The Underworld of Oregon Caves
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The Raw Material — Rock

If we could turn back some 180 million years into geologic time, we would find the North American continent a much different place. This was the Triassic Period. Early dinosaurs thrived in primitive forests over much of the United States. The area around southwestern Oregon was not yet part of the continent; it was a shallow arm of the sea. Smoldering volcanoes jutted out as cone-shaped islands or poured forth fumes and lava from the distant mainland.

During quieter centuries the age-old process of life and death went on within the sea waters. Fish, clams, coral—even tiny one-celled creatures too small to be seen—extracted a mineral called calcium carbonate from the water. With it they built bones, shells and skeletons. When these animals died, their hard parts settled to the ocean bottom. Gradually, layers of calcium carbonate were built up.

At the same time certain chemical functions of ocean plants extracted carbon dioxide from the water and caused still more calcium carbonate to precipitate and add to the sediments. The layers deepened. Eventually the weight of overlying sediments and the ocean above compressed them into a rock called limestone.

In different parts of the sea, and under varied conditions, other ocean sediments were deposited. Near the shore, wave-swept sand accumulated and eventually became sandstone. Fine silt and clay carried to the sea by rivers settled in bluish layers which were to become shale. Near rocky headlands, course gravel deposits became cemented into a hard mass called conglomerate.

This steady formation of sedimentary layers was periodically interrupted by volcanic activity. Heavy clouds of volcanic ash and fragments settled into the sea. Molten lava poured into shallow bays or welled up from subsurface volcanoes to mix with calcium carbonate muds. When volcanism subsided, the seas went back to the quiet deposition of limestone. Today at Oregon Caves we find evidence of this interbedding of sedimentary and volcanic materials. An example of such interbedding can be seen above the parking area near the Chateau.

Thus mixed deposits of volcanic and ocean sediments continued to collect for several million years. Apparently this steady transfer of material from one part of the earth's crust to another created a crustal imbalance. The edge of the continent was under a strain. Then, like an accordion, a tremendous folding of the lands along the Pacific Coast occurred.

The floor of the sea was lifted above the ocean's surface to form a new coast line in this vicinity. Violent stresses in the earth's crust created intense heat and pressure which changed, or metamorphosed, the rocks. Shales were altered to slate. Sandstone became quartzite. Limestone became the marble so important to Oregon Caves. Even the volcanic materials were altered considerably from their original form. The resultant geological belt composed of inter-bedded layers of slate, quartzite, marble, and metamorphosed volcanics is known as the Applegate Group.

After the uplift, there was a long period of crustal stability. The folded mountains were eroded away and the area became a flattened plain near sea level. As a result, its streams were sluggish and meandered slowly to the ocean. Then, in various stages, the plain was uplifted in another period of crustal adjustments which produced a flat-topped plateau, so to speak, known as the ancient Klamath Peneplane. This restored the vigor of the stream erosion, which helped at times by glacier sculpture, dissected the plateau into the mountains we know today. The Siskiyou Range surrounding Oregon Caves National Monument is part of the Klamath Mountain System.

Let us focus on one of the ancient marble layers of the Applegate Group, for this is the rock strata in which Oregon Caves were formed. It is actually a narrow, tilted belt, varying in thickness up to 400 feet. It dips eastward into the earth at an angle of about 60° and can be followed in a southwest-northeast direction for about 4 miles along the west shoulder of Mt. Elijah (see illustrations on the opposite page). Examination of the marble layers inside the exit tunnel or the outcrop at the beginning of Cliff Nature Trail reveals many fractures caused by the stresses of upheaval. Some are vertical cracks, but there are also many cross fractures at varying angles.

marble outcrouop
Marble outcrop at cave exit.

Piece of Oregon Caves marble—8 inches wide

When tested for chemical composition, Oregon Caves marble samples have averaged 93 percent pure calcium carbonate (CaCO3). Its bluish color is derived from the remaining percentage of impurities. Without these, it would be white. A good example of nearly pure calcium carbonate is the white chalk used on blackboards.

Without this belt of soluble marble, and without the fractures within it, natural processes could not have produced the "Marble Halls of Oregon." It is the foundation, the framework, and the raw material of the caves.

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Last Updated: 10-May-2006