Cochise College Student Papers in Geology

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Roger Weller, geology instructor

Coronado National Forest
Carol Hardin

Physical Geology

Fall 2006

Coronado National Forest

Sierra Vista, Arizona

Previously the road was known to carry travelers in search of gold and silver. Presently, Carr Canyon Road brings up campers, hikers and sight seers. Miles and miles of rocky dirt road take you high up on the Huachuca Mountains and it gives you a great look over Sierra Vista and the San Pedro valley. Coronado National Forest reaches 7,200 feet in elevation. As you travel up Carr Canyon road to Coronado National Forest there are many interesting geological sights.

Right at the base of the mountain on the side of the road there are several talus slopes which are inclined slopes that are made up of loose material of all sizes.

Talus Slope Photograph taken by Carol Hardin

As you climb higher and higher in elevation you can see some trees on either side of the road that slant toward the bottom of the mountain.

Mass Wasting Photograph taken by Carol Hardin

These trees are demonstrating a very slow form of mass wasting called “creep.” As they grow on a slope with a creep occurring they get pushed in the direction of the mass wasting. The trees, as they continue to grow, try to correct the angle by growing upwards. They then produce the “hockey stick” tree look. Some of these trees look like they’re holding on for dear life by their roots.

At some points on your way up to the forest you can see signs of physical weathering, which is the breaking down of rocks.

Root Pressure Photo by C. Hardin

One example of physical weathering would be root pressure. As roots grow outward from the plant they can get into cracks of rocks. They push through expanding the cracks and sometimes breaking the rock all the way through.

Physical Weathering Photograph by Carol Hardin

For some rocks it’s harder to tell which kind of weathering process is present, but you can tell that the rock used to be whole and that something must have fractured it by seeping into its cracks.

Along the side of the mountain there are several areas that show signs of metamorphism. Layers and layers of rock here are just squeezed very tightly together.

Squeezed Rock Photographs by Carol Hardin

Concordant and discordant intrusions pop up every where along the path. All the softer material around it washes away and the stronger material stays in place. They look as if they just popped up above the ground.

Intrusion Photo by C. Hardin

At the end of the drive up the terrain consists of hard, dry dirt with lots of rocks and other loose material, and the color is a dusty yellow/gold. As you travel further up to the forest on the trail the terrain softens and becomes darker. This pathway showed me how it progressed from the hard rocky dirt to the softer, darker dirt. It started with the hard dry rocky dirt, to softer more powdery dirt, then to the forest floor which was covered in pine needles, then last to the dark dirt that had some rocks in it.

Hard Terrain Soft Terrain

Pine Needle Terrain Dark Terrain Photos by C. Hardin

At the top of the forest, giant pine trees are everywhere. Many of these tall trees are standing, but some have fallen. Those fallen trees show some decomposing, animal use, and some have burned spots on them.

Pine Trees Pine Trees

Burned Pine Tree Photos by C. Hardin

Many of the rocks at the top of the mountain are quartz. The side of the roads are splattered all over with this bright white quartz. This material is derived mainly quartz veins.

Quartz Quartz

Quartz Photos by C. Hardin

The rocks sometimes have white quartz within their fractures.

Quartz filling fractures Photo by C. Hardin

There are many more sights to see at Coronado National Forest. In Sierra Vista travel south on highway 92 down to Carr Canyon Road up to the Huachuca’s to a great panoramic view of Sierra Vista, and much more!