Towards the middle of the Rockies the tilted blocks of the eastern side give place to broad folds, more or less dome shaped at times, where there are wide anticlines; while at others the anticlines have been destroyed and shallow synclinal forms make the mountain tops. Here the carving of the rivers, perhaps helped by faults in some places, has cut the nearly flat-lying beds into castles, minsters and cathedrals, magnificent types of architecture, with towers and unscalable walls, supported here and there by mighty buttresses.
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
