Effect of Marine Currents on Climate.

Pleistocene Submergence of the Sahara.

On the North American continent, between the arctic circle and the 42nd parallel of lat.i.tude, we meet with signs of ice-action on a scale as grand as, if not grander than, in Europe; and there also the excess of cold appears to have been first felt at the close of the Tertiary, and to have continued throughout a large portion of the Pleistocene period.

[36]

The general absence of organic remains in the North American glacial formation makes it as difficult as in Europe to determine what mammalia lived on the continent at the time of the most intense refrigeration, or when extensive areas were becoming strewed over with glacial drift and erratic blocks, but it is certain that a large proboscidean now extinct, the Mastodon giganteus, Cuv., together with many other quadrupeds, some of them now living and others extinct, played a conspicuous part in the post-glacial era. By its frequency as a fossil species, this pachyderm represents the European Elephas primigenius, although the latter also occurs fossil in the United States and Canada, and abounds, as I learn from Sir John Richardson, in lat.i.tudes farther north than those to which the mastodon has been traced.

In the state of New York, the mastodon is not unfrequently met with in bogs and lacustrine deposits formed in hollows in the drift, and therefore, in a geological position, much resembling that of Recent peat and sh.e.l.l-marl in the British Isles, Denmark, or the valley of the Somme, as before described. Sometimes entire skeletons have been discovered within a few feet of the surface, in peaty earth at the bottom of small ponds, which the agriculturists had drained. The sh.e.l.ls in these cases belong to freshwater genera, such as Limnaea, Physa, Planorbis, Cyclas, and others, differing from European species, but the same as those now proper to ponds and lakes in the same parts of America.

I have elsewhere given an account of several of these localities which I visited in 1842,* and can state that they certainly have a more modern aspect than almost all the European deposits in which remains of the mammoth occur, although a few instances are cited of Elephas Primigenius having been dug out of peat in Great Britain.

(* "Travels in North America" volume 1 page 55 London 1845; and "Manual of Geology" chapter 12 5th edition page 144.)

Thus I was shown a mammoth's tooth in the museum at Torquay in Devonshire which is believed to have been dredged up from a deposit of vegetable matter now partially submerged beneath the sea. A more elevated part of the same peaty formation const.i.tutes the bottom of the valley in which Tor Abbey stands. This individual elephant must certainly have been of more modern date than his fellows found fossil in the gravel of the Brixham cave, before described, for it flourished when the physical geography of Devonshire, unlike that of the cave period, was almost identical with that now established.

I cannot help suspecting that many tusks and teeth of the mammoth, said to have been found in peat, may be as spurious as are the horns of the rhinoceros cited more than once in the "Memoirs of the Wernerian Society" as having been obtained from sh.e.l.l-marl in Forfarshire and other Scotch counties; yet, between the period when the mammoth was most abundant and that when it died out, there must have elapsed a long interval of ages when it was growing more and more scarce; and we may expect to find occasional stragglers buried in deposits long subsequent in date to others, until at last we may succeed in tracing a pa.s.sage from the Pleistocene to the Recent fauna, by geological monuments, which will fill up the gap before alluded to as separating the era of the flint tools of Amiens and Abbeville from that of the peat of the valley of the Somme.

How far the lacustrine strata of North America above mentioned may help to lessen this hiatus, and whether some individuals of the Mastodon giganteus may have come down to the confines of the historical period, is a question not so easily answered as might at first sight be supposed. A geologist might naturally imagine that the fluviatile formation of Goat Island, seen at the falls of Niagara, and at several points below the falls,* was very modern, seeing that the fossil sh.e.l.ls contained in it are all of species now inhabiting the waters of the Niagara, and seeing also that the deposit is more modern than the glacial drift of the same locality.

(* "Travels in North America" by the author, volume 1 chapter 2 and volume 2 chapter 19.)

In fact, the old river bed, in which bones of the mastodon occur, holds the same position relatively to the boulder formation as the strata of sh.e.l.l-marl and bog-earth with bones of mastodon, so frequent in the State of New York, bear to the glacial drift, and all may be of contemporaneous date. But in the case of the valley of the Niagara we happen to have a measure of time which is wanting in the other localities, namely, the test afforded by the recession of the falls, an operation still in progress, by which the deep ravine of the Niagara, 7 miles long, between Queenstown and Goat Island has been hollowed out.

This ravine is not only post-glacial, but also posterior in date to the fluviatile or mastodon-bearing beds. The individual therefore found fossil near Goat Island flourished before the gradual excavation of the deep and long chasm, and we must reckon its antiquity, not by thousands, but by tens of thousands of years, if I have correctly estimated the minimum of time which was required for the erosion of that great ravine.*

(* "Principles of Geology" 9th edition page 2; and "Travels in North America" volume 1 page 32 1845.)

The stories widely circulated of bones of the mastodon having been observed with their surfaces pierced as if by arrow-heads or bearing the marks of wounds inflicted by some stone implement, must in future be more carefully inquired into, for we can scarcely doubt that the mastodon in North America lived down to a period when the mammoth co-existed with Man in Europe. But I need say no more on this subject, having already explained my views in regard to the evidence of the antiquity of Man in North America when treating of the human bone discovered at Natchez on the Mississippi.

In Canada and the United States we experience the same difficulty as in Europe when we attempt to distinguish between glacial formations of submarine and those of supra-marine origin. In the New World, as in Scotland and England, marine sh.e.l.ls of this era have rarely been traced higher than 500 feet above the sea, and 700 feet seems to be the maximum to which at present they are known to ascend. In the same countries, erratic blocks have travelled from north to south, following the same direction as the glacial furrows and striae imprinted almost everywhere on the solid rocks underlying the drift. Their direction rarely deviates more than fifteen degrees east or west of the meridian, so that we can scarcely doubt, in spite of the general dearth of marine sh.e.l.ls, that icebergs floating in the sea and often running aground on its rocky bottom were the instruments by which most of the blocks were conveyed to southern lat.i.tudes.

There are, nevertheless, in the United States, as in Europe, several groups of mountains which have acted as independent centres for the dispersion of erratics, as, for example, the White Mountains, lat.i.tude 44 degrees north, the highest of which, Mount Washington, rises to about 6300 feet above the sea; and according to Professor Hitchc.o.c.k some of the loftiest of the hills of Ma.s.sachusetts once sent down their glaciers into the surrounding lower country.

GREAT SOUTHERN EXTENSION OF TRAINS OF ERRATIC BLOCKS IN BERKSHIRE, Ma.s.sACHUSETTS, U.S., LAt.i.tUDE 42 DEGREES NORTH.

Having treated so fully in this volume of the events of the glacial period, I am unwilling to conclude without laying before the reader the evidence displayed in North America of ice-action in lat.i.tudes farther south by about ten degrees than any seen on an equal scale in Europe.

This extension southwards of glacial phenomena in regions where there are no snow-covered mountains like the Alps to explain the exception, nor any hills of more than moderate elevation, const.i.tutes a feature of the western as compared to the eastern side of the Atlantic, and must be taken into account when we speculate on the causes of the refrigeration of the northern hemisphere during the Pleistocene period.

[Ill.u.s.tration: Figure 50. Erratic Blocks In Berkshire, Ma.s.sachusetts]

(FIGURE 50. MAP SHOWING THE RELATIVE POSITION AND DIRECTION OF SEVEN TRAINS OF ERRATIC BLOCKS IN BERKSHIRE, Ma.s.sACHUSETTS, AND IN PART OF THE STATE OF NEW YORK.

Distance in a straight line, between the mountain ranges A and C, about eight miles.

A. Canaan range, in the State of New York. The crest consists of green chloritic rock.

B. Richmond range, the western division of which consists in Merriman's Mount of the same green rock as A, but in a more schistose form, while the eastern division is composed of slaty limestone.

C. The Lenox range, consisting in part of mica-schist, and in some districts of crystalline limestone.

d. k.n.o.b in the range A, from which most of the train Number 6 is supposed to have been derived.

e. Supposed starting point of the train Number 5 in the range A.

f. Hiatus of 175 yards, or s.p.a.ce without blocks.

g. Sherman's House.

h. Perry's Peak.

k. Flat Rock.

l. Merriman's Mount.

m. Dupey's Mount.

n. Largest block of train, Number 6. See Figures 51 and 52.

p. Point of divergence of part of the train Number 6, where a branch is sent off to Number 5.

Number 1. The most southerly train examined by Messrs. Hall and Lyell, between Stockbridge and Richmond, composed of blocks of black slate, blue limestone and some of the green Canaan rock, with here and there a boulder of white quartz.

Number 2. Train composed chiefly of large limestone ma.s.ses, some of them divided into two or more fragments by natural joints.

Number 3. Train composed of blocks of limestone and the green Canaan rock; pa.s.ses south of the Richmond Station on the Albany and Boston railway; is less defined than Numbers 1 and 2.

Number 4. Train chiefly of limestone blocks, some of them thirty feet in diameter, running to the north-west of the Richmond Station, and pa.s.sing south of the Methodist Meeting-house, where it is intersected by a railway cutting.

Number 5. South train of Dr. Reid, composed entirely of large blocks of the green chloritic Canaan rock; pa.s.ses north of the Old Richmond Meeting-house, and is three-quarters of a mile north of the preceding train (Number 4).

Number 6. The great or princ.i.p.al train (north train of Dr. Reid), composed of very large blocks of the Canaan rock, diverges at p, and unites by a branch with train Number 5.

Number 7. A well-defined train of limestone blocks, with a few of the Canaan rock, traced from the Richmond to the slope of the Lenox range.)

In 1852, accompanied by Mr. James Hall, state geologist of New York, author of many able and well-known works on geology and palaeontology, I examined the glacial drift and erratics of the county of Berkshire, Ma.s.sachusetts, and those of the adjoining parts of the state of New York, a district about 130 miles inland from the Atlantic coast and situated due west of Boston in lat.i.tude 42 degrees 25 minutes north.

This lat.i.tude corresponds in Europe to that of the north of Portugal.

Here numerous detached fragments of rock are seen, having a linear arrangement or being continuous in long parallel trains, running nearly in straight lines over hill and dale for distances of 5, 10, and 20 miles, and sometimes greater distances. Seven of the more conspicuous of these trains, from 1 to 7 inclusive, Figure 50, are laid down in the accompanying map or ground plan.*

(* This ground plan, and a farther account of the Berkshire erratics was given in an abstract of a lecture delivered by me to the Royal Inst.i.tution of Great Britain, April 27, 1855 and published in their Proceedings.)

It will be remarked that they run in a north-west and south-east direction, or almost transversely to the ranges of hills A, B, and C, which run north-north-east and south-south-west. The crests of these chains are about 800 feet in height above the intervening valleys. The blocks of the northernmost train, Number 7, are of limestone derived from the calcareous chain B; those of the two trains next to the south, Numbers 6 and 5, are composed exclusively in the first part of their course of a green chloritic rock of great toughness, but after they have pa.s.sed the ridge B, a mixture of calcareous blocks is observed. After traversing the valley for a distance of 6 miles these two trains pa.s.s through depressions or gaps in the range C, as they had previously done in crossing the range B, showing that the dispersion of the erratics bears some relation to the acutal inequalities of the surface, although the course of the same blocks is perfectly independent of the more leading features of the geography of the country, or those by which the present lines of drainage are determined. The greater number of the green chloritic fragments in trains 5 and 6 have evidently come from the ridge A, and a large proportion of the whole from its highest summit d, where the crest of the ridge has been worn into those dome-shaped ma.s.ses called "roches moutonnees," already alluded to, and where several fragments having this shape, some of them 30 feet long, are seen in situ, others only slightly removed from their original position, as if they had been just ready to set out on their travels. Although smooth and rounded on their tops they are angular on their lower parts, where their outline has been derived from the natural joints of the rock. Had these blocks been conveyed from d by glaciers, they would have radiated in all directions from a centre, whereas not one even of the smaller ones is found to the westward of A, though a very slight force would have made them roll down to the base of that ridge, which is very steep on its western declivity. It is clear, therefore, that the propelling power, whatever it may have been, acted exclusively in a south-easterly direction. Professor Hall and I observed one of the green blocks--24 feet long, poised upon another about 19 feet in length. The largest of all on the west flank of m, or Dupey's Mount, called the Alderman, is above 90 feet in diameter, and nearly 300 feet in circ.u.mference. We counted at some points between forty and fifty blocks visible at once, the smallest of them larger than a camel.

