Cochise College Student Papers in Geology

Geology Home Page physical geology historical geology planetary gems

Roger Weller, geology instructor

Rincon Mountains
Tucson, Arizona
Jennifer Lewis

Physical Geology

Fall 2006

THE RINCON MOUNTAINS

(Saguaro National Park-East and Colossal Cave Mountain Park)

The Rincon Mountain range is located on the eastside of Tucson, Arizona. The Rincon Mountains cover more than 66,000 acres. They are one of fiveinor mountain ranges that outline the Tucson Valley. The other four mountain ranges include the Santa Catalina Mountains, the Santa Rita Mountains, the Tucson Mountains, and the Tortolita Mountains. The Rincon Mountains began forming over 30 million years ago, by the movement of the North American Plate overlapping the Pacific Plate. The movement of the plates caused the earths crust to thin and crush onto itself producing earthquakes, volcanic eruptions, and tectonic features. Heat and pressure caused by the vertical rising along the Catalina fault caused a tectonic feature, called a metamorphic core complex, to form the Rincon Mountain range (Bezy, 2005). As the metamorphic core complex, made up of the metamorphic rock gneiss, the Rincon Mountains were formed. The bedrock that slid down during this uplifting formed the Tucson valley floor (Bezy, 2005).

Gneiss is formed when the sedimentary rock of shale is subjected to great heat and pressure. The mountain range is also composed of large areas of limestone, a sedimentary rock.

Rincon Mountains

Although the mountain range was formed over 30 million years ago the face of the mountains is continuously changing due to weathering. Such geological formations of dikes, mass wasting, and caves may be easily experienced through two visitor areas of the Rincon Mountain range. Also a variety of plant species and regions may be viewed in the Rincon Mountains. These two parks are the Saguaro National Park-East and the Colossal Cave Mountain Park.

The Saguaro National Park-East

To get to the Saguaro National Park-East from I-10 East from Tucson exit onto Houghton Road and go north. Turn right onto Escalante Road and then left onto Old Spanish Trail. The visitor center and park will be on the right side of the road.

Saguaro

Saguaro

The park was named for the cacti species that is prominently found there, the Saguaro Cacti. Other cacti species such as the Prickly Pear, Barrel Cactus, and Ocotillo may be found in the park. Also mesquite trees and paloverdes are prominent in the park. At higher elevations pines and firs thrive.

Prickly Pear

Barrel Cactus

Ocotillo

Paloverde

The most common rock found in the Rincon Mountains is gneiss. It is created from the sedimentary rock of shale that has been exposed to great pressure and heat. It appears to be made up of many compacted layers. It also has a high concentration of Mica crystals in it. Thus newly exposed surfaces have a shimmering appearance.

gneiss

gneiss

The Rincon Mountains are continuously changing due to weathering. Dikes, intrusive hard material is exposed when surrounding softer material is eroded away, is a geological feature found in Saguaro National Park. It appears has a horizontal protrusion on the side of the mountain.

dike

Other changes occur because of rainfall seeping into cracks of the rocks. Because of changes in temperature causes this water to freeze thus creating frost wedging. The rocks may split apart into smaller rocks and may trigger a rockslide.

evidence of frost wedging

evidence of frost wedging and rockslides

Stream channels are formed as rainfall works its way down the mountain. The movement and water mass cuts channels into the native rocks of the Rincon Mountains. Also the streams are able to carry boulders twice the size of the depth of the stream. Large boulders may be pushed to the outside of the channel, causing them to rest on the bank.

Stream channels are formed from rainfall at higher elevations

Stream Channel in a valley

Saguaro National Park offers visitors about 128 miles of trails. The difficulty of the trails range from paved surfaces, which are wheelchair accessible, to steep climbing trails. Some trails are multiuse (hiking, biking, and equestrian) trails and other trails are strictly limited to hiking. There is also a paved eight mile road with many parking areas to view the Rincon Mountains. Camping is allowed in certain areas of the park. Also backpacking permits may be obtained at the visitors’ center. The park has two picnic areas that have tables, fire grills, and pit toilets. It is an important note that visitors need to carry their own water while partaking in park attractions. There is a variety of fees to enter the park. The most common is a vehicle fee of $10.00 which allows the visitor to return during the next seven days.

Colossal Cave Mountain Park

To get to Colossal Cave Mountain Park take I-10 east from Tucson to exit 279 (Vail/Wentworth exit), turn north , and follow the signs for abut 7 miles.

The most common rock type found in Colossal Cave Mountain Park is limestone, a very soft sedimentary rock. Due to the softness of this rock it is easily eroded by moving water, which allows caves to form. There are three recorded caves in the park. Two caves, the Arkenstone and La Tetera, are live caves which are not accessible to the public. The third cave, Colossal Cave, is a dry cave which offers tours to the public.

Limestone

Cave entrance

Dry caves occur when arid climates protrude inside the cave. Needed moisture is no longer available to continue the growth of cave formations. Formations that may be seen in Colossal Cave include stalactites, stalagmites, columns, shields, and flowstone. A large flowstone formation, in Colossal Cave, has been named the Silent Waterfall. The inside of the cave consist of a very porous surface and passages which once flowed with water.

Stalactites: photo courtesy of M. Rozenbaum

columns: photo courtesy of M. Rozenbaum

shield formation

“The Silent Waterfall”: Photograph courtesy of M. Rozenbaum.

Besides multiuse (hiking, biking, and equestrian) tails the park offers other family adventures. The park offers camping and picnicking. Historical La Posta Quemada Ranch is also located in Colossal Cave Mountain Park. The ranch offers tours of the mountain side via guided horseback rides. The ranch also offers hay rides and youth programs about working ranches. There is also a butterfly garden, tortoise enclosure, a research library, snack shops, and also museums located in the park. There is a variety of fees to enter the park. The most common is a vehicle fee of $10.00 vehicle fee. The cave offers a variety of tours. The most common is a generic tour costing approximately $10.00 per person. There are independent fees for activities offered at the La Posta Quemada Ranch. A price and activities list may be obtained from La Posta Quemada Ranch by telephoning 520-647-3450.

Photographs

Where noted the photographs are courtesy of M. Rozenbaum.

All other photographs provided by Jennifer L. Lewis.

Works Cited

Bezy, John V. A Guide to the Geology of Saguaro National Park. Tucson:

The Arizona Geological Survey, 2005.

*I highly recommend this text to all who plan to visit the Saguaro National Park East and/or West. Besides a general overview of the formation of the mountains it describes exact places to view geological features. The book may be purchased for about $8.00 at the visitors’ center or from the Western National Parks Association ( www.wnpa.org ).

Other information gathered from the following: