A Manual of Elementary Geology Part 21

On Moel Tryfane, near the Menai Straits, Mr. Trimmer met with sh.e.l.ls of the species commonly found in the drift at the height of 1392 feet above the level of the sea.

It is remarkable that in the same neighbourhood where there is evidence of so great a submergence of the land during part of the glacial period, we have also the most decisive proofs yet discovered in the British Isles of subaerial glaciers. Dr. Buckland published in 1842 his reasons for believing that the Snowdonian mountains in Caernarvons.h.i.+re were formerly covered with glaciers, which radiated from the central heights through the seven princ.i.p.al valleys of that chain, where striae and flutings are seen on the polished rocks directed towards as many different points of the compa.s.s. He also described the "moraines" of the ancient glaciers, and the rounded "bosses" or small flattened domes of polished rock, such as the action of moving glaciers is known to produce in Switzerland, when gravel, sand, and boulders, underlying the ice, are forced along over a foundation of hard stone. Mr. Darwin, and subsequently Prof. Ramsay, have confirmed Dr. Buckland's views in regard to these Welsh glaciers. Nor indeed was it to be expected that geologists should discover proofs of icebergs having abounded in the area now occupied by the British Isles in the Pleistocene period without sometimes meeting with the signs of contemporaneous glaciers which covered hills even of moderate elevation between the 50th and 60th degrees of lat.i.tude.

In Ireland the "drift" exhibits the same general characters and fossil remains as in Scotland and England; but in the southern part of that island, Prof. E. Forbes and Capt. James found in it some sh.e.l.ls which show that the glacial sea communicated with one inhabited by a more southern fauna. Among other species in the south, they mention at Wexford and elsewhere the occurrence of _Nucula Cobboldiae_ (see fig. 120. p. 149.) and _Turritella incra.s.sata_ (a crag fossil); also a southern form of _Fusus_, and a _Mitra_ allied to a Spanish species.[131-A]

FOOTNOTES:

[122-A] Chap. xvi. and the references there given.

[122-B] Voyage in 1822, p. 233.

[123-A] T. L. Hayes, Boston Journ. Nat. Hist. 1844.

[124-A] See paper by the author, Phil. Trans. 1835, p. 15.

[125-A] See above, section, p. 48.

[125-B] Geol. of Fife, &c. p. 220.

[129-A] For a full account of the drift of East Norfolk, see a paper by the author, Phil. Mag. No. 104. May, 1840.

[130-A] Quart. Journ. Geol. Soc. vol. vii. p. 22.

[131-A] Forbes, Memoirs of Geol. Survey of Great Britain, vol. i. p. 377.

CHAPTER XII.

BOULDER FORMATION--_continued_.

Difficulty of interpreting the phenomena of drift before the glacial hypothesis was adopted--Effects of intense cold in augmenting the quant.i.ty of alluvium--a.n.a.logy of erratics and scored rocks in North America and Europe--Bayfield on sh.e.l.ls in drift of Canada--Great subsidence and re-elevation of land from the sea, required to account for glacial appearances--Why organic remains so rare in northern drift--Mastodon giganteus in United States--Many sh.e.l.ls and some quadrupeds survived the glacial cold--Alps an independent centre of dispersion of erratics--Alpine blocks on the Jura--Whether transported by glaciers or floating ice--Recent transportation of erratics from the Andes to Chiloe--Meteorite in Asiatic drift.

It will appear from what was said in the last chapter of the marine sh.e.l.ls characterizing the boulder formation, that nine-tenths or more of them belong to species still living. The superficial position of "the drift" is in perfect accordance with its imbedded organic remains, leading us to refer its origin to a modern period. If, then, we encounter so much difficulty in the interpretation of monuments relating to times so near our own--if in spite of their recent date they are involved in so much obscurity--the student may ask, not without reasonable alarm, how we can hope to decipher the records of remote ages.

To remove from the mind as far as possible this natural feeling of discouragement, I shall endeavour in this chapter to prove that what seems most strikingly anomalous, in the "erratic formation," as some call it, is really the result of that glacial action which has already been alluded to.

If so, it was to be expected that so long as the true origin of so singular a deposit remained undiscovered, erroneous theories and terms would be invented in the effort to solve the problem. These inventions would inevitably r.e.t.a.r.d the reception of more correct views which a wider field of observation might afterwards suggest.

The term "diluvium" was for a time the popular name of the boulder formation, because it was referred by some geologists to the deluge.

Others retained the name as expressive of their opinion that a series of diluvial waves raised by hurricanes and storms, or by earthquakes, or by the sudden upheaval of land from the bed of the sea, had swept over the continents, carrying with them vast ma.s.ses of mud and heavy stones, and forcing these stones over rocky surfaces so as to polish and imprint upon them long furrows and striae.

But no explanation was offered why such agency should have been developed more energetically in modern times than at former periods of the earth's history, or why it should be displayed in its fullest intensity in northern lat.i.tudes; for it is important to insist on the fact, that the boulder formation is a _northern_ phenomenon. Even the southern extension of the drift, or the large erratics found in the Alps and the surrounding lands, especially their occurrence round the highest parts of the chain, offers such an exception to the general rule as confirms the glacial hypothesis; for it shows that the transportation of stony fragments to great distances, and the striation, polis.h.i.+ng, and grooving of solid floors of rock, are here again intimately connected with acc.u.mulations of perennial snow and ice.

That there is some intimate connection between a cold or northern climate and the various geological appearances now commonly called glacial, cannot be doubted by any one who has compared the countries bordering the Baltic with those surrounding the Mediterranean. The smoothing and striation of rocks, and the erratics, are traced from the sea-sh.o.r.e to the height of 3000 feet above the level of the Baltic, whereas such phenomena are wholly wanting in countries bordering the Mediterranean; and their absence is still more marked in the equatorial parts of Asia, Africa, and America; but when we cross the southern tropic, and reach Chili and Patagonia, we again encounter the boulder formation, between the lat.i.tude 41 S. and Cape Horn, with precisely the same characters which it a.s.sumes in Europe. The evidence as to climate derived from the organic remains of the drift is, as we have seen, in perfect harmony with the conclusions above alluded to, the former habits of the species of mollusca being accurately ascertainable, inasmuch as they belong to species still living, and known to have at present a wide range in northern seas.

But if we are correct in a.s.suming that the northern hemisphere was considerably colder than now during the period under consideration, owing probably to the greater area and height of arctic lands, and to the quant.i.ty of icebergs which such a geographical state of things would generate, it may be well to reflect before we proceed farther on the entire modification which extreme cold would produce in the operation of those causes spoken of in the sixth chapter as most active in the formation of alluvium. A large part of the materials derived from the detritus of rocks, which in warm climates would go to form deltas, or would be regularly stratified by marine currents, would, under arctic influences, a.s.sume a superficial and alluvial character. Instead of mud being carried farther from a coast than sand, and sand farther out than pebbles,--instead of dense stratified ma.s.ses being heaped up in limited areas,--nearly the whole materials, whether coa.r.s.e or fine, would be conveyed by ice to equal distances, and huge fragments, which water alone could never move, would be borne for hundreds of miles without having their edges worn or fractured; and the earthy and stony ma.s.ses, when melted out of the frozen rafts, would be scattered at random over the submarine bottom, whether on mountain tops or in low plains, with scarcely any relation to the inequalities of the ground, settling on the crests or ridges of hills in tranquil water as readily as in valleys and ravines. Occasionally, in those deep and uninhabited parts of the ocean, never reached by any but the finest sediment in a normal state of things, the bottom would become densely overspread by gravel, mud, and boulders.

In the Western Hemisphere, both in Canada and as far south as the 40th and even 38th parallel of lat.i.tude in the United States, we meet with a repet.i.tion of all the peculiarities which distinguish the European boulder formation. Fragments of rock have travelled for great distances from north to south; the surface of the subjacent rock is smoothed, striated, and fluted; unstratified mud or _till_ containing boulders is a.s.sociated with strata of loam, sand, and clay, usually devoid of fossils. Where sh.e.l.ls are present, they are of species still living in northern seas, and half of them identical with those already enumerated as belonging to European drift 10 degrees of lat.i.tude farther north. The fauna also of the glacial epoch in North America is less rich in species than that now inhabiting the adjacent sea, whether in the Gulf of St. Lawrence, or off the sh.o.r.es of Maine, or in the Bay of Ma.s.sachusetts. At the southern extremity of its course, moreover, it presents an a.n.a.logy with the drift of the south of Ireland, by blending with a more southern fauna, as for example at Brooklyn near New York, in lat. 41 N., where, according to MM. Redfield and Desor, _Venus mercenaria_ and other southern species of sh.e.l.ls begin to occur as fossils in the drift.

The extension on the American continent of the range of erratics during the Pleistocene period to lower lat.i.tudes than they reached in Europe, agrees well with the present southward deflection of the isothermal lines, or rather the lines of equal winter temperature. Formerly, as now, a more extreme climate and a more abundant supply of floating ice prevailed on the western side of the Atlantic.

Another resemblance between the distribution of the drift fossils in Europe and North America has yet to be pointed out. In Norway, Sweden, and Scotland, as in Canada and the United States, the marine sh.e.l.ls are confined to very moderate elevations above the sea (between 100 and 700 feet), while the erratic blocks and the grooved and polished surfaces of rock extend to elevations of several thousand feet.

[Ill.u.s.tration: Fig. 118. Cross section.

K. Mr. Ryland's house.

_h_. Clay and sand of higher grounds, with _Saxicava_, &c.

_g_. Gravel with boulders.

_f_. Ma.s.s of _Saxicava rugosa_, 12 feet thick.

_e_. Sand and loam with _Mya truncata_, _Scalaria Groenlandica_, &c.

_d_. Drift, with boulders of syenite, &c.

_c_. Yellow sand.

_b_. Laminated clay, 25 feet thick.

A. Horizontal lower Silurian strata.

B. Valley re-excavated.]

I described in 1839 the fossil sh.e.l.ls collected by Captain Bayfield from strata of drift at Beauport near Quebec, in lat. 47, and drew from them the inference that they indicated a more northern climate, the sh.e.l.ls agreeing in great part with those of Uddevalla in Sweden.[134-A] The sh.e.l.ly beds attain at Beauport and the neighbourhood a height of 200, 300, and sometimes 400 feet above the sea, and dispersed through some of them are large boulders of granite, which could not have been propelled by a violent current, because the accompanying fragile sh.e.l.ls are almost all entire.

They seem, therefore, said Captain Bayfield, writing in 1838, to have been dropped down from melting ice, like similar stones which are now annually deposited in the St. Lawrence.[134-B] I visited this locality in 1842, and made the annexed section, fig. 118., which will give an idea of the general position of the drift in Canada and the United States. I imagine that the whole of the valley B was once filled up with the beds _b_, _c_, _d_, _e_, _f_, which were deposited during a period of subsidence, and that subsequently the higher country (_h_) was submerged and overspread with drift. The partial re-excavation of B took place when this region was again uplifted above the sea to its present height. Among the twenty-three species of fossil sh.e.l.ls collected by me from these beds at Beauport, all were of recent northern species, except one, which is unknown as living, and may be extinct (see fig. 119.). I also examined the same formation farther up the valley of the St. Lawrence, in the suburbs of Montreal, where some of the beds of loam are filled with great numbers of the _Mytilus edulis_, or our common European mussel, retaining both its valves and purple colour. This sh.e.l.ly deposit, containing _Saxicava rugosa_ and other characteristic marine sh.e.l.ls, also occurs at an elevated point on the mountain of Montreal, 450 feet above the level of the sea.[135-A]

[Ill.u.s.tration: Fig. 119. _Astarte Laurentiana._

_a._ Outside.

