Cochise College Student Papers in Geology
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Roger Weller, geology instructor regional geology planetary gems
wellerr@cochise.edu
Brown Canyon
Huachuca Mountains
Vincent Festa
physical Geology
Spring 2008
*SLOW TO LOAD BUT WORTH IT*
Brown Canyon-Huachuca Mountains,
Arizona
Brown Canyon is located in the Huachuca Mountains in
Southeastern Arizona, approximately 8 miles south of Sierra Vista. The Huachucas
are located in what geologists refer to as the Basin and Range Province. The
Huachuca Mountains are a complex mix of Precambrian granites, and younger
quartzites, sandstones, limestones, shales, conglomerates and volcanic rocks.
The entire range has been subjected to folding, faulting and deformation. Brown
Canyon contains examples of many of these rock types and evidence of these
tectonic forces. This student presentation is part of Dr. Roger Weller’s Virtual
Geology Museum web site. You can find additional information and photos of the
rocks and minerals mentioned in this presentation on Dr. Weller’s site.
http://skywalker.cochise.edu/wellerr/aawellerweb.htm
The canyon is popular with hikers, mountain bikers and
horseback riders. The elevation at the beginning of the trail up the canyon is
approximately 5100 feet and the trail climbs to about 6300 feet over the next
3.4 miles. There is shade along much of the trail. Motor vehicles are
prohibited.
Access to Brown Canyon is generally made from Ramsey Canyon
Road. (It is possible to enter Brown Canyon from the west by hiking up and
through Ramsey Canyon. However you must go through the Ramsey Canyon Preserve to
access Ramsey Canyon from this route. You can also hike in on Brown Canyon Road
although the road has signs declaring it a “Private Road.”) Take State Route 92
south to Ramsey Canyon Road. Go west on Ramsey Canyon Road for 2.1 miles. There
is a “gate” on the north side of the road and a parking area inside the opening.
It is possible to drive closer to the canyon if you have a four wheel drive or
high clearance vehicle. Trail access to Brown Canyon is located about .8 miles
from the parking area. Follow the road from the parking area north and west
toward the ridgeline. The road reaches the ridge and goes west up the ridge for
a short distance. The path into the canyon is marked with signs. Photo 1.
photo 1
The path joins what was apparently an old road on the canyon
floor. There is a normally dry wash cutting the canyon bottom which is lined
with sycamores. Follow the trail to the left, up the canyon. In about .5 miles
the trail goes through an opening in a fence line. On the right rocks rise
sharply toward a ridge which runs north. If you get off the trail at this point
and walk up the hill, you will see that some of the rocks have been bend and
twisted. These appear to be sedimentary rocks that has been changed or
metamorphosed. Sedimentary rocks are normally created deep within the earth
under the pressure of overlying layers of material and heat from inside the
earth. The process is called lithification. The sedimentary rock was then
metamorphosed probably as the result of intense directed pressure on the rock.
Photo 2. Under such pressures the rock becomes plastic and actually bends and
twists.
photo2
As you continue up the canyon about 100 yards, the trail
divides at a small wash. The Brown Canyon Trail goes to the left according to
Taylor, Hiker’s Guide to the Huachuca Mountains. However both trails
rejoin each other in less than a mile and the path to the right is much more
interesting because it gives you a closer view of the ridge. After the path
crosses a small wash, you come to a small stock tank. The ridge is visible all
along the path. As you look toward the top of the ridge, you see several white
outcroppings of rock along the ridgeline protruding at about a 45 degree angle.
Photo 3. It is well worth the climb to get a closer look at these formations.
There is no path to the ridge so you must “bushwhack” up the hill. Exercise
extreme caution because, in addition to the normal dangers associated with
climbing, the Huachucas are home to several varieties of rattlesnakes.
photo 3
These rocks are a metaconglomerate, formed by extreme
metamorphic activity. Tectonic forces exerted shearing pressure on the
existing, or parent, rock which crushed, reformulated and flattened the rock.
Photos 4 through 10.
photo 4
photo 5
photo 6
photo 7
photo 8
photo 9
photo 10
A simple illustration of this process can be seen by
taking a rolling pin and crushing Oreo cookies onto a flat surface. The rolling
pin exerts shearing pressure, crushes the cookies into finer pieces, combines
them together and flattens the recombined cookies parallel to the direction of
the applied pressure. These rocks were primarily limestones which were
metamorphosed into marble. The metaconclomerate also contains large pieces of
chert which appear to have been crushed by the same pressures. Photos 11 and
12.
photo 11
photo 12
Phyllites, an intermediate metamorphic rock, which were also subjected to
shearing pressure, are also found on the ridge. Photo 13. As you head back
down to the trail, again be extremely cautious. Gravity often makes the descent
more hazardous than the climb.
photo 13
When you return to the canyon floor, continue on the
trail. You will come to a short, steep incline. In this area of the trail you
see some of the Precambrian granite exposed. Photo 14.
photo 14
This granite has been subject to chemical and physical weathering for hundreds
of millions of years. Although not as spectacular as some geologic events such
as earthquakes or volcanic eruptions, weathering and erosion are among the
principles forces that shape our physical environment. Along the trail on the
left, you can see examples of how the weathering process works. Photo 15 shows
jointing in the granite.
photo 15
Granite is an igneous rock which forms under ground. As overlying pressure is
removed either by the removal of the overlying rock or by the granite being
thrust to the surface by tectonic forces, the granite breaks along joints. This
process increases the surface area of the granite and, therefore, exposes more
of the granite to weathering processes. Weathering wears off the sharper
surfaces such as corners first, eventually leaving rounded boulders. Photo16.
This process is known as ‘spheroidal weathering.”
photo 16
Further along the trail are more examples of weathered granite. You can
easily crumble this granite with you bare hands. The large 1-2 inch crystals in
the granite or which have weathered out are called phenocrysts. These
phenocrysts are made up of feldspar and feldspars are the most common mineral
group in the earth’s crust. Photo 17.
Photo 17.
There is other exposed granite as you continue on the trail. Photo 18.
photo 18
The two trails join at the ruins of a house. The trail
continues along the north side of the canyon on the remnants of the road. A
little more than 2 miles into the hike, there is large, milky white rock which
intrudes on the trail. This rock is composed of quartz, the most abundant
mineral in the earth’s crust. Quartz occurs in numerous forms. This is massive
quartz but quartz also has a crystalline and cryptocrystalline form. Amethyst
and citrine are examples of crystalline quartz while agate, