Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle, Vol. 3 [of 3] · Charles Darwin
STRAIT OF MAGELLAN.
In considering this table, and beginning from the south, we observe, that through the first twelve degrees, the height of the snow-line rises only a little more than 2000 feet. In this space the climate and productions of the country are in many respects very uniform. In the succeeding nine degrees the rise is no less than nine thousand feet. Before any one pronounces this to be impossible, let him reflect well that the height of the snow-line very much depends on the heat of summer. In Chiloe no fruit, excepting apples and strawberries, comes to perfection; it is even oftentimes necessary to carry the barley and corn into the houses to be ripened:[21] on the other hand, in central Chile, even the sugar-cane[22] has been cultivated out of doors, and during a long summer of seven months the sky is seldom clouded, and rain never falls. The island of Chiloe, as well as the neighbouring mainland, is concealed by one dense forest, dripping with moisture, and abounding with ferns and other plants that love a humid atmosphere: while the soil of central Chile, where not irrigated, is arid and nearly desert. These two countries, so remarkably opposed to each other in every character, blend together rather suddenly near Concepcion, in lat. 37°. I do not doubt, the plain of perpetual snow undergoes an extraordinary flexure in the district where the forest ceases; for trees indicate a rainy climate, and hence a clouded state of atmosphere.[23]
From central Chile to Bolivia, a space of 16°, the rise of the snow-line is only 2000 feet. If Bolivia possessed an atmosphere as clear as that of Chile, the limit in all probability would be even higher than the present 17,000. The cause why the limit in the equatorial regions should be lower than in a latitude seventeen degrees to the southward, I leave to those to explain, who have more means of information respecting the dryness and clouded state of the atmosphere in the respective regions. The presence of glaciers depends on the accumulation of a large mass of snow, subject to some variations of temperature, sufficient partially to thaw, and then reconsolidate the mass in its downward course. They have been aptly compared to gigantic icicles. The lower limit of glaciers, must depend on that of the parent snow, greatly affected by the form of the land: in Tierra del Fuego the snow-line descends very low, and the mountain sides are abrupt; therefore we might expect to find glaciers extending far down their flanks.[24] Nevertheless, when on first beholding, in the middle of summer, many of the creeks on the northern side of the Beagle channel terminated by bold precipices of ice overhanging the salt water, I felt greatly astonished. For the mountains from which they descended, were far from being very lofty. Captain FitzRoy from angular measurements considers the general range to have an elevation rather under 4000 feet, with one point called Chain Mountain rising to 4300. Further inland, there is indeed a more lofty mountain of 7000 feet, but it is not directly connected with the glaciers to which I now allude. This range, which exceeds by so little the height of some mountains in Britain, which yet sends down in the middle of summer its frozen streams to the sea-coast, is situated in the latitude of the Cumberland hills.
I was much interested by observing the great difference between the matter brought down by torrents and by glaciers. In the former case a spit of gravel is formed, but in the latter a pile of boulders. On one occasion, the boats being hauled on shore, within the distance of half a mile from a glacier, we were admiring the perpendicular cliff of blue ice, and wishing that some more fragments would fall off, like those we saw floating on the water, at a distance of more than a mile from their source. At last, down came a mass with a roaring noise, and immediately we saw the smooth outline of a wave travelling towards us. The men ran down as quickly as they could to the boats; for the chance of their being dashed to pieces was evident. One of the seamen just caught hold of the bows, as the curling breaker reached it: he was knocked over and over but not hurt; and the boats, though thrice lifted on high and let fall again, received no damage. This was most fortunate for us, for we were a hundred miles distant from the ship, and we should have been left without provisions or fire-arms.
I had previously observed that some large fragments of rock on the beach had been lately displaced: but until seeing this wave I did not understand the cause. The structure of the creek in which this happened was very curious. One side was formed by a spur of mica slate (of which rock the surrounding mountains were composed); the head by a cliff of ice about forty feet high; and the other side by a promontory which was built up of huge rounded fragments of granite and mica slate, and was more than fifty feet in height. To account for the present position of these blocks, where they must have long remained, for old trees were growing on the upper parts; we must suppose, either that the glacier formerly advanced half a mile further outward, or that the land stood at a rather different level. Whether we are able fully to account, or not, for the height and size of this promontory of boulders, certainly it must have been the work of the glacier. One semi-rounded fragment of granite lying just above high-water mark, was of enormous dimensions. It projected six feet above the sand, and was buried to an unknown depth: its shape was oval with a circumference of thirty yards, so that the longer axis probably measured about ten or eleven. This fragment must have come from the higher parts of the range; for the base of the mountain was entirely composed of mica slate.
The waves caused by the fall of the ice must be a most powerful agent in rounding and heaping together these huge fragments, and likewise in wearing away projecting points of the solid rock. In Georgia, situated in the very same latitude, Cook, speaking of the great ice-cliffs at the head of every harbour, says, “pieces were continually breaking off, and floating out to sea, and a great fall happened while we were in the bay, which made a noise like cannon.” He adds, “It can hardly be doubted, that a great deal of ice is formed here in winter, which in the spring is broken off, and dispersed over the sea.” Mr. Sorrell, the boatswain of the Beagle, who has long been accustomed to these seas, informs me, that at this season he has seen small icebergs, with mud and gravel on them, floating from the shores. I have heard from another quarter of the same circumstance. Captain Hunter[25] says, he met numerous ice-islands in this neighbourhood, and that “many were half black apparently with earth from the land, to which they had adhered, or else with mud from the bottom on which they had been formed.” By the latter method large fragments might easily be transported, and unless the iceberg should be upset, they would never be discovered. Nevertheless, the islands of ice floating in the southern ocean, and especially those occurring far south, appear generally to be quite free from all impurities excepting the dung of seafowl. Captain Biscoe, who extended his enterprising researches so far towards the antarctic pole, informs me in a letter that he never observed in a single instance[26] any mud or fragments of stone on the numerous icebergs which he encountered during his voyage.
Glaciers occur at the head of the sounds along the whole western coast of the southern part of South America. Looking at the chart I find sixteen places mentioned: besides these I know of several others, such as those in the Beagle channel and at the foot of Mount Sarmiento. The sounds, moreover, were not all traced to the head, and it is in this part that the glaciers most frequently occur. Of the sixteen referred to, many include several frozen arms coming down from one vast body of ice. In the Canal of the Mountains, for instance, no less than nine descend from a mountain, the whole side of which, according to the chart, is covered by a glacier of the extraordinary length of twenty-one miles, and with an average breadth of a mile and a half. It must not be supposed that the glacier merely ascends some valley for the twenty-one miles, but it extends apparently at the same height for that length, parallel to the sound; and here and there sends down an arm to the sea-coast.[27] There are other glaciers having a similar structure and position, with a length of ten and fifteen miles.
I will now specify a few of the more remarkable cases, taken from Captain King’s paper, to which I have so often referred. The canal of St. Andrew is said by Lieutenant Skyring to be “suddenly and boldly closed by tremendous and astonishing glaciers.” The highest mountain in this part (Mount Stokes) was ascertained during our ascent of the river of St. Cruz to be 6200 feet, and this certainly exceeds considerably the height of the general range. About ninety miles to the northward, Sir G. Eyre’s Sound, in the latitude of Paris, has its several arms terminated by glaciers. Mr. Bynoe, the surgeon of the Beagle, who accompanied the boat when this part was surveyed, informs me that about mid-channel, and more than _twenty_ miles from the head of the sound, there were great numbers of floating masses of ice. Standing in the boat he supposes he saw about fifty: he, together with four of the boat’s crew, landed on one, which although only two or three feet above the surface of the water, felt quite steady, and easily supported their weight. On the surface, in the central part, a mass of granite, of an angular form, was partly embedded; and the ice had thawed all round it, so as to form a shallow pool of water. It was a cube of nearly two feet; and Mr. Bynoe with a maul knocked off, and brought away, a piece as large as a man’s head. The iceberg was still floating, and drifting outwards: even if it had been stranded in the immediate neighbourhood, the block of granite would have rested on the clay-slate of the surrounding mountains. For the parent rock we must look to the higher parts of the range, near the head of the sound.
Again, a few miles to the northward I see in the chart an _Iceberg_ Sound, which no doubt was so called from the number of floating masses of ice. It may be recollected that in this latitude, on the opposite side of the Cordillera, the plains of St. Cruz, at the distance of fifty and sixty miles from the mountains, were strewed with great fragments of rock. Of these, one was sixty feet in circumference, and another, which was angular, measured five yards square;--both being partly buried in the gravel, so that their thickness was unknown. As it is probable that the plains were covered by the sea within a period geologically recent, and as we absolutely know, that icebergs at the present day, both in the same latitude and even further northward, are transporting angular blocks from the opposite side of the Cordillera, the explanation of the St. Cruz case through the same means of transport, is rendered so evidently probable, that we are not justified in doubting to receive it: more especially as the unbroken surface of those plains, and the terrace-formed valley, opposes a very great difficulty to the admission of any violent debacle. The latitudes which we have now been talking of, correspond to the southern extremity of Cornwall, and the northern provinces of France.
I will add only one other case; namely, the occurrence of glaciers at the level of the sea, in the gulf of Penas, latitude 46° 40′. A glacier is represented in the charts as in one part abutting on _a flat swamp often inundated_, and in another reaching to the head of Kelly Harbour. The accompanying woodcut is copied from the published charts. Captain King says its length is fifteen miles, and from the chart one part is seven broad; it is also described as being lofty; so that we here have an enormous mountain, covering a wide area, composed of ice. If we compare its situation with countries in the northern hemisphere, the corresponding parallel crosses the Alps of Switzerland. Or we may state the case stronger, by saying that glaciers here descend to the sea within less than nine degrees of latitude, from where palms grow, less than two and a half from arborescent grasses, and (looking to the westward in the same hemisphere) less than two from orchideous parasites, and within a single degree of tree-ferns! In Norway, Von Buch found glaciers descending to the sea at Kunnen in latitude 67°; that is twenty degrees nearer the pole, than in this hemisphere;--a difference of latitude rather greater than that between the snow-lines of equal altitude in the same two countries.
The survey of the _inner_ coast terminated at the gulf of Penas, so that I am far from knowing whether glaciers are not found much further northward: and considering the immense size of the one just described, it is extremely improbable that it should be the last. On the island of Chiloe, which fronts the Cordillera as the Jura does the Alps, many angular fragments of granite, of an enormous size, which appear to have crossed the inland arm of sea, lie scattered at different heights over the country. Although situated between the parallels of 41° and 43°, I know of no sound objection to the supposition that these might formerly have been floated across, on icebergs produced by the fall of glaciers. We are not bound to suppose that the latitude 46° 40′ has always been the northern limit of such phenomena, even if it _should_ be so at present. We have endeavoured to show that the snow-line in the parallel of Chiloe has an elevation of about 6000 feet; and since on Mont Blanc the glaciers descend 5160 feet beneath the line of perpetual snow, we might at present expect to find them in front of Chiloe at a very small altitude above the level of the sea.
With respect to the position of the glaciers, they seem to occur only within the deep sounds which penetrate the central Cordillera. This may be attributed chiefly to the subordinate elevation of the outer lines. When we consider the vast dimensions and number of these glaciers, the effect produced on the land must be very great. Every one has heard of the mass of rubbish propelled by the glaciers of Switzerland, as they slowly creep onwards. In the same manner in Tierra del Fuego, on a still night the cracking and groaning of the great moving mass may be distinctly heard. The same force, which is known to uproot whole forests of lofty trees, must, when grating over the surface, tear from the flanks of the mountain many huge fragments of rock. Beneath each glacier, also, a roaring torrent drains the upper part of the ice. To these effects, which are common to all cases, there must be added, in this country, the wear and tear of the waves produced by each successive fall. Nor can this agency be inconsiderable, when we remember that it goes on night and day, century after century. We must look at every portion of the mountain as having, during the gradual rising of the land, been successively exposed to the action of these combined forces.
It is, perhaps, useless to speculate on the effects of earthquakes without some positive data. But as we find in the immediate neighbourhood of that great glacier, which stands in the latitude of the Alps, Byron[28] mentioning with surprise the quantities of sea-shells lying on all the hill-tops (a fact which may be taken as a proof of recent continental elevations); and Bulkeley,[29] in his narrative, saying, “This day we felt four great earthquakes, three of which were very terrible;” we may feel well assured, that the same power, which in Chile causes such vast masses of rock and soil to fall from the sea-cliffs, has oftentimes precipitated fragments far more immense, of a mass traversed by great fissures, already in motion, and resting on an inclined plane. I cannot imagine any scene of more terrific violence, than the waves produced by such a fall: we know that they are very bad from the mere oscillation, consequent on the movements of the ground; but in this case I can readily believe that the water would be fairly beaten back out of the deepest inlet, and then returning with an overwhelming force, would whirl about rocks of vast size like so much chaff.
In after ages, with a climate modified by the process of such physical changes as are now going on throughout the greater part of this continent, the effects which had been produced by these glaciers would appear inexplicable, to a person who doubted the possibility of their occurrence in such latitudes. He would see in the most retired and protected valleys (the present channels) beaches composed of great rounded boulders, such as those heaped up on the shore of the most turbulent ocean. Then perhaps he would speculate, either that the outer chain of mountains had been elevated subsequently to the interior ones, so as to protect a coast hitherto exposed, or that overwhelming deluges had swept down the valleys, and in some manner produced, in one day, the effects of attrition which on ordinary occasions require the prolonged action of centuries.
If we could at the present day submerge the greater part of Tierra del Fuego, or leave unelevated that which we know has recently been gained, an island with a few small outliers would be formed, similar to Georgia, and situated in exactly the same latitude. Have we in such case the slightest right to deny the probability that the snow-line would descend nearly to the water’s edge, and that every valley would be “terminated by a wall of ice,” and that “in winter masses would be broken off and dispersed over the sea?”--all of which circumstances are now happening in Georgia. The currents, which always set from the westward towards the east, would drift these floating masses through the channels towards the eastern side. And as we know that icebergs at the present day, in both hemispheres, occasionally transport fragments of rock, so we cannot deny that those of Tierra del Fuego might formerly have done so. When the land was elevated, the fragments of rock would be found deposited on the eastern side of the continent, in bands representing the ancient channels. Whether or not the hypothesis of their transport be true, such is the position of the erratic blocks in Tierra del Fuego.
With respect to the general theory of the transport by great fragments of ice, especially of such as are angular, I may add a few remarks. Humboldt having observed that none occurred over the vast intertropical plains of the eastern side of South America, believed that they were entirely absent from the whole continent. As far as I am able to discover from the works of travellers, and from what I have myself seen, the remark holds good in the countries on both sides of the Cordillera as far south as central Chile. Azara has particularly stated such to be the case in Chaco. With respect to the tributaries of the Amazons, nothing can more strongly prove it than La Condamine’s[30] story. He says, “Below Borja even for four or five hundred leagues, a stone, even a single flint, is as great a rarity as a diamond would be. The savages of those countries don’t know what a stone is, and have not even a notion of it. It is diversion enough to see some of them when they come to Borja, and first meet with stones, express their admiration at them with signs, and be eager to pick them up, loading themselves therewith as with a valuable merchandise.” It is therefore a remarkable circumstance that as soon as we reach the colder latitudes in the southern hemisphere (from 41° to Cape Horn), the same phenomenon occurs, almost on as grand a scale and with similar limits, as in the northern parts both of the Old and New World. Neither in the southern nor in the northern hemisphere do the fragments, coming from the polar regions, or from other mountain groups, arrive within a considerable distance of the lines of the tropics.
We must couple the absence of erratic blocks along that part of the Andes which is situated under a warmer climate, with the similar non-occurrence, as I am informed by Professor Royle, in Northern India round the flanks of the Himmalaya;--those loftiest pinnacles on the face of the globe. With regard to Southern Africa, from lat. 35° to the tropic, Dr. Andrew Smith, who has visited as a naturalist so large a portion of the interior, assures me he has never seen any thing of the kind. Nor do I recollect meeting with any mention of them, in the works of the numerous travellers in the equatorial regions of the same continent. The same remark certainly holds good with Australia in the parallel of Sydney, but perhaps is more doubtful with respect to Van Diemen’s Land.[31] To my mind these negative facts[32] have very great weight in support of the mass of positive evidence which Mr. Lyell[33] has brought to bear upon the question.
The circumstance of a luxuriant vegetation with a tropical character so largely encroaching on the temperate zones, under the same kind of climate that allows of a limit of perpetual snow of little altitude, and consequent descent of the glaciers into the sea, is very important; because it has been argued, with great apparent truth, that as there is the strongest presumptive evidence of a gradual cooling down of the climate (or rather of a less favourable state for tropical productions) in Europe, it is most unphilosophical to imagine that formerly glaciers could have acted where they do not now occur. It may be asked; what are the circumstances in the southern hemisphere that produce such results? Must we not attribute them to the large proportional area of water; and do not plain geological inferences compel us to allow, that during the epoch anterior to the present, the northern hemisphere more closely approached to that condition, than it now does?
We are all so much better acquainted with the position of places in our own, than in any other quarter of the globe, that I will recapitulate what is actually taking place in the southern hemisphere,[34] only transporting in imagination each part to a corresponding latitude in the north. On this supposition, in the southern provinces of France, magnificent forests, intwined by arborescent grasses, and the trees loaded with parasitical plants, would cover the face of the country. In the latitude of Mont Blanc, but on an island as far eastward as central Siberia, tree-ferns and parasitical orchideæ would thrive amidst the thick woods. Even as far north as central Denmark, humming-birds might be seen fluttering about delicate flowers, and parrots feeding amidst the evergreen woods, with which the mountains would be clothed down to the water’s edge. Nevertheless, the southern part of Scotland (only removed twice as far to the westward) would present an island “almost wholly covered with everlasting snow,” and having each bay terminated by ice-cliffs, from which great masses yearly detached, would sometimes bear with them fragments of rock. This island would only boast of one land-bird, a little grass and moss; yet in the same latitude the sea might swarm with living creatures.
Text from Project Gutenberg, public domain in the United States.