# Canadian Alpine Journal/Volume 1/Number 2/The Causes of Mountain Forms in the Canadian Rockies

SCIENTIFIC SECTION.
THE CAUSES OF MOUNTAIN FORMS IN THE CANADIAN ROCKIES.
By A. P. Coleman.
The "Everlasing Hills" have become proverbial, so that we are apt to think of mountains as the very emblems of stability and permanence, and very few except geologists ever inquire into their past to see how they were lifted up and sculptured to their present shape, or look into the future to forecast their ultimate fate. There is nothing more certain, however, than the fact that mountains, like every other creature of earth, have their birth, their youth, and middle age, and at last sink into decrepitude. The loftiest mountains must always be geologically young, for, once elevated, every century means wear and tear and loss, till finally only stumps remain, as in the most ancient ranges of America, the Laurentide Mountains of northeastern Canada.
The raising of mountains is a difficult bit of engineering to explain, and geologists are by no means agreed as to the causes that thrust one part of the earth's crust skyward and sink other parts into ocean depths. The most commonly accepted cause is the shrinkage of the earth's interior, by cooling, or by the loss of gases, or by condensation due to gravity. In this process the solid outer crust becomes too large for the interior and must be crushed and crumpled to adjust itself. This crushing and crumpling takes place along the lines of weakness, usually where sea and land meet, as in our Pacific coast region. There, from time to time, through the geologic ages the ocean floor has pushed itself inland, thrusting up the parallel ranges of British Columbia and Alberta, the Selkirks first and the Rocky Mountains last.
​It was not until the end of Mesozoic times, when the dawn of the recent ages had begun, that the Rockies were elevated. No volcanic forces took part in the work. All the rocks that compose them were laid down as sediment on a sea bottom, mud and sand and gravel and the lime of shells accumulating until the beds were five miles or more in thickness and were slowly transformed into slate and quartzite and conglomerate and limestone, the building materials of the mountains that were to be.
What gave the signal for the raising of the new range no one knows, but after the Cretaceous sediments forming the coal-bearing rocks of the prairie provinces had been deposited, the thrust from the Pacific became irresistible, the earth's crust yielded and step by step the thick layers of rock were pushed inwards, rising as folds or breaking off strip by strip, tilted up and riding upon one another like ice-cakes when a great floe is driven ashore. We must not think of this process as taking place suddenly in one mighty convulsion, but very deliberately, a small push with its earthquake shock, followed by a long quiescence before the next instalment of elevation; so that age by age the mountains grew, perhaps are growing even yet.
During all this time of slow growth in height the destructive forces were at work, frost and weather and running water and ice, tearing down the structures just as they do now, though the constructive forces were more than a match for them, at least in the earlier history of the mountains. The present forms of the Rockies are due then to a balance between the upbuilding and the down-tearing influences which have been at work during the past millions of years since the end of Cretaceous times.
Having discussed general causes shaping the mountains, let us turn now to some of the special features. The fundamental structure of the Rockies is simple when ​contrasted with the complex foldings and faultings in different directions shown by the Alps. The folds in our mountains are often rather broad and uncomplicated, especially toward the Pacific side of the range; while toward the northeast vast blocks of the sediments. 25 to 40 miles long, from nothwest to southeast, several miles wide and sometimes more than 15,000 feet thick, seem to have been split off from the earth's crust, the southwest side being tipped down, and the northeast side slid up over the next block toward the prairies, the last block riding seven miles out over the region of the foothills before the thrust from behind ceased. Mr. McConnell, of the Geological Survey, whose work I am following here, estimated that along Bow Pass this over-riding or telescoping of range after range sums up to a shrinkage of 25 miles. If the blocks were set back in their place again and the strata ironed out flat Golden would be 25 miles farther from Calgary than now.
The blocks which build the eastern ranges have various tilts. In the Brazeau Valley I found the inclinations run from 28° to about 50°, blocks with the lower dip presenting a steep cliff of 3,000 or 4,000 feet toward the northeast, and a gentler slope following the surface of the strata, toward the southwest. These rather gently tilted blocks provide the "writing-desk" type of mountain so common in the eastern Rockies, e.g., near Banff, rather scorned by certain English mountain climbers. The steeper blocks, with a dip of 50° or more, make very rugged, striking mountains, however, often with two or three more resistant layers of quartzite standing out as sharp ridges, while the softer slates have been carved away by the weather.
The great faults that separate block from block sometimes run out into sharp folds at one end, as in Sentinel Mountain, near the Kootenay Plains on the Saskatchewan.
A. P. Coleman, Sketch
FOLDING UNDER THRUST FAULT, CLEARWATER RIVER
ANTICLINE AND SYNCLINE, SENTINEL MT., KOOTENAY PLAINS
The folding is not always on broad lines with domes and gentle synclines, however; for sometimes, as on Kananaskis Pass, the folds have been pushed so far as to be overturned and lie flat on their side, to be carved up in various ways by frost and running water. Along with predominant folding, faulting occurred also in many places, splitting up the folded structures into large or small fragments. On the other hand when the great fault blocks of the eastern side of the range rode upon the next block to the east the softer strata beneath were often crumpled into small folds, as may be seen along the Clearwater Valley. In the eastern half of the Rockies we have then chiefly fault blocks with minor folds, and in the western half chiefly broad folds with faults of a less important kind.
All of the fundamental structures described are supposed to be due to thrusting from the direction of the Pacific, thus furnishing the rough and massive forms from which were to be carved the splendid variety of slopes and cliffs and ridges and pinnacles that give the mountains their present wild variety of surface. Above the snow-line the sculptors which shaped them are chiefly frost, the avalanche and the glacier; on the lower slopes frost and rain and torrents have done most of the work; while the larger rivers have sawn their way down through the rocks, hollowing canyons and broad valleys, and sweeping downwards toward the plains or the sea ​all the debris, the rocks and pebbles, the sand and the clay, delivered to them by the agents working at higher levels. The main valleys have generally been cut right across the direction of the great ranges, as shown by the Bow, the Saskatchewan and the Brazeau on the east, and the Kicking Horse on the west.
Were the rivers there before the mountains, and did they carve their valleys downwards as fast as the upheaving forces pushed the mountains aloft; or did great lines of faulting provide channels that the rivers merely had to deepen? I am inclined to think that the main rivers at least were earlier than the mountain ranges and simply held their ground during the ages of uplift.
Passing through the Rockies by the lower valleys as in the Kicking Horse Pass, one sees mainly the work of running water. Where the river has a somewhat gentle slope, like the Bow, the valley which it has cut is broad and open, with terraces on each side sweeping with a curve up to the foot of the cliffs, which have their bases buried under vast heaps of talus blocks from above, mainly quarried by frost. The broad valleys seem peaceful enough, and it is hard to imagine the relentless war of the river and its tributary torrents upon the mountains until one works out the cross-section which they have cut from the summits on one side to those on the other, and figures the many cubic miles of rock which have been destroyed and carried down to the plains by the flow of water.
Where the slopes are steeper we have turbulent rivers, like the Kicking Horse, rapidly cutting down their V-shaped valleys into canyons, and our sense of the endless strife grows more vivid as we watch them leaping down thousands of feet in a few miles, dragging with them the rocks which have rolled from the sides and using them as powerful tools to cut the canyon still deeper.
​As one climbs out of the main valleys, especially on the western side of the Rockies, when timber-line is passed, snow begins to show itself, and at length there are snow-fields draining into glaciers, which creep thousands of feet down into the valleys. Finally the warmth of the lower elevation balances their slow advance and from an ice cave at the end there flows a milky mountain torrent, loaded with the stones and gravel and rock flour ground from the rock floor of the glacier above. Here there is a splendid chance to study the carving power of ice in its downward motion urged by gravity. Where the mountain torrent cuts sharp-walled canyons or V-shaped gorges, the glacier carves broad U-shaped valleys with smoothly rounded surfaces; and one notices that these broad U valleys often run far below the present glacier and are crossed by crescent-shaped moraines, perhaps now tree-covered, monuments of former ice extension. In general our glaciers seem to be retreating as if the warming up of the climate after the Ice Age were still slowly going on.
In many cases the old ice-carved valleys have been hollowed into rock basins or have their outlet blocked by moraines; and this gives rise to some of our most delightful mountain scenery, where forest slopes and precipices and snow-fields are reflected in lakes of the most marvellous turquoise blue in deeper parts, running into clear green in the shallows. These ravishing colors appear to be due to the last remnants of glacial mud from the ice-fed streams flowing into the lakes, the finest possible particles settling almost infinitely slowly, and reflecting the short blue rays of light, just as infinitessimal particles in the air give the paler blue to the sky. The intense blue or green of these mountain lakes contrasts strongly with the much paler blue of clear lakes like Superior or Ontario,, unfed with glacial mud, and makes it certain that the minute remaining particles are the real cause of the color.
​Rising out of the snow-fields and rock ridges and isolated peaks, nunataks, as they are called in Greenland, and as one ascends above the glaciers a new type of scenery shows itself, no longer smoothed and rounded surfaces of rock with here and there a moraine, but rugged forms where weathering and frost have rudely done the shaping. In this higher region the character of the rock has much to do with its forms. Hard quartzite or solid limestone resist best and stand out as cliffs and ridges, while softer slates and sandstones crumble and slide, giving long slopes of loose scree into which the foot sinks, the whole surface often slipping with the climber.
In these upper regions the jointage of the rocks plays a large part, those with numerous joints, into which the water from thawing snow may sink by day, only to freeze at night and pry asunder the blocks, are quickly shattered even if of hard materials; while rocks with few open fissures stand their ground far better and rise amidst the slopes of debris as walls or pinnacles.
From the higher levels one sees, too, how the glaciers eat back their U-shaped valleys into the solid rocks of the central mountain blocks, even little "cliff glaciers" carving for themselves nests shaped like a half kettle, cirques, as they are called in the Alps. When two of these cirques have been gnawed inwards toward each other very narrow ridges of rock with knife edges may result. From the lips of empty cirques or hanging valleys hollowed during the Ice Age bridal-veil falls now spring hundreds or thousands of feet over precipices into some deeply cut main valley carved by a glacier of the first rank.
A. P. Coleman, Sketch
OVERTURNED FOLD, CLEARWATER RIVER
A. P. Coleman, Sketch
CIRQUES, HEAD OF SOUTH FORK, BRAZEAU RIVER
One type of mountain scenery is lacking in our Rockies. No eruptive rocks have reached the surface in their elevation, so that none of the forms belonging to massive rocks can be seen.
From the comparative simplicity of their structure our Rockies make a splendid school for the study of folds and faults on a large scale, and it is well worth while for the members of our Club to add this geological interest to the many other attractions of the mountains.
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