_b._ Inside of right valve.

_c._ Inside of left valve.]

In my account of Canada and the United States, published in 1845, I announced the conclusion to which I had then arrived, that to explain the position of the erratics and the polished surfaces of rocks, and their striae and flutings, we must a.s.sume first a gradual submergence of the land in North America, after it had acquired its present outline of hill and valley, cliff and ravine, and then its re-emergence from the ocean. When the land was slowly sinking, the sea which bordered it was covered with islands of floating ice coming from the north, which, as they grounded on the coast and on shoals, pushed along such loose materials of sand and pebbles as lay strewed over the bottom. By this force all angular and projecting points were broken off, and fragments of hard stone, frozen into the lower surface of the ice, had power to scoop out grooves in the subjacent solid rock. The sloping beach, as well as the floor of the ocean, might be polished and scored by this machinery; but no flood of water, however violent, or however great the quant.i.ty of detritus or size of the rocky fragments swept along by it, could produce such long, perfectly straight and parallel furrows, as are everywhere visible in the Niagara district, and generally in the region north of the 40th parallel of lat.i.tude.[135-B]

By the hypothesis of such a slow and gradual subsidence of the land we may account for the fact that almost everywhere in N. America and Northern Europe the boulder formation rests on a polished and furrowed surface of rock,--a fact by no means obliging us to imagine, as some think, that the polis.h.i.+ng and grooving action was, as a whole, anterior in date to the transportation of the erratics. During the successive depression of high land, varying originally in height from 1000 to 3000 feet above the sea-level, every portion of the surface would be brought down by turns to the level of the ocean, so as to be converted first into a coast-line, and then into a shoal; and at length, after being well scored by the stranding upon it of thousands of icebergs, might be sunk to a depth of several hundred fathoms. By the constant depression of land, the coast would recede farther and farther from the successively formed zones of polished and striated rock, each outer zone becoming in its turn so deep under water as to be no longer grated upon by the heaviest icebergs. Such sunken areas would then simply serve as receptacles of mud, sand, and boulders dropped from melting ice, perhaps to a depth scarcely, if at all, inhabited by testacea and zoophytes. Meanwhile, during the formation of the unstratified and unfossiliferous ma.s.s in deeper water, the smoothing and furrowing of shoals and beaches is still going on elsewhere upon and near the coast in full activity. If at length the subsidence should cease, and the direction of the movement of the earth's crust be reversed, the sunken area covered with drift would be slowly reconverted into land. The boulder deposit, before emerging, would then for a time be brought within the action of the waves, tides, and currents, so that its upper portion, being partially disturbed, would have its materials re-arranged and stratified. Streams also flowing from the land would in some places throw down layers of sediment upon the _till_. In that case, the order of superposition will be, first and uppermost, sand, loam, and gravel occasionally fossiliferous; secondly, an unstratified and unfossiliferous ma.s.s, for the most part of much older date than the preceding, with angular erratics, or with boulders interspersed; and, thirdly, beneath the whole, a surface of polished and furrowed rock. Such a succession of events seems to have prevailed very widely on both sides of the Atlantic, the travelled blocks having been carried in general from the North Pole southwards, but mountain chains having in some cases served as independent centres of dispersion, of which the Alps present the most conspicuous example.

It is by no means rare to meet with boulders imbedded in drift which are worn flat on one or more of their sides, the surface being at the same time polished, furrowed, and striated. They may have been so shaped in a glacier before they reached the sea, or when they were fixed in the bottom of an iceberg as it ran aground. We learn from Mr. Charles Martins that the glaciers of Spitzbergen project from the coast into a sea between 100 and 400 feet deep; and that numbers of striated pebbles or blocks are there seen to disengage themselves from the overhanging ma.s.ses of ice as they melt, so as to fall at once into deep water.[136-A]

That they should retain such markings when again upraised above the sea ought not to surprise us, when we remember that rippled sands, and the cracks in clay dried between high and low water, and the foot-tracks of animals and rain-drops impressed on mud, and other superficial markings, are all found fossil in rocks of various ages.

On the other hand, it is not difficult to account for the absence in many districts of striated and scored pebbles and boulders in glacial deposits, for they may have been exposed to the action of the waves on a coast while it was sinking beneath or rising above the sea. No s.h.i.+ngle on an ordinary sea-beach exhibits such striae, and at a very short distance from the termination of a glacier every stone in the bed of the torrent which gushes out from the melting ice is found to have lost its glacial markings by being rolled for a distance even of a few hundred yards.

The usual dearth of fossil sh.e.l.ls in glacial clays well fitted to preserve organic remains may, perhaps, be owing, as already hinted, to the absence of testacea in the deep sea, where the undisturbed acc.u.mulation of boulders melted out of very large bergs may take place. In the aegean and other parts of the Mediterranean, the zero of animal life, according to Prof. E.

Forbes, is approached at a depth of about 300 fathoms. In tropical seas it would descend farther down, just as vegetation ascends higher on the mountains of hot countries. Near the pole, on the other hand, the same zero would be reached much sooner both on the hills and in the sea. If the ocean was filled with floating bergs, and a low temperature prevailed in the northern hemisphere during the glacial period, even the shallow part of the sea might have been uninhabitable, or very thinly peopled with living beings. It may also be remarked that the melting of ice in some fiords in Norway freshens the water so as to destroy marine life, and famines have been caused in Iceland by the stranding of icebergs drifted from the Greenland coast, which have required several years to melt, and have not only injured the hay harvest by cooling the atmosphere, but have driven away the fish from the sh.o.r.e by chilling and freshening the sea.

If the cold of the glacial epoch came on slowly, if it was long before it reached its greatest intensity, and again if it abated gradually, we may expect to find the earliest and latest formed drift less barren of organic remains than that deposited during the coldest period. We may also expect that along the southern limits of the drift during the whole glacial epoch, there would be an intimate a.s.sociation of transported matter of northern origin with fossil-bearing sediment, whether marine or freshwater, belonging to more southern seas, rivers, and continents.

That in the United States, the _Mastodon giganteus_ was very abundant after the drift period is evident from the fact that entire skeletons of this animal are met with in bogs and lacustrine deposits occupying hollows in the drift. They sometimes occur in the bottom even of small ponds recently drained by the agriculturist for the sake of the sh.e.l.l marl. I examined one of these spots at Geneseo in the state of New York, from which the bones, skull, and tusk of a Mastodon had been procured in the marl below a layer of black peaty earth, and ascertained that all the a.s.sociated freshwater and land sh.e.l.ls were of a species now common in the same district. They consisted of several species of _Lymnea_, of _Planorbis bicarinatus_, _Physa heterostropha_, &c.

In 1845 no less than six skeletons of the same species of Mastodon were found in Warren County, New Jersey, 6 feet below the surface, by a farmer who was digging out the rich mud from a small pond which he had drained.

Five of these skeletons were lying together, and a large part of the bones crumbled to pieces as soon as they were exposed to the air. But nearly the whole of the other skeleton, which lay about 10 feet apart from the rest, was preserved entire, and proved the correctness of Cuvier's conjecture respecting this extinct animal, namely, that it had twenty ribs like the living elephant. From the clay in the interior within the ribs, just where the contents of the stomach might naturally have been looked for, seven bushels of vegetable matter were extracted. I submitted some of this matter to Mr. A. Henfrey of London for microscopic examination, and he informs me that it consists of pieces of small twigs of a coniferous tree of the Cypress family, probably the young shoots of the white cedar, _Thuja occidentalis_, still a native of North America, on which therefore we may conclude that this extinct Mastodon once fed.

Another specimen of the same quadruped, the most complete and probably the largest ever found, was exhumed in 1845 in the town of Newburg, New York, the length of the skeleton being 25 feet, and its height 12 feet. The anchylosing of the last two ribs on the right side afforded Dr. John C.