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Rocks Important in Forming Soil

Although zonal plant growth is mainly influenced by elevation and exposure, the chemical and physical makeup of soils have an important influence. These properties are derived largely from the rocks which have decomposed and become converted into soil by forces of nature.

By far the largest part of the monument consists of very old recrystallized rocks, classified by geologists as gneisses and schists of Precambrian age. These rocks form the Tanque Verde and Rincon Mountains which are, geologically, a part of the same ancient uplift as the nearby Santa Catalina Mountains. Hundreds of millions of years ago, deep within the earth's crust, these older rocks were invaded by molten materials which cooled slowly to form granite. Numerous alternating bands, readily observable today, represent this preexisting host mass, together with the readily recognizable "injected" material.

Thus the Tanque Verdes and Rincons represent the "roots" of very old mountains, the overlying parts of which have been carried away through the ages by erosion—the wearing away process of water, wind, and weather. Some of these removed materials form deeply buried layers of outwash sands and gravels covering much of the Cactus Forest area of the monument at the western base of the Tanque Verdes. There is an abundance of mica in the granitic rocks of the Rincons, especially in the vicinity of Mica Mountain, Manhead Lookout, and Manning Camp. Some of these old rocks contain garnets in profusion, as well.


Along the flanks of the Tanque Verdes, outcroppings of ancient quartzite are noticeable in the Cactus Forest area. They are very tough resistant rocks made by the partial welding together of quartz sand grains. These quartzite layers belong to the Apache group of rock beds believed to be of Cambrian age—part of the second (Paleozoic) of four great chapters of earth history. Cambrian, and, somewhat later Devonian, limestone deposits made during periods when this region was beneath the sea are to be found here too. In the Cactus Forest area, there are also small outcroppings of two kinds of lava flows, dark basalt and the lighter colored rhyolite, indicating local volcanic activity during Tertiary times—part of the fourth and current era of earth history, the Cenozoic.

The Rincon limestone, which although of Cambrian age, is one of the topmost sedimentary formations of southern Arizona, occurs only sparingly in the monument. However, a much larger deposit occurs in the vicinity of Colossal Cave a few miles south of the eastern part of the monument. This Rincon formation is composed of about 40 feet of thick-bedded, coarse-grained, pink limestone containing numerous fragments of ancient crablike trilobites and other fossils. These had lived here in ancient seas before the advent of today's plants and air-breathing animals.

Quaking aspen and other vegetation of the Canadian Life Zone at Spud Rock Ranger Station—7,400 feet.

Ancient movements in this part of the earth's crust are indicated along several faults or cracks within the rocks of the monument. One of these is brought to your attention by the self-guiding leaflet available at monument headquarters and also by a small wayside marker where the faultline is crossed by the roadway of the Cactus Forest Loop Drive.

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Last Modified: Sat, Nov 4 2006 10:00:00 pm PST

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