To the north, beyond the groves of aspen, stretches the pine-covered north lateral moraine, its western end banked against the mountain at the mouth of the canyon down which the ice flowed. The top of this moraine, like that of the south lateral, slopes to the east, the inclined line of its crest being plainly visible against the mountain side.

The flat valley of Moraine Park (fig. 12) was the melting basin of the glacier and was occupied as the ice melted back by a glacial lake. The waters of this lake, impounded by the terminal moraine, were later drained away when the outlet cut through this natural dam. The present stream swings in broad meanders across the flat which was the lake bed, because it does not have enough fall to give direction to its waters. Rising from the flat in the distance is a rock island which the glacier was unable to destroy, although it ground down the west side to a gentle slope.

Figure 12.—Moraine Park from the museum. The even-topped pine-covered ridge on the left is the south lateral moraine, or deposit of boulders, gravel, and sand, left by the former Thompson Glacier. Its height indicates the height of the glacial ice. The flat is the bed of a former lake, impounded as the glacier melted, by the terminal moraine which the glacier threw across the valley. Note the U-shaped gorge, in distance on right, down which the glacier moved. Photograph by Carroll H. Wegemann

Around the east side of the lake the road follows what appears to be an old lake terrace or shore line formed by the waves when the lake stood at its initial high stage before the lowering of its outlet. As the outlet was deepened and the waters drained away, the lake established a new and lower shore line leaving the old terrace dry, to be cut away, in part, by the waves and stream erosion.

DEER RIDGE JUNCTION TO FALL RIVER PASS VIA FALL RIVER

There is glacial polish on the rock ledge just above the parking area.

The schist and granite were at one time so deeply buried within the earth that the pressure upon them from all sides exceeded the crushing strength of the rock. Under such conditions no crevasses could form and during earth movement the rocks bent and flowed like thick tar. Note that much of the bending took place after the intrusion of the light granite. At one point a band of granite has been folded and the fold, by continued lateral pressure, has been overturned, broken, and one side thrust over the other. This type of thrust-folding and faulting is characteristic of the mountain building which took place at the end of the Mesozoic era when the Rockies were first formed. It often happens that a single rock may show in miniature a type of folding which is characteristic of an entire mountain range.

In the next mile are numerous intrusions of coarse-grained granitic rock known as pegmatite. The molten rock, deep within the earth from which these intrusions solidified, probably contained, under great pressure, much steam which made it thinly fluid. It cooled slowly deep below the surface, giving the large crystals time to grow.

From this point an excellent view is to be had of three cirques or basins in which glacial snow and ice accumulated at the head of Fall River. From these the great glacier flowed down the valley.

NOTE.—Left hand column of figures below is for the use of travelers coming up from Poudre Lakes.)

FALL RIVER PASS TO POUDRE LAKES

There is not time on this circle trip to drive to Grand Lake, but if you have an hour to spare turn right on the road to Grand Lake, and use the right hand column of figures in log. Drive down as far as Poudre Lakes on the Continental Divide. If you do not care to take this trip but wish to go east down Trail Ridge Road, turn to page 24, point 0.0 miles in right-hand column.

Figure 14.—Volcanic Rocks of Specimen Mountain, and the Never Summer Range, Specimen Mountain is an extinct volcano, its sides, as shown in the foreground, formed by black pitchstone or volcanic glass, white volcanic ash, fragments of rock blown out by the explosions, and mud flows. The ancient crater has been destroyed by erosion. Photograph by Carroll H. Wegemann

That the mountain was a volcano may be inferred from the nature of the rocks which are exposed on its top and sides. Like many volcanic cones, the mountain is built up of alternating layers of black volcanic glass (pitchstone), lava, white volcanic ash, mud flows, breccia, and pumice (fig. 14). Notes on the formation of these rocks follow:

Volcanic glass is produced when molten rock flowing out upon the surface is chilled so quickly that mineral crystals do not have time to form in it.

Ordinary lava is cooled more slowly, but not slowly enough for the rock to become completely crystalline like granite, which is cooled very slowly far below the surface.

Volcanic ash is rock dust blown out by the terrific explosions of the eruption.

Mud flows are formed when this dust is washed out of the atmosphere by the torrential rains which frequently accompany the eruption, the rains being due to the condensation of clouds of steam emitted from the volcano.

Breccia is formed of fragments of the sides of the crater, blown out during the explosions.

Pumice is lava charged with gas, the bubbles of which expand when the pressure upon the lava is reduced as it reaches the surface. The resulting small cavities when filled with air cause the pumice to float on water.

All these rocks are to be seen on Specimen Mountain, mute reminders of a time, in the distant past, when volcanoes were active in this region.

The mountain was probably named for the opal, agate, and delicate crystals of topaz which are found lining some of the small cavities in the volcanic deposits.

(NOTE.—If you have entered the park from the west, set your speedometer at 0.0 at the parking area at Poudre Lakes and read paragraphs in reverse order up to 4.5 in the left-hand column of figures. At Fall River Pass reset at 0.0 and continue as below.)

Returning to Fall River Pass, reset the speedometer at 0.0 and use the right-hand column of figures. The left-hand column is for the use of those driving in the opposite direction.

TRAIL RIDGE ROAD BETWEEN FALL RIVER PASS AND DEER RIDGE JUNCTION

Looking north along the ridge, which is a continuation of the cliff, it will be noted that there is pre-Cambrian rock next to the lava and that it forms the north end of the ridge, being rather poorly exposed in, and below, the north point on the skyline. Across the highway, just south of Iceberg Lake, pre-Cambrian rock is also exposed. The pre-Cambrian. therefore, borders the lava on both sides. It is evident that the molten lava which solidified to form the rock of the cliff must have flowed down a valley cut in the older rock.

Southeast from Iceberg Lake there is an excellent view of the Upper Flattop peneplain (fig. 4). When it was formed it was not quite a plain, for above it rose a line of low rounded mountains, the remnants of the first Rockies. The Mummy Range to the northwest was part of the line. The northwest side of this range shows the old erosion surface merging with the mountain slope. This surface is, however, being rapidly destroyed by the streams which are cutting canyons into it from the east. Farther to the south the group of mountains—Ida, Julian, and Terra Tomah—appear to have been one rounded mountain rising above the plain before the erosion of water and ice cut the canyon of the Gorge Lakes into its heart. Stones Peak may have been part of the same mass before it was separated from it by the cutting of the gorge of Hayden Creek, the next gorge to the east.

On Trail Ridge, about half a mile to the southeast, where the road follows the narrowest part of the ridge, the slopes on either side look as if they were roughly terraced. The soil is but a few feet thick and rests on a sloping surface of bedrock. In the spring when the snow melts, or during heavy rains, the soil becomes saturated with water which not only increases its weight but makes it slippery. Masses of soil, bound together by plant roots, tend to slide gradually for a few inches, or a few feet, down the slope, breaking away from the turf above and overlapping the turf below, thus producing the terraces.

Below lies Horseshoe Park in the broad valley of Fall River. Beyond the tree less meadow in the distance, which is a former lake bed, a low wooded ridge crosses the valley (fig. 15, C—4). This is one of the terminal moraines of the Fall River Glacier (Wisconsin stage). It was this natural dam which impounded the lake waters and through which the outlet finally cut its channel, draining the lake and leaving several small ponds in the irregularities of its bed.

Figure 15.—Horseshoe Park from Rainbow Curve, Trail Ridge Road. A—1, Big-Horn Mountain; A—4, crest of north lateral moraine of former Fall River Glacier, slopes east; B—2, The Needles, part of an old erosional surface at 10,000 feet elevation; B—6, bed of former lake impounded by terminal moraine; C—3; McGregor Mountain; C—5, terminal moraine of Fall River Glacier; C—7; south lateral moraine which merges at east end with terminal moraine; D—3 Castle Mountain, part of old erosional surface at 8,700 feet elevation; D—7, beaver pond in Hidden Valley; E—2, flat top of Deer Mountain, part of same erosional surface as that of The Needles. Photograph by Carroll H. Wegemann

On either side of the meadow rise the abrupt gravel slopes of the lateral moraines, 800 feet in height, the tops of which indicate the height of the ice in the valley (fig. 15, A—4). They slope eastward to the point where they merge with the crest of the terminal moraine (see p. 29, point 8.2 miles).

From the north end of the terminal moraine the north lateral moraine can be traced to the point where it crosses the valley of Roaring River. This valley also was occupied by a glacier, but one which apparently lacked the cutting power of the longer and straighter Fall River Glacier, It was unable to lower the bed of its valley as rapidly as did the glacier of Fall River, and the mouth of the side stream was left hanging 400 feet above the bed of the major stream to which it falls by a series of cascades, It is probable that the glacier of Fall River continued its flow longer than its smaller tributary, for it seems to have thrown its north lateral moraine across the Roaring River Valley. After the glaciers disappeared the stream was able to cut away part of this obstruction.

Note that almost no morainal gravel or sand was deposited on the point of Bighorn Mountain which projects into the valley and which felt the full force of the ice movement. The ice did drop its load, where it was moving with less power, in the more sheltered parts of the valley both above and below this point.

Turning now to the south lateral moraine (fig. 15, C—7), it is evident that this also threw a dam across the mouths of the side valleys coming in from the south. These valleys did not carry glaciers. Their waters were impounded by the moraine, back of which the road now runs, and were forced to flow eastward south of the moraine. The moraine is lower in this direction and the stream, which is called Hidden Valley Creek, at length reached a point where its waters could overflow the barrier and cascade down the moraine to the floor of the main valley. This little stream is crossed on the road from Deer Ridge Junction to Horseshoe Park. The beaver have built several dams across the stream back of the moraine, their ponds (fig. 15, D—7) being plainly visible from this point.

The flat-topped mountain in line with the road is Deer Mountain, along the base of which the road from Deer Ridge Junction to Beaver Park is located. Its top and the top of The Needles are parts of an old erosion surface similar in origin to the Flattop peneplain, but formed somewhat later. A remnant of a still lower plain of erosion is to be seen on the top of Castle Mountain, beyond and just to the left of Deer Mountain. As explained on page 10, these old erosion surfaces were formed, when the region stood at lower altitudes, by stream which had reached their limit of down cutting and were widening their valleys. These erosion surfaces have been for the most part destroyed by the streams which produced them, after further uplift gave the streams more fall and enabled them once more to deepen their valleys.

DEER RIDGE JUNCTION TO FALL RIVER ENTRANCE VIA HORSESHOE PARK

Figure 16.—West side of Longs Peak with Glacier Gorge below. Photograph by Carroll H. Wegemann

BEAR LAKE ROAD

A short but interesting drive is that to Bear Lake. Set your speedometer at 0.0 miles.

The ridge ends on the west against the side of Flattop Mountain for which the Upper Flattop peneplain has been named. From this observation point it is difficult to appreciate fully the flatness of the mountain's top.

The first sharp peak on the skyline to the left of Flattop Mountain is Hallett Peak and in the cirque to the right of it lies Tyndall Glacier.

Left, or south of Hallett Peak, is Otis Peak, and just left and back of the latter, but hidden from view in the gorge, lies Andrews Glacier.

Note that the smooth side of Otis Peak slopes to the north and that a similar surface on Hallett Peak slopes to the south. These smooth surfaces appear to be the sides of one of the old erosion valleys cut in the Flattop peneplain after the initial uplift of that surface when the bench was formed at the head of Forest Canyon (see point 1.0 mile, p. 24). A similar surface appears on Thatchtop Mountain which is the second peak south of Otis and is nearer this point. To the right of Thatchtop and farther away is Taylor Peak and between them is the glacial valley of Loch Vale, U-shaped in cross section, containing a series of glacial lakes and Taylor Glacier hidden from view at its head.

East of Thatchtop Mountain lies Glacier Gorge, and east of that is Half Mountain against which rests the west end of the south lateral moraine of the Bartholf Glacier which rims Glacier Basin on the south. Beyond the long even crest of the moraine rises in majestic grandeur the flat summit of Longs Peak, 14,255 feet in elevation. Its perpendicular cliffs are the walls of ancient cirques.

Returning from Glacier Basin Campground to the main road, reset the speedometer at 3.3 miles. Turning left (west), continue the log. On the right rises the long ridge of the north lateral moraine of the Bartholf Glacier.