Man and the Glacial Period

by G. Frederick Wright

Fig. 13.—Shows central part of the front of Muir Glacier one half mile distant. Near the lower left hand corner the ice is seen one mile distant resting for about one half mile on gravel which it had overrun. The ice is now retreating in the channel. (From photograph.)

The main body of the Muir Glacier occupies a vast amphitheatre, with diameters ranging from thirty to forty miles, and covers an area of about one thousand square miles. From one of the low mountains near its mouth I could count twenty-six tributary glaciers which came together and became confluent in the main stream of ice. Nine medial moraines marked the continued course of as many main branches, which becoming united formed the grand trunk of the glacier. Numerous rocky eminences also projected above the surface of the ice, like islands in the sea, corresponding to what are called “nunataks” in Greenland. The force of the ice against the upper side of these rocky prominences is such as to push it in great masses above the surrounding level, after the analogy of waves which dash themselves into foam against similar obstructions. In front of the nunataks there is uniformly a depression, like the eddies which appear in the current below obstacles in running water.

Over some portions of the surface of the glacier there is a miniature river system, consisting of a main stream, with numerous tributaries, but all flowing in channels of deep blue ice. Before reaching the front of the glacier, however, each one of these plunges down into a crevasse, or moulin, to swell the larger current, which may be heard rushing along in an impetuous course hundreds of feet beneath, and far out of sight. The portion of the glacier in which there is the most rapid motion is characterised by innumerable crags and domes and pinnacles of ice, projecting above the general level, whose bases are separated by fissures, extending in many cases more than a hundred feet below the general level. These irregularities result from the combined effect of the differential motion (as illustrated in the diagram on [page 4]), and the influence of sunshine and warm air in irregularly melting the unprotected masses. The description given in our introductory chapter of medial moraines and ice-pillars is amply illustrated everywhere upon the surface of the Muir Glacier. I measured one block of stone which was twenty feet square and about the same height, standing on a pedestal of ice three or four feet high.

The mountains forming the periphery of this amphitheatre rise to a height of several thousand feet; Mount Fairweather, upon the northwest, from whose flanks probably a portion of the ice comes, being, in fact, more than fifteen thousand feet high. The mouth of the amphitheatre is three miles wide, in a line extending from shoulder to shoulder of the low mountains which guard it. The actual water-front where the ice meets tide-water is one mile and a half.[AI] Here the depth of the inlet is so great that the front of the ice breaks off in icebergs of large size, which float away to be dissolved at their leisure. At the water’s edge the ice presents a perpendicular front of from two hundred and fifty to four hundred feet in height, and the depth of the water in the middle of the inlet immediately in front of the ice is upwards of seven hundred feet; thus giving a total height to the precipitous front of a thousand feet.

[AI] These are the measurements of Professor Reid. In my former volume I have given the dimensions as somewhat smaller.

The formation of icebergs can here be studied to admirable advantage. During the month in which we encamped in the vicinity the process was going on continuously. There was scarcely an interval of fifteen minutes during the whole time in which the air was not rent with the significant boom connected with the “calving” of a berg. Sometimes this was occasioned by the separation of a comparatively small mass of ice from near the top of the precipitous wall, which would fall into the water below with a loud splash. At other times I have seen a column of ice from top to bottom of the precipice split off and fall over into the water, giving rise to great waves, which would lash the shore with foam two miles below.

This manner of the production of icebergs differs from that which has been ordinarily represented in the text-books, but it conforms to the law of glacial motion, which we will describe a little later, namely, that the upper strata of ice move faster than the lower. Hence the tendency is constantly to push the upper strata forwards, so as to produce a perpendicular or even projecting front, after the analogy of the formation of breakers on the shelving shore of a large body of water.

Evidently, however, these masses of ice which break off from above the water do not reach the whole distance to the bottom of the glacier below the water; so that a projecting foot of ice remains extending to an indefinite distance underneath the surface. But at occasional intervals, as the superincumbent masses of ice above the surface fall off and relieve the strata below of their weight, these submerged masses suddenly rise, often shooting up considerably higher than they ultimately remain when coming to rest. The bergs formed by this latter process often bear much earthy material upon them, which is carried away with the floating ice, to be deposited finally wherever the melting chances to take place.

Numerous opportunities are furnished about the front and foot of this vast glacier to observe the manner of the formation of kames, kettle-holes, and various other irregular forms into which glacial débris is accustomed to accumulate. Over portions of the decaying foot of the glacier, which was deeply covered with morainic débris, the supporting ice is being gradually removed through the influence of subglacial streams or of abandoned tunnels, which permit the air to exert its melting power underneath. In some places where old moulins had existed, the supporting ice is melting away, so that the superincumbent mass of sand, gravel, and boulders is slowly sliding into a common centre, like grain in a hopper. This must produce a conical hill, to remain, after the ice has all melted away, a mute witness of the impressive and complicated forces which have been so long in operation for its production.