Page images
PDF
EPUB

surface. Before the lower fall is reached nearly all traces of the thousands of fissures formed in the rapids above have disappeared.

On looking down on the rapids from any commanding point, the definite arrangement of the crevasses along the center of the ice-stream at once attracts attention, and their order suggests a rapid central current in the stream.

Below Camp 14, for at least two or three miles, as well as at many places above that point, the Seward glacier flows between banks of snow. Along its border there are marginal crevasses trending up-stream, and in the adjacent banks there are similar breaks trending down-stream. Where the two systems meet there is a line of irregular crevasses, exceedingly difficult to cross, which mark the actual border of the flowing ice. A similar arrangement of marginal crevasses and of shore crevasses has been referred to in connection with the Marvine glacier, and was observed in many other instances.

While occupying Camp 14 we could hear the murmur of waters far down in the glacier below our tent, but there were no surface streams visible. Crashing and rumbling noises made by the slowly moving ice frequently attracted our attention, and sometimes at night we would be awakened by a dull thud, accompanied by a trembling of the rocks beneath us, as if a slight earthquake had occurred. Occasionally a pinnacle of ice would fall and be engulfed in the crevasses at its base. These evidences of change indic ed that movements in Seward glacier were constantly in progress. A short base-line was measured and sights taken to well-marked points in the Seward glacier for the purpose of measuring its motion. The angles between the base-line and lines of sight to the chosen points were read on several successive days, but when these observations were compared they gare discrepant results. The measurements which seemed most reliable indicate that the central part of the ice-stream has a movement of about twenty feet a day. This is to be taken only as an approximation, which needs to be verified before much weight can be attached to it.

CHARACTERISTICS OF ALPINE GLACIERS ABOVE THE SNOW-LINE.

The surface of the névé is white, except near its lower limit in late summer, where it frequently becomes covered with dust

[blocks in formation]

blown from neighboring cliffs. It is almost entirely free from moraines, but at the bases of steep slopes small areas of débris sometimes appear at the surface when the yearly melting has reached its maximum. The absence of moraines is accompanied by an absence of glacial tables, sand-cones and other details of glacial surfaces due to differential melting. Streams seldom appear at the surface, for the reason that usually the water produced by surface melting is quickly absorbed by the porous strata beneath; yet the crevasses are frequently filled with water, and sometimes shallow lakes of deep blue occur at the bottoms of the amphitheatres and form a marked contrast to the even white of the general surface. Crevasses are present or absent according to the slope of the surface on which the névé rests. In the crevasses the edges of horizontal layers of granular ice are exhibited, showing that the névé down to a depth of at least one or two hundred feet is horizontally stratified. In the St. Elias region the strata are most frequently from ten to fifteen feet thick, but in a few instances layers without partings over fifty feet thick were seen. The surface is always of white, granular ice, but in the crevasses the layers near the bottom appear more compact and bluer in color than those near the surface.

Some of the most striking features of the névé are due to the crevasses that break their surfaces. The orderly arrangement of marginal crevasses and of the interior crevasses at the rapids in the Seward glacier have already been referred to; but there are still other crevasses, especially in the broad, gently sloping portions of the snow-fields where the motion is slight, which, although less regular in their arrangement, are fully as interesting. The crevasses on such slopes generally run at right angles to the direction in which the snow is moving. On looking down on such a surface, the breaks look like long clear-cut gashes which have stretched open in the center, but taper to a sharp point at each end. The ability of the névé ice to stretch to a limited extent is thus clearly shown. The initiation of the crevasses seems to be due to the movement of the névé ice over a surface in which there are inequalities of such magnitude that the ice cannot stretch sufficiently to allow it to accommodate itself to them, so that strains are produced which result in fractures at right angles to the line of general movement. Crevasses found where the grade is gentle vary from a fraction of an inch to 10 or 15 feet in width, and are sometimes two or three thou

25-NAT. GEOG, MAG., VOL. III, 1891.

sand feet long. Broader gulfs are seldom formed unless the slope has an inclination of 15° or 20o.

The grandest crevasses are in the higher portions of the névé, and occur especially on the borders of the great amphitheatres. In such situations the crevasses are usually fewer in number but are of greater size than in equal areas lower down. A length of three or four thousand feet and a breadth of fifty feet or more is not uncommon. The finest and most characteristic glacial scenery is found among these great cañon-like breaks. Standing on the border of one of the gulfs, as near the brink as one cares to venture, their full depth cannot usually be seen. In some instances they are partially filled with water of the deepest blue, in which the ice-walls are reflected with such wonderful distinctness that it is impossible to tell where the ice ends and its counterfeit begins. The walls of the crevasses are most frequently sheer cliffs of stratified ice, with occasional ornamentations, formed of ice-crystals or a pendent icicle. After a storm they are frequently decorated in the most beautiful manner with fretwork and cornice of snow. The bridges spanning the crevasses are usually diagonal slivers of ice left where the clefts overlap; but at times, especially in the case of the larger crevasses, there are true arches resembling the Natural Bridge of Virginia, but on a larger scale, spanning the blue cañons and adding greatly to their strange, fairy-like beauty. The most striking feature of these cracks is their wonderful color. All tints, from the pure white of their crystal lips down to the deepest blue of their innermost recesses, are revealed in each gash and rent in the hardened snow.

Above the snow-line all of the mountain tops that are not precipitous are heavily loaded with snow. Where the snow breaks off at the verge of a precipice and descends in avalanches a depth of more than a hundred feet is frequently revealed, but in the valleys and amphitheatres the snow has far greater thickness. Pinnacles and crests of rock, rising through the icy covering, indicate that the thickness of the névé must be many hundreds of feet.

There are no evidences of former glaciation on the mountain crests which project above the névé fields. There are no polished and striated rock surfaces or glaciated domes to indicate that the mountains were ever covered by a general capping of ice, as has been postulated for similar mountains elsewhere. When the

[blocks in formation]

glaciers had their greatest expansion the higher mountains were in about their present condition. The increase in the volume of the glaciers was felt almost entirely in their lower courses.

CHARACTERISTICS OF ALPINE GLACIERS BELOW THE SNOW-LINE.

The first feature that attracts attention on descending from the névé region to the more icy portion of the glaciers is the rapid melting everywhere taking place. Every day during the summer the murmur and roar of rills, brooks and rivers are to be heard in all of the ice-fields. The surface streams are usually short, on account of the crevasses which intercept them. They plunge into the gulfs, which are many times widened out by the flowing waters so as to form wells, or moulins, and join the general drainage beneath. The streams then flow either through caverns in the glaciers or in tunnels at the bottoms. While traversing the glacier one may frequently hear the subdued roar of rivers coursing along in the dark chambers beneath when no other indication of their existence appears at the surface. When these subglacial streams emerge, usually near the margin of the ice, they issue from archways forming the ends of tunnels, and perhaps flow for a mile or two in the sunlight before plunging into another tunnel to continue their way as before.

The best example of a glacial river seen during our exploration was near the western border of the Lucia glacier. It is shown in the illustration forming plate 12, which is reproduced mechanically from a photograph. This Styx of the ice-world has been described on an earlier page. The lakes formed at the southern end of nearly every mountain spur projecting into the Malaspina glacier discharge through tunnels in the ice, which are similar in every way to those formed by the stream already mentioned.

In the beds of the glacial streams there are deposits of sand and gravel, and when the streams expand into lakes these deposits are spread over their bottoms in more or less regular sheets. When streams from the mountains empty into the lakes, deltas are formed. While these deltas have the same characteristics as those built in more stable water bodies, many changes in detail occur, owing to the fluctuation of the water level.

One of the tunnels leading to a dry lake-bed at the end of the Hitchcock range was explored for several rods and found to be a high, arching cavern following a tortuous course, and large enough to allow one to drive a coach and four through it without danger of collision. Its floor was formed of gravel and bowlders, and its arching roof was clear ice. Here and there the courses of crevasses could be traced by the stones and finer débris that had fallen in from above, giving the appearance of veins in a mine. The deposit on the floor of the tunnel rested upon ice, and would certainly be greatly disturbed and broken up before reaching a final resting place in case the glacier should melt. In the lake basins, also, the sand and gravel forming their bottoms frequently rested upon substrata of ice, and are greatly disturbed when the ice melts.

At the ends of the glaciers the subglacial and intraglacial drainage issues from tunnels and forms muddy streams. These usually flow out from the foot of a precipice of ice, down which rills are continually trickling. The streams flowing away from the glaciers are usually rapid, owing to the high grade of their built-up channels, and sweep away large quantities of débris which is deposited along their courses. The streams widen and bifurcate as they flow seaward, and spread vast quantities of bowlders, sand, and gravel over the country to the right and left, not infrequently invading the forests and burying the still upright trees. The deposits formed by the streams are of the nature of alluvial fans, over which the waters meander in a thousand channels. Where this action has taken place long enough the alluvial fans end in deltas; but should there be a current in the sea, the débris is carried away and formed into beaches and bars along adjacent shores. Should these glaciers disappear, it is evident that these great bowlder washes would form peculiar topographic features, unsupported at the apexes, and it might be perplexing to determine from whence came the waters that deposited them. I am not aware that similar washes have been recognized along the southern border of the Laurentide glaciers, but they should certainly be expected to occur there.

Another very striking difference in the appearance of the glaciers above and below the snow-line is due to the prevalence of débris on the lower portion. The melting that takes place

« PreviousContinue »