Part 21 (1/2)
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.