[Ill.u.s.tration: Figure 51. Dome-Shaped Block]

(FIGURE 51. ERRATIC DOME-SHAPED BLOCK OF COMPACT CHLORITIC ROCK (n in map in Figure 50), near the Richmond Meeting-house, Berkshire, Ma.s.sachusetts, lat.i.tude 42 degrees 25 minutes North. Length, 52 feet; width, 40 feet; height above the soil, 15 feet.)

The annexed drawing (Figure 51) represents one of the best known of train Number 6, being that marked n on the map (Figure 50). According to our measurement it is 52 feet long by 40 in width, its height above the drift in which it is partially buried being 15 feet. At the distance of several yards occurs a smaller block, 3 or 4 feet in height, 20 feet long, and 14 broad, composed of the same compact chloritic rock, and evidently a detached fragment from the bigger ma.s.s, to the lower and angular part of which it would fit on exactly. This erratic n has a regularly rounded top, worn and smoothed like the "roches moutonnees"

before mentioned, but no part of the attrition can have occurred since it left its parent rock, the angles of the lower portion being quite sharp and unblunted.

[Ill.u.s.tration: Figure 52. Position of Block in Figure 51]

(FIGURE 52. SECTION SHOWING THE POSITION OF THE BLOCK IN FIGURE 51.

a. The large block in Figure 51 and n in the map in Figure 50.

b. Fragment detached from the same.

c. Unstratified drift with boulders.

d. Silurian limestone in inclined stratification.)

From railway cuttings through the drift of the neighbourhood and other artificial excavations, we may infer that the position of the block n, if seen in a vertical section, would be as represented in Figure 52. The deposit c in that section consists of sand, mud, gravel, and stones, for the most part unstratified, resembling the till or boulder clay of Europe. It varies in thickness from 10 to 50 feet, being of greater depth in the valleys. The uppermost portion is occasionally, though rarely, stratified. Some few of the imbedded stones have flattened, polished, striated, and furrowed sides. They consist invariably, like the seven trains above mentioned, of kinds of rock confined to the region lying to the north-west, none of them having come from any other quarter. Whenever the surface of the underlying rock has been exposed by the removal of the superficial detritus, a polished and furrowed surface is seen, like that underneath a glacier, the direction of the furrows being from north-west to south-east, or corresponding to the course of the large erratics.

As all the blocks, instead of being dispersed from a centre, have been carried in one direction and across the ridges A, B, C and the intervening valleys, the hypothesis of glaciers is out of the question.

I conceive, therefore, that the erratics were conveyed to the places they now occupy by coast ice, when the country was submerged beneath the waters of a sea cooled by icebergs coming annually from arctic regions.

[Ill.u.s.tration: Figure 53. Canaan And Richmond Valleys]

(FIGURE 53. SECTION THROUGH CANAAN AND RICHMOND VALLEYS AT A TIME WHEN THEY WERE MARINE CHANNELS.

d, e. Ma.s.ses of floating ice carrying fragments of rock.)