GLACIATION, PAST AND PRESENT: by T. Henry
THE stupendous effects of ice in ages long gone by have been elaborately studied by geologists, who have given us fascinating descriptions thereof. The enormous power of ice as an agent in transforming the land is shown by the study of its doings at the present day. Much has been done in this direction in the Alps, but in America we have Alaska, which, besides the prospects of material resources which it holds out to the future, is already affording a fine field for the observer of nature. Here we may see glaciers at work; and though the action of the ice-sheet at its bottom is hid from view, what goes on at the advancing margin is evident from year to year, and even from day to day. All the phenomena of moraines, the pushing forward of rocks and trees, the damming up of valleys to form lakes, the scraping up of boulder-clay, the rounding-off of the rocks, etc., may be witnessed; together with many details that could not easily have been inferred from a study of the sites of past glaciation. One of the most interesting of these effects is the way in which the glacier acts indirectly through the force of the huge waves it produces when it enters a river. Vast blocks from the ice-front fall off with a splash and send up a wave and a series of waves that sweep over the bank and into the forest beyond, achieving more erosion than ever rain or river did. The greater erosive effects follow on brief sudden movements.
In the National Geographical Magazine (Washington) for June, 1911, there is a most interesting article recording the field-studies of the National Geographical Society in Alaska. Many of the glaciers which they studied had advanced during the last year or two, and others had been retreating. The reasons why some should advance while others retreat were not satisfactorily determined, and further study must precede a decision in this respect. But earthquakes, of which there were twenty-six days in September, 1899, are assigned a chief rôle. The effect of an earthquake was to produce a sudden advance and great but brief transformations.
One of the largest glaciers in Yakutat Bay, the Nunatak, had changed a great deal since the year before. It had advanced decidedly, different parts of its front having come out 700 to 1000 feet up to June 17, 1910. From 1890 to 1909 the Nunatak Glacier receded steadily, going back over two miles and a half in this time.... The forward movement commenced between July 6, 1909 and June 1910. This was due to the accession of unusually large quantities of snow to the reservoirs of this glacier by avalanches during the twenty-six days of severe earthquakes of September, 1899.
The size of glaciers is illustrated by the following description:
On the lower Copper River is Childs Glacier, which is seriously threatening to destroy a steel railway bridge just completed. The rate of forward motion in Childs Glacier increased during the winter of 1909-10 so that part of the margin of the glacier changed its forward movement from nothing to two and as much as eight feet a day.... Childs Glacier is ten to twelve miles long, not much over a mile wide in the mountain valley, but it widens to over three miles in Copper River Valley.
Its front is a precipitous white wall 250 to 300 feet high, and is swept at the base by Copper River....
In August, 1909, Childs Glacier was advancing at about its normal rate—four feet a day at a point near the north side and perhaps six or seven feet a day in midglacier. The melting and the many icebergs discharged from the terminal cliff at that time just about balanced this advance, so that the front of the glacier remained in about the same place.... During the winter and early spring of 1909-10, however, the glacier began to advance more rapidly, buckling up the ice of the frozen river. In June 1910 the ice-front had moved forward from 920 to 1225 feet, narrowing the river to 400 or 500 feet.
Every time the ice cliff was sufficiently undercut by the river, great masses of ice would cascade down the front, raising a gigantic wave in the river.... During the advance the waves washed up over a bank five to twenty-five feet in height and rushed back 100 or 200 feet into the alder thicket. Ice blocks, up to ten tons in weight were thrown in among the trees. Stones a foot or two in diameter were hurled into the thicket. Alders nine to eleven inches in diameter were stripped of leaves and bark and bent backward or broken off short, or uprooted or buried beneath the gravel and boulders and macerated trunks of other trees.
The river bank, which was cut back some in the preceding year was in 1910 being fairly eaten up by the iceberg waves which crossed the river, fifty to sixty feet by actual measurement having been removed along the bank of the stream facing the glacier.
It was a rare opportunity to see the visible forward movement of Childs Glacier into the forest. A series of lobes developed, though some of them were not persistent, and at the end of these lobes the day-to-day changes were most pronounced. Ice blocks were sliding down the frontal slope some of them being rolled many feet into the forest; trees were overturned, turf and grass were ploughed up and carried on the ice of the glacier. Yet one saw and heard little of a spectacular nature while traversing the ice-front. It was an irresistible steady movement, but slow, as the movement of the hour hand of a clock is slow. As impressive as anything was to find tons of ice resting where one stood to take a photograph the day before, or to find some great tree, 100 years old, prone on the ground with the butt beneath the glacier, where the day before the tree was upright with the ice just touching it.
A whole grove ... was overturned between 1909 and 1910, ... practically not a tree remaining which was not overturned or leaning. Peat bogs were rolled up in great bolsters five or six feet high. Isolated trees in the peat were pushed forward a hundred feet or more without being overturned.... In the bay east of Heather Island marine deposits with shells are being pushed up above sea-level.
On the east margin of the glacier a lake was formed where there was only a marginal stream.
It is evident that in ice we have an agent which in the past has played a great part in cosmic changes and cataclysms, and may do so at any time in the future. When we consider the changes in climate to which the earth is believed to be liable, owing to certain cyclic changes in the gearing of its revolving pinions, the conviction becomes stronger. It is now generally admitted that the words "Ice Age" or "Glacial Age" should be spelt with a final s indicating the plural number; for if there was one there were many. What we study in the north of America and Europe is the effects of the last, or the last few, of these periodic phenomena.