodically, though scarcely approaching to a regular stratified arrangement. Some of the small fragments and blocks are perfectly angular, as they were on the day they fell from their proper bed, but others, small and large, have been ground and rubbed, and scored and polished, and striated in a manner not to be described by words, but which some of the specimens exhibited will enable any one better to comprehend. These specimens not only show that numerous fragments of different qualities of coal and shale, both which occur in small pieces, and are often beautifully striated, but also that occasional pieces of Cheviot porphyry, quartz-pebbles, and fragments of granite, derived, probably, the two latter from the Old-red-sandstone conglomerate of the North Tyne valley, and the former from the valley of Reedwater, are also presented to us in these rocks. But the most abundant blocks, and those that are best polished, striated, and rounded, are of Mountain-limestone, and it is this rock that forms the most extensive beds in the valleys from which these blocks are assumed to be derived. Immense blocks, also, of coarse Millstone-grit, and whinstone or basalt from the Great Whin-sill, are too numerous to reckon. The iron-ore and fossils from the Mountain-limestone series-the grits and sandstone of the Millstone-grit formation-the shales, coals, ironstone, and shellbands of the Coal-measures, are all represented in this typical deposit at Tynemouth. Lately I have been so fortunate as to detect in this boulder clay small pieces of a marine shell Cyprina Islandica, which is, I believe, the only Tertiary fossil that has been found in this deposit.
Proceeding southwards we find another thick deposit of boulder-clay, stretching from the coast near Whitburn nearly to Roker, or in other words filling up the denudation between the Cleadon and the Fulwell Hills. Near Fulwell its thickness, in probably not the deepest part of the valley, has been proved to be about 66 feet at the Fulwell water-works. Further to the South the boulder-clay occupies the coast-line from the North side of the Wear to a distance of more than a mile beyond Hendon. In the two latter localities, in addition to the materials enumerated in the Tynemouth section, there are also numerous fragments of Magnesian limestone, indicating very clearly the direction of the transporting agent. The fragments of Cheviot porphyry also appear to be more numerous than in more northern localities. As it is quite impossible to enumerate all the localities where this boulder-clay occurs, we may, having drawn attention to the most conspicuous examples occurring on the coast-line of this neighbourhood, mention that it is generally present in all the denuded valleys that run across the Magnesian-limestone terrace, and is seen in cliff section in many places along the coast and near the mouth of the Tees. Great quantities of fine gravel and beds of sand are often intercalated in the clay of this deposit.
The boulder-clay passes up in many sections on the coast-line into a much purer deposit of tenacious clay, containing few or no pebbles or fragments of rocks; but in a few localities the deposit succeeding the boulder clay is remarkable for the numerous angular fragments of chalk-flints which it contains. It is this deposit which covers the rocks to the depth of five or six feet at the quarries near South Shields as above mentioned, and the same beds appear also in the same relative position on the North side of the Tyne near Tynemouth. It is also found reposing on the boulder-clay in the neighbourhood of Whitburn. The nearest places from which chalk-flints can have been derived are South from the neighbourhood of Flamborough Head, West from Giant's Causeway, North from the neighbourhood of Aberdeen, and East from Denmark; and it is to this latter locality that in all probability these flints will be traced the deposit in which they occur appearing to be an extension of the great Scandinavian drift to this Eastern coast of England.
In the beds of clay, stratified beds of sand become more numerous, and in many places-especially along the river valleys-immense deposits of this material, containing stratified lines and false beddings of small coal, form the most conspicuous physical feature. Perhaps there is no place in these counties where this peculiar deposit is seen to greater advantage than in the neighbourhood of Durham. Some of the sections of the " Wash " paper show the thickness of this deposit in this interesting neighbourhood in a clear manner.
In a few localities, on the flanks and at the base of some of the eminences of the Magnesian-limestone terrace, beds of rounded gravel, with rocks derived from the boulder-clay, are deposited in beds round the base of the cliff. Along the coast, near Seaham and Hawthorn, deposits of this kind occur. A remarkable instance of one of these ancient beach formations was exposed on the North escarpment of Fulwell Hills. This has been carefully described by Mr. Kirkby, who has kindly furnished a diagram showing the position of this old cliff and elevated beach, and its relationship to the boulder-clay. A corresponding deposit occurs on the opposite escarpment of the Cleadon Hills, in the form of a bed of gravel, which underlies the village of Cleadon.
An elevated beach, composed of fragments of Magnesian-limestone chiefly, with remains of flints and also pebbles from the boulder-clay, interstratified with beds of sand and grit and a very fine glacial silt, occurs in a section lately made in the Castle-yard at Tynemouth. It contains also the remains of a marine shell, Cyprina Islandica, some fragments of which have been also detected in the boulder-clay. This beach deposit rests on a thin bed of clay with pebbles, which appears to be a re-construction of the boulder-clay under which is the proper rock surface. It reposes on a slope to the west, and contains, as before stated, materials derived from some Magnesian-limestone cliff-from a cliff, in fact, which must have been situated considerably East of the present line of coast. Other deposits of this kind occur in many other localities in the county of Durham.
It is well known to the members of this society that the whole of the South-eastern part of the county of Durham is covered, to a great depth, with mounds, or heaps of coarse gravel, rubble, and sand; some of these forming hills of remarkable conformation, and the whole deposit concealing the rock-surface over nearly the whole extent of that district. This brief description of most of the superficial deposits will help to illustrate the theory I wish to explain. The origin of these different deposits, and the mode in which they have been accumulated and arranged, we hope to show as we proceed.
In as brief a manner as possible I must now mention the localities in these counties where glaciated rocks have been observed.
The first instance I find recorded in the Tyneside Transactions, Vol. L, p. 273, by our lamented friend Mr. Loftus. He mentions in this paper, " that during a geological excursion in the neighbourhood of Belsay, he was so fortunate as to obtain an opportunity of seeing a surface of the stone exposed to view, over an extent of about half an acre, which had previously been covered by a quantity of earth and rubbish. The strata appeared to dip at an angle of nearly thirty degrees, and on the lowest part of the quarry a bed of detritus rested, upwards of twenty feet in height. My attention was soon directed to the extraordinary appearance of the exposed surface. In the direction of the dip throughout, it was deeply scored and scratched by longitudinal and nearly parallel lines of various width-so deeply, in fact, that on standing at the bottom of the quarry, and looking upwards, one could see the surface irregularly furrowed in a transverse direction, which was evidently occasioned by the depth of the longitudinal grooves or scratches, which had acted with more or less force on different portions of the surface. Many of the furrows were six inches in depth. On looking around in hopes of finding some explanation of the cause of the phenomenon, we perceived that the mass of earth and rubbish resting on the lowest part of the quarry was filled with stones of various sizes-from a yard in diameter to a few inches. Lying on the surface of the quarry were several large blocks, of too great size for the workmen to remove, and which were consequently left remaining there to be broken up. The whole of these ' had their edges and angles completely worn down, and were scratched on all sides and in every direction, and were frequently also polished; evidently a proof that in causing the scratches and grooves on the surface of the quarry, they had likewise mutually rubbed and ground each other to the state in which we found them. As these boulders are, I believe, all of the same limestone as the quarry, and similar beds among the Cheviot Hills, we may fairly presume that they have not been transported from any considerable distance."
The next recorded observation of glaciated surface is by the eminent geologist, Mr. Tate, of Alnwick, in the same volume of the Tyneside Transactions, Vol. I., p. 348, as follows:
"The polished and scratched surfaces now to be described were found in a limestone quarry on Hawkhill farm, belonging to Earl Grey. The rocks in this neighbourhood belong to the Carboniferous or Mountain limestone formation. Tile section presented at the Hawkhill quarry is as follows, beginning with the uppermost bed:
|
Ft. In. |
1. Red tough clay with large and many smaller boulders |
12 0 |
2. Five beds of blue Carboniferous-limestone with thin shale partings |
19 0 |
|
|
"The quarry is situated on the high ground which runs nearly parallel with the coast, and which has apparently been elevated by the basaltic protrusion. The slope is towards the river Aln, and on the opposite bank of the river, nearly a mile distant, a similar clay bed to that which lies on the top of Hawkhill quarry is found, but at a lower level.
Now, immediately below the red tough clay the surface of the limestone bed is polished, scratched, and grooved. An area, twenty feet by six, has been bared in this state; and the same polished and scratched surface appears to extend under the clay. One part of the surface was flat and even, presenting a smooth bright face, like marble artificially polished ; other portions were rounded and undulating, but still exhibiting the same mirror-like polish. One part, in particular, was one foot below the general level; but in this and similar cases the angular corners of the higher portions were removed, and a smooth and rounded outline was formed. It is important to notice that the polishing of this surface is very different from the rounding and smoothing of rocks; arising from their attrition on each other by the driving action of tides and currents.
Besides being polished, the Hawkhill limestone was more or less scratched, the scratches varying both in depth and in length, some being very fine striae, and a few being grooves one-quarter of an inch in depth.
These grooves were parallel, one inch apart, and from six to twelve inches long. Many of the scratches were 1-10th of an inch in breadth, and from one to six inches in length, having a general direction of from N. to S., pretty nearly in the dip of the quarry; but there were also other scratches, several being broad and deep, which were more or less oblique to the general direction; those on the rounded corners of the higher parts of the surface had a tolerably regular direction of from N.W. to S.E. Notwithstanding, however, the exceptional cases, the general direction of the scratches, when observed over the whole surface, could not be mistaken.
The appearances described are undoubtedly connected with the boulder formation of the district; for in the bed of clay above the polished limestone there are polished blocks and fragments. • A large block of limestone, measuring three feet long by two feet broad and two feet thick, embedded in the clay three feet above the limestone bed, was scratched and polished on its under surface, the scratches having a direction as the stone lay of from S.E. to N.W. ; this block was not rounded like a water-worn stone. Smaller polished and scratched rocks are numerous on the clay near to the limestone bed; but the number of such polished fragments proportionally diminishes as we ascend higher in the clay deposit. In Scotland, in the Isle of Plan, and other parts of England, and also in Scandinavia the same connection is manifested, where the polished and striated surfaces are, there is also the boulder formation."
Much other interesting information is contained in Mr. Tate's valuable paper, and also the mode in which the glaciation has been caused is carefully discussed. The theory of glaciation by glaciers is objected to, and the more generally received one of glaciation by the stranding of icebergs is advocated.
Since the year 1855 repeated opportunities have occurred to me of observing, during the process of baring the Magnesian-limestone rock at South Shields, extensive portions of glaciated rock-surface extending over several acres. The phenomena presented to view here are much the same in detail as those observable in the Mountain-limestone district; but the more extensive exposure of the surface has given greater interest to the observations.
The rock-surface at this locality presents a somewhat uniform level for several miles inland, extending with slight undulations to the foot of the Cleadon Hills, which rise up rather abruptly from this extensive flat, and with a strongly marked ancient cliff-line. There is a very gentle rise from the interior valley towards the coast at this place. So far as the excavations have proceeded these rocks are covered with a deposit of coarse clay, harsh-looking, broken up into prisms, and of a cold bluish colour. containing fragments of sandstone, quartz, porphyry and angular flints. It contains no boulders nor large pieces of rock, but the former are very numerous immediately beneath its under surface. It is generally about six feet thick, and the upper surface presents, when uncovered from the blown sand resting on it, a remarkably level appearance. The rock surface under this deposit of clay, though showing a general level is, from the variable nature of the limestone and the destruction of the surface by the erosion of water through the bed of clay, somewhat irregular. In the cavities are left small accumulations of true boulder-clay of the usual appearance, large Mountain-limestone and whinstone-boulders glaciated very strongly, and patches of moraine gravel and silt. Over one part of the rock-surface the gravel and blocks formed an elevated lengthened mound for many yards. The rock-surface on the South side of this mound was conspicuously and strongly glaciated, the direction of the striae being East, and the surface gently sloping towards the sea. The glaciation continued onwards close to the extreme edge of the cliff. On the other side of the mound the glaciated rocks, following the slope of the surface, showed the striae running to the N.E. Further westward the striae have a North direction, or towards the bed of the Tyne valley, Not the highest points showed the glacial markings to greatest perfection, but the sides of depressed surfaces, the rising edges of which were always more distinctly striated and polished than the more level portions, at least, these markings are best preserved in such positions. Rocky masses projecting above the general level, roche moutonné, were beautifully polished and grooved all round. One side of a large specimen of this character is deposited in the Newcastle Museum.
At the northern extremity of the denudation between the Cleadon and Fulwell Hills, on the coast near Whitburn, conspicuous striations of the rock-surface have been observed which are the more interesting, as at this locality, those peculiar conglobated forms of Magnesian-limestone form the surface rock. Nevertheless the upper surfaces of some of these are ground clown and scratched in the direction of the valley in a very conclusive manner. The deposit resting on these has more the character of glacial-moraine than in some other localities. It is composed chiefly of fragments of Magnesian- and Mountain-limestone, etc., the former predominating, also shale, coal, and porphyry. The boulder-clay is at this point distinctly capped with a bed of clay containing flints, the upper part of which is finely laminated, and containing intercalations of sand with false-beddings of coal very finely and distinctly exhibited.
In the South, near Ryhope, the Magnesian-limestone is seen to be distinctly grooved and rubbed down where the clay and sand have been removed from it, the direction of the striae following the natural slope of the locality.
The glaciation of the rock-surface under the boulder-clay at Tynemouth and Whitley has already been mentioned. More recently glaciations have been observed in the quarries in the neighbourhood of Belford, by Messrs. Tate and Douglas, presenting much the same appearances as before described. In the valuable paper on the"' Wash" deposit of the Wear valley, the authors distinctly state that the surfaces of some of the harder sandstones, forming the side of this valley, are scored and scratched as if by some heavy body transported over them.
The fine specimens of glaciated rocks belonging to this Institution, now deposited in the Museum of the Natural History Society, show that further to the West, at Ryal, the rock-surface, where it can be examined, bears the strongest evidences of glaciation. These specimens, which are ground down perfectly flat, are reported to have been obtained from a tunnel, the rock-surface of which was covered with 00 feet of superficial deposits. Under this deposit, the rock-surface was, when examined, as level as a slab of polished marble.
Many of the physical features of these counties must have struck most observers with their peculiarities. The Magnesian-limestone plateau with its ancient terrace lines; its rounded hills, sometimes almost conical; its valleys of denudation, hopes, as they are called ; its own peculiar surface-drainage, in the form of deep secluded denes; its abrupt, irregular escarpment, stretching to the south-west, through the county of Durham, and forming one side of the ancient glacier valley through which flows the Wear; all these features must have been noticed by most observers. And, standing on the edge of this elevated, limestone escarpment, and looking across this broad Wear valley to the elongated, swelling, elevated sandstone ridges to the West; ridges due to the harder material of which they are composed, who can fail to observe the generally flat appearance of the upper Coal-measures deeply denuded, and broken only by those long swelling ridges of sandstone.
The Millstone-grit rears itself as a huge barrier when in its most characteristic form, and crowns the summits of the highest hills. The Mountain-limestone district, traversed only by few, has its own peculiar features. These have more the character of Alpine valleys and miniature river-basins. Standing on any of the heights of this elevated district, it does not require much reflection or comparison to comprehend the agencies that formerly were busily at work excavating the present river channels, and converting the excavated materials for future use, and carrying them onwards into the plains below. The similarity of the heads of some of these valleys to those of more mountainous districts cannot be overlooked, and there can, we think, be little doubt that the agencies by which they have been scooped out have been identically the same.
The phenomena presented to view in countries still under the process of glaciation, and more especially those districts of North America so long visited by our own intrepid seamen in search of a N.W. passage, Greenland, Iceland, Spitzbergen, and the Scandinavian peninsula, are best fitted for comparison with our own country, being more congenially situated, and more on a level with the sea. Elevated Alpine districts in lower latitudes, though presenting similar phenomena, are scarcely so eligible for comparison with a country so flat as our own, and situated also in a much higher latitude, as they represent only the extreme end of river valleys, and do not, now at least, protrude themselves into the lower grounds of the country and never reach the sea-coast; also, they pass over steeper inclinations, and the glaciation is limited to the sides of the glacier valleys. The reverse of this is the case in the Arctic regions, where many of the larger glaciers discharge themselves into the sea, and the inclination of their channels is very trifling compared with those of more southern mountainous districts. Also, the accumulations of snow are so great that the slopes and elevated ridges of land that border the valleys are covered more or less with thick accumulations of ice or half melted frozen snow, forming enormous masses of land- or continental-, ice, which move forward towards the valleys or sea-coast, and glaciate the rock-surface over which they move.
Most of the moisture that falls from the atmosphere in the Arctic regions, whether in summer or winter, is in the form of snow. In the latter season, so abundantly is this material deposited that the whole surface of the country, inlets, bays, and seas, are concealed under it. With the change of the season, and especially on slopes exposed more directly to the sun's rays, this covering gradually disappears. When entirely thawed, it is drained off over the rock-covered surface of the country, or it is only partially thawed, and then, in the form of névé, or in plain English, “sludge," it subsides into the hollows and depressions of the surface, to be afterwards more solidly frozen. On the more elevated ridges, and in the higher depressions of the land, this half frozen snow or névé accumulates year after year, and by its increasing weight a pressure is formed towards the lower parts of the country, and any lower surface on the. boundary of this accumulated half-frozen mass eventually becomes, by accumulated pressure from the higher portion, the natural outlet or channel for the accumulated mass. In this manner, by increased pressure from accumulation above, the partially frozen mass gradually extends itself forward. It removes, according to its power, any resistance from the rock-surface it may meet with, widening or expanding itself where any broad depression occurs in its course. It fills up enclosed valleys, and gradually rising up above the level of their boundary, seeks for itself a new outlet, and pushes its icy mass forward and onward until it reaches the sea-coast, where enormous masses-the accumulations of centuries-are urged forward into the sea, forming perpendicular cliffs of ice, until the depth of water is sufficient to float them away into a new mode of existence. During the slow motion of this gradually-increasing mass of moving ice from the higher districts to the shore, it has not only lost much of its original composition, from the gradual wasting of its under- and upper-surfaces during periodical changes of temperature, but it has also been much increased in its progress by incorporating into its mass much of the annual deposits of snow. It has also been burdening itself with mud and rubbish, and enormous fragments of rock, which are annually loosened from its boundaries and discharged on to its surface by the powerful agencies of cold and heat, or frost and thaw. These rocky fragments, strewed over its surface and embedded into its sides, or deposited in some instances as long lines of rubbish, extending along its entire length, together with the sand and gravel deposited on the sides of its bed, give to the glacier its power to wear away and enlarge, and also to polish, scratch, and score the surface of the channel over which it passes. And not only the rocks in shit are thus marked, but the fragmentary materials embedded into the icy stream are ground and polished and striated in a similar manner. It is unnecessary to suppose that the channel or bed of a glacier must be much sloped or inclined, or that a glacier valley must be perfectly level; like running water, if it meets with obstructions, it accumulates till it rises above or overpowers them, and likewise it does not hesitate to throw itself over a precipice or down a steep inclination, if such should come in its way. But the glaciers of Polar regions are, from their longer course and excessive thickness, generally less precipitous than those of Alpine districts, and preserve the even tenour of their way from the mountains which gave them birth to the sea, into which eventually they are launched, and in which finally they disappear. But in Arctic lands, not only are most of the lower valleys, glacier channels, but much of the flat country becomes covered with accumulations of continental-ice or coast-glacier, which are gradually pressed downwards, either to glacial valleys, when it becomes incorporated with it partially, or to the sea coast, where it is precipitated over the headlands on to the coast-ice below : such masses of ice carry with them detached rocky fragments, and by their aid polish and striate the surface over which they pass. There is always a strong current of water flowing underneath the glacier, which periodically increases in quantity, and which on such occasions is generally very muddy and turbid. The melting of the snow on the surface of the glacier also forms another system of waste and drainage. Most glaciers are crevassed or fissured transversely, either from compression of its mass within narrow limits, or by its passage over an uneven surface, or, when pushed into the sea, by the rapid erosion of its under surface. There are many other interesting details connected with the movements of glaciers, but the above particulars serve, perhaps, to illustrate the kind of agency by which our district has been formerly glaciated, and the manner in which the striated and polished boulders have been . formed, and also the mode in which the materials forming our boulde rclay have been produced.
It was stated before that the depth of the superficial deposits at the mouth of the Tyne are of unknown thickness; but there are a few sections up the Tyne valley, which, by showing the thickness of this deposit, also indicate that the land was formerly much higher than at present. In the 11 Wash " paper, the authors state that at the mouth of the Team valley the thickness of the deposit was 140 feet below the present high water level. There is an old section of the Percy Main Engine Pit, nearer the present mouth of the Tyne valley, dated 1800, in which the superficial deposits, consisting chiefly of boulder-clay, are estimated at 183 feet. This fact and proof of the former elevation of the land being much higher than at present, is important, for it is necessary, in order to account for the deposition of the boulder-clay, and the beds of clay, sand, and gravel which occur all over these counties, to assume that the whole, or nearly so, of the entire island was submerged beneath the sea. It was during this period of submergence that the boulder-clay, which we have so many opportunities of inspecting, was deposited. We also infer that it was during the period of submergence that portions of the glacier of the Wear valley overlapping, or breaking through its time honoured boundary, sought a shorter course to the sea through those numerous valleys of denudation which we see along the Magnesian-limestone escarpment. Can we doubt, when we look at the surface of this county from any considerable eminence, that the numerous isolated hills formed, at this period, numerous small islands, which were in their turn submerged, and covered with deposits still brought down from the west on floating masses of ice detached from the glacier face of the upper valley ? Only one or two points along the whole extent of the Pennine range seem to have been exempted from this submergence,-Cheviot, perhaps, and the much-terraced Crossfell, higher than Cheviot, and two or three points further to the south. Should anyone question the fact of this submergence, we ask how were the large blocks of Shap granite brought across the Pennine range into the valley of the Tees? The block in the North Street, Darlington, being one of these. How does it happen that we have Criffle granite distributed among the most superficial deposits of the valley of the Tyne and the Wear? It is impossible to answer these questions satisfactorily, unless we admit the subsidence of nearly the whole of the Pennine range.
If the subsidence be admitted, as we think it must be, the re-elevation is evident; but, during this re-elevation, extensive modification of the formerly submerged surface, must have taken place. Enough observations have not yet been made to determine anything about the mode, whether by a gradual rise or by saltations, in which this re-elevation took place. In our own neighbourhood, and chiefly in the Magnesian-limestone district, the best indications of raised beaches-formed, probably, during this reelevation-have been observed, and one of these has been particularly described. It was probably during this change that extensive floods, bearing rocky materials, washing through the valleys of denudation, spread the south-eastern parts of Durham with moraine-rubble and sand.. During this time, also, excessive denudations of the boulder-clay would disengage the materials contained in it, and re-deposit them in the form of gravel, brick-clay, laminated-clay, and sand, of which such remarkable examples occur in the Wear valley.
A word must be said on the change of climatal conditions which this theory involves. Let it be remembered that the British Islands are situated between the 50th and 60th degrees of N. lat.; that Cape Farewell, in Greenland, is in the same latitude as the Shetland Islands, and that Newcastle is in the same latitude as that part of Labrador which is permanently frozen two feet below the surface in summer; and that icebergs, in the spring, drift on the American coast further south than the latitude of the English Channel; and that at present all the Arctic drift ice passes to the South on the West side of the Atlantic. If we bear these facts in mind, I think the explanation of a change in the Oceanic currents the most feasible and probable one than can be offered, especially when we see that by its instrumentality the isothermal line of South Labrador in the latitude of Newcastle is carried to South Greenland, North of Iceland and Lapland. That we are not entirely quit of Arctic currents and Arctic influences, seems to be indicated by the presence on the coasts of the North Sea of so many Northern forms of animal and vegetable life, and the bitter life-destroying air-currents of our spring months is a most unwelcome proof of our nearness to Arctic regions and Arctic influences.
In order then, satisfactorily to account for the formation of such extensive valleys as those ancient ones of the Tyne, Wear, and Tees: to explain the abrasion, rounding and terracing of the sides of these valleys, and the polishing and scoring described in that of the "Wash"; and the polishing and striation of the rock-surface already observed. in so many parts of these counties, it is necessary to call in aid the existence of such a climate and such physical conditions as exist now in Greenland, Spitzbergen, and other Arctic regions. It is necessary also, in order to account for the carriage of so many large blocks of Mountain-limestone and Millstone-grit, from the higher land, extending from the monoclinal ridge of the Pennine chain to the coast, to assume an agent, similar to a glacier accumulating in its early course, masses of rock and deposits of mud, derived from the rocky boundary of its channel, and bearing these onwards to the coast-line; and then, as the face of the glacier breaks off and floats out as an iceberg to sea, depositing its burden of rock and mud at different distances from the axis of distribution, or highest portion of the glacier valley. The theory of stranded icebergs, and masses of floating ice drifting along the coast-line, is quite insufficient and entirely inadequate to produce the appearances above enumerated, for it is pretty clear that these rocks were glaciated and covered over with a thick deposit of boulder-clay before the land was submerged deep enough for icebergs to pass over them. Stranded ice would, even on exposed rock surface, be insufficient to wear and rub them down in the manner observed in these counties; in short, the effects produced by the stranding of an iceberg, are at present, merely conjectural, as no one has yet pretended to point out any special glaciations, produced by such an agent. And, if these bergs came from distant northern shores, they certainly ought to have brought as burden, rocks peculiar to the country they were formed on, but, we have seen that the transported materials embedded in the boulder-clay, are all of local valley origin : that is, they have been brought down the valleys of the Tyne or Wear, or some of the tributaries of these rivers.
Further, it is necessary, in order to account for the deposition of the boulder-clay not only along the valley-channels but on the tops of some of the hills, to infer a gradual subsidence with the last glaciation of the surface. The distribution of Criffie granite in the most superficial deposits of the Tyne valley, and the occurrence of massive blocks of Shap-fell granite along the course of the Tees, supports this inference.
The great depth of the old valleys below the present level of the sea certainly proves the former higher elevation of the land.
And with regard to climatal change, from Arctic to more temperate conditions, this inference is supported not only by the former prevalence of a more Arctic marine-fauna, but by the occurrence in these islands of fossil remains of the mammalia peculiar to the Arctic regions of the present period.
The best proofs and illustrations of this theory of glaciation by continental- or land-ice can be obtained by a careful perusal and study of the works of most of the Arctic navigators, especially of the "Arctic Explorations" of Dr. Kane.