PART I

Much of Rocky Mountain National Park is underlain by rocks that are more than 1,800 million years old. These ancient rocks are chiefly gneiss and schist. They are prominently displayed in the steep headwalls of glacial basins along the Continental Divide and in cliffs along the valley walls in the central and western parts of the Park. They can be observed at close hand in rock outcrops along Trail Ridge Road. Gneiss is a light-colored rock, banded in shades of black, gray, and pink. It is made up mostly of small chunky crystals of gray quartz, pink or gray feldspar, and a small quantity of dark minerals such as black mica. The minerals tend to be crudely arranged in the same direction as the banding, and give the rock a somewhat streaked appearance. Schist is a dark-colored rock composed mostly of shiny black minerals and a little light-colored quartz and feldspar. A parallel orientation of the minerals is also very noticeable in this rock. Interlayering of the dark schist and light gneiss is common, and is easily seen in both cliffs and glacially smoothed rock surfaces in the upper parts of the canyons.

Banded schist and gneiss in cliffs on southeast side of Ypsilon Mountain. (Fig. 2) (National Park Service)

Banded gneiss near Bear Lake. (Fig. 3) (Willis T. Lee)

Dark schist cut by vein of white, coarse-grained pegmatite along Fall River Pass road. (Fig. 4) (Wayne B Alcorn)

Originally some of these old rocks were sediments—clays, silts, and sands—of ancient streams, lakes, and oceans. Some were of volcanic origin—probably black basalt lava flows. Together, they were many thousands of feet thick. Gradually, as they became covered by successively younger deposits, forces within the earth drew them down to depths as much as 5 to 10 miles under the surface. There, at tremendous pressures, and temperatures as high as 1000° F., the original sedimentary and volcanic rocks were changed by compaction, folding, and the formation of new minerals to harder rocks, like the near vertical strata seen along the highway in Big Thompson Canyon east of the Park. Later, as we shall see, both in the Park and elsewhere in the Front Range, these rocks were further changed to become the gneiss and schist they are today.

About 1,720 million years ago, while the ancient rocks were still deep within the earth, they were folded and changed for a second time by pressure and temperature. Simultaneously, they were invaded from beneath by a number of masses of molten rock, called magma, each a few miles in diameter. In general, these magma masses invaded parallel to the banding in the older rocks. Eventually, they cooled, crystallized, and solidified into masses of granite. Granite is a gray rock composed of chunky crystals of gray quartz, pink and white feldspar, and black mica, all easily visible. For the most part, it lacks the distinct streaky appearance characteristic of gneiss and schist. During a late phase of the cooling, mineral liquids were squeezed from between the crystals of the solidifying granite and forced out into the surrounding ancient rocks where they formed large pods and lenses parallel to the banding. Gradually, under high enclosing pressures that permitted minerals to grow to a large size, they cooled and solidified into a rock known as pegmatite. Pegmatite is a light-colored rock, composed chiefly of the same minerals as granite. It differs in that the individual mineral crystals are larger, in places several inches in diameter.

About 1,450 million years ago, the ancient rocks were folded and changed by intensive pressure and high temperature for a third time. This time, they were simultaneously invaded by a very large mass of molten magma.

The margins of the invading magma tended to parallel the banding in the ancient rocks but here and there they cut across the banding. Where the pressure of the invasion shattered the surrounding rocks, the magma intruded along the fractures, widening and extending them. Blocks and fragments of the ancient rocks floated off into the magma where they were partly absorbed and changed to dark clots and streaks. Along the borders of the magma the ancient rocks were similarly changed.

Gradually the magma cooled and solidified to form a large mass of granite, some 30 miles across as seen at the surface today. This granite is similar to the older one, but is lighter gray and contains larger chunky crystals of pink feldspar. From it, an intricate maze of fractures filled with granite extended outward into the surrounding rocks.

Later, the mineral liquid of the magma was forced out into fractures where it crystallized as veins of light-colored pegmatite both in the granite and in the surrounding ancient rocks whose conversion to the gneiss and schist they are today was now complete.

Granite (left) cut by coarse-grained pegmatite (right). (Fig. 5) (Dwight L. Hamilton)

Both granite invasions probably pushed up the rocks above them to form mountains at the surface of the earth. How high such mountains may have been or what they looked like is unknown, for none remain. Today, after uplift and erosion of overlying rock several miles thick both granites are at the surface. The older is exposed in the mountains about 7 miles east-northeast of Estes Park. It also forms the hills east of Grand Lake and Shadow Mountain Lake in the western part of the Park. Southeast of the Park, it is exposed in the canyon of North St. Vrain Creek, along the highway west of Lyons. The younger light-gray granite forms the mountains north and south of Estes Park, Estes Cone and Twin Sisters. It forms Hagues Peak to the north and is spectacularly displayed in the east face of Longs Peak. A gently sloping contact between the granite and the older overlying gneiss and schist can be seen in cliffs along canyons east of the Continental Divide, where also light-colored pegmatite veins extend in all directions through the gneiss and schist. All these rock types can be observed at close hand along trails in these areas.

From about 1,450 million years ago to about 530 million years ago almost nothing is known of the geologic history of the Park. To the south, a third granite, of which Pikes Peak is formed, invaded the ancient rocks of that region about 1,050 million years ago. But in the northern Front Range there is a great hiatus, or gap, in the record. Possibly, a prominent band of black basalt, known in the Park as the "Iron Dike," intruded along a fracture in the older rocks during this time. The dike, several yards wide, cuts vertically across these rocks in a southeasterly direction from Mount Chapin nearly to the Longs Peak Ranger Station. It can be seen easily along Trail Ridge Road below the crossing of the ski lift, in the bank of the Big Thompson River at "the Pool" near the crossing of the Fern Lake Trail, and in Storm Pass along the trail northwest of Estes Cone.

At least the latter part of the long hiatus is thought to have been chiefly a time of extensive erosion. It is known that by about 530 million years ago, central Colorado had been reduced to a nearly featureless plain at sea level, for a sea deposited sands (now sandstone) of this age over an essentially flat terrain eroded across the old gneisses, schists, and granites. Exposures of the sandstone resting on granite can be seen in the canyon west of Colorado Springs.