present altitude the revived streams of the current cycle of erosion
have not entrenched themselves deep enough to develop strong
relief. This idea is confirmed 80 m. farther south, where Pike's
Peak (14,108 ft.), a conspicuous landmark far out on the plains,
has every appearance of being a huge monadnock, surmounting
a rough peneplain of 10,000 ft. in general elevation. The idea is
still better confirmed farther north in Wyoming, where the Laramie
Range, flanked with upturned strata on the east and west, is for
the most part a broad upland at altitudes of 7000 or 8000 ft., with no
strong surmounting summits and as yet no deep carved valleys.
Here the first of the Pacific railways chose its pass. When the summit
is reached, the traveller is tempted to ask, “Where are the
mountains?” so small is the relief of the upland surface. This low
range turns westward in a curve through the Rattlesnake Mountains
towards the high Wind River Mountains (Gannett Peak, 13,775 ft.),
an anticlinal range within the body of the mountain system, with
flanking strata rising well on the slopes. Flanking strata are even
better exhibited in the Bighorn Mountains, the front range of
northern Wyoming, crescentic in outline and convex to the north-east,
like the Laramie Range, but much higher; here heavy sheets of
limestone arch far up towards the range crest, and are deeply notched
where consequent streams have cut down their gorges.
Farther north in Montana, beyond the gorge of the Missouri river, the structure of the Front Range is altogether different; it is here the carved residual of a great mass of moderately bent Palaeozoic strata, overthrust eastward upon the Mesozoic strata of the plains; instead of exposing the oldest rocks along the axis and the youngest rocks low down on the flanks, the younger rocks of the northern range follow its axis, and the oldest rocks outcrop along its eastern flanks, where they override the much younger strata of the plains; the harder strata, instead of lapping on the mountain flanks in great slab-like masses, as in the Bighorns, form out-facing scarps, which retreat into the mountain interior where they are cut down by outflowing streams.
The structure of the inner ranges is so variable as to elude simple description; but mention should be made of the Uinta range of broad anticlinal structure in north-east Utah, with east-west trend, as if corresponding to the east-west Rattlesnake Mountains, already named. The Wasatch Range, trending north-south in central Utah, is peculiar in possessing large east-west folds, which are seen in cross-section in the dissected western face of the range, because the whole mass is there squarely cut off by a great north-south fault with down-throw to the Basin Range province, the fault face being elaborately carved.
Volcanic action has been restricted in the Rocky Mountains proper. West Spanish Peak (13,620 ft.), in the Front Range of southern Colorado, may be mentioned as a fine example of a deeply dissected volcano, originally of greater height, with many unusually strong radiating dike-ridges near its denuded flanks. In north-western Wyoming there are extensive and heavy lava sheets, uplifted and dissected, and crowned with a few dissected volcanoes. It is in association with this field of extinct volcanic activity that a remarkable group of geysers and hot springs has been developed, from which the Yellowstone river, a branch of the Missouri, flows north-eastward, and the Snake river, a branch of the Columbia, flows south-westward. The geyser district is held as a national domain, the Yellowstone Park.
Travellers whose idea of picturesqueness is based upon the abnormally sharpened peaks of the ice-sculptured Alps are disappointed with the scenery of the central and southern ranges of the Rocky Mountains. It is true that many of these ranges are characterized by the rounded tops and the rather evenly slanting, waste-covered slopes which normally result from the long-continued action of the ordinary agencies of erosion; that they bear little snow in summer and are practically wanting in glaciers; that forests are often scanty on the middle and lower slopes, the more so because of devastation by fires; and that the general impression of great altitude is much weakened because the mountains are seen from a base which itself is 5000 or 6000 ft. above sea-level. Nevertheless the mountains are of especial interest to the physiographer who wishes to make a comparative study of land forms as affected by normal and by glacial sculpture, in order to give due attention to “process” as well as to “structure and stage” in the analysis and description of mountain topography. A journey along the range from south to north reveals most strikingly a gradual increase in the share of sculpture due to Pleistocene glaciers. In New Mexico, if glaciers were formed at all in the high valleys, they were so small as not greatly to modify the more normal forms. In central Colorado and Wyoming, where the mountains are higher and the Pleistocene glaciers were larger, the valley heads were hollowed out in well-formed cirques, often holding small lakes; and the mountain valleys were enlarged into U-shaped troughs as far down as the ice reached, with hanging lateral valleys on the way. Different stages of cirque development, with accompanying transformation of mountain shape, are finely illustrated in several ranges around the headwaters of the Arkansas river in central Colorado, where the highest summit of the Rocky Mountains is found (Mt Massive, 14,424 ft., in the Sawatch range); and perhaps even better in the Bighorn range of Wyoming. In this central region, however, it is only by way of exception that the cirques were so far enlarged by retrogressive glacial erosion as to sharpen the preglacial dome-like summits into acute peaks; and in no case did glacial action here extend down to the plains at the eastern base of the mountains; but the widened, trough-like glaciated valleys frequently descend to the level of the elevated intermont basins, where moraines were deployed forward on the basin floor. The finest examples of this kind are the moraines about Jackson Lake on the basin floor east of the Teton Range (Grand Teton, 13,747 ft.), a superb north-south range which lies close to the meridional boundary line between Wyoming and Idaho. Farther north in Montana, in spite of a decrease of height, there are to-day a few small glaciers with snowfields of good size; and here the effects of sculpture by the much larger Pleistocene glaciers are seen in forms of almost alpine strength.
The intermont basins which so strongly characterize the Rocky Mountain system are areas which have been less uplifted than the enclosing ranges, and have therefore usually become the depositories of waste from the surrounding mountains.
Some of the most important basins may be mentioned. San Luis “Valley” is an oval basin about 60 m. long near the southern end of the mountain system in New Mexico and Colorado; its level, treeless floor, at an altitude of 7000 ft.. is as yet hardly trenched by the Rio Grande, which escapes through an impassable canyon southward on its way to the Gulf of Mexico. The much smaller basin of the upper Arkansas river in Colorado is well known because the Royal Gorge, a very narrow cleft by which the river escapes through the Front Range to the plains, is followed by a railroad at river-level. South Park, directly west of Pike's Peak, is one of the highest basins (nearly 10,000 ft.), and gains its name from the scattered, park-like growth of large pine trees; it is drained chiefly by the South Platte river (Missouri-Mississippi system), through a deep gorge in the dissected mass of the plateau-like Front Range. The Laramie Plains and the Green river basin, essentially a single structural basin between the east-west ranges of Rattlesnake Mountains on the north and the Uinta Range on the south, measuring roughly 260 m. east-west by 100 m. north-south, is the largest intermont basin; it is well known from being traversed through its greatest length by the Union Pacific railway. Its eastern part is drained north-eastward through a gorge that separates the Laramie and Rattlesnake (Front) ranges by the North Platte river to the Missouri-Mississippi; its western part, where the basin floor is much dissected, often assuming a bad-land expression, is drained southward by the Green river, through a deep canyon in the Uinta Range to the Colorado river and then to the Pacific. The Bighorn basin has a moderately dissected floor, drained north-eastward by Bighorn river through a deep canyon in the range of the same name to the Missouri. Several smaller basins occur in Montana, all somewhat dissected and drained through narrow gorges and canyons by members of the Missouri system.
The Plateau province, next west of the southern Rocky Mountains, is characterized for the most part by large-textured forms, developed The Plateau Province. on a great thickness of nearly horizontal Palaeozoic, Mesozoic and Tertiary formations, and by a dry climate. The province was uplifted and divided into great blocks by faults or monoclinal flexures and thus exposed to long-lasting denudation in a mid-Tertiary cycle of erosion; and then broadly elevated again, with renewed movement on some of the fault lines; thus was introduced in late Tertiary time the current cycle of erosion in which the deep canyons of the region have been trenched. The results of the first cycle of erosion are seen in the widespread exposure of the resistant Carboniferous limestone as a broad platform in the south-western area of greater uplift through central Arizona, where the higher formations were worn away; and in the development of a series of huge, south-facing, retreating escarpments of irregular outline on the edges of the higher formations farther north. Each escarpment stands forth where a resistant formation overlies a weaker one; each escarpment is separated from the next higher one by a broad step of weaker strata. A wonderful series of these forms occurs in southern Utah, where in passing northward from the Carboniferous platform one ascends in succession the Vermilion Cliffs (Triassic sandstones), the White Cliffs (Jurassic sandstones, of remarkably cross-bedded structure, interpreted the dunes of an ancient desert), and finally the Pink Cliffs (Eocene strata of fluviatile and lacustrine origin) of the high, forested plateaus. Associated with these irregular escarpments are occasional rectilinear ridges, the work of extensive erosion on monoclinal structures, of which Echo Cliffs, east of the Painted Desert (so called from its many-coloured sandstones and clays), is a good example.
With the renewal of uplift by which the earlier cycle of erosion was interrupted and the present cycle introduced, inequalities of surface due to renewed faulting were again introduced; these still appear as cliffs, of more nearly rectilinear front than the retreating escarpments formed in the previous cycle. These cliffs are peculiar in gradually passing from one formation to another, and in having a height dependent on the displacement of the fault rather than on the structures in the fault face; they are already somewhat battered and dissected by erosion. The most important line of cliffs of this class is associated with the western and southern boundary of the plateau province, where it was uplifted from the lower ground. The few rivers of the region must have reached the quiescence of old age in the earlier cycle, but were revived by uplift to a vigorous youth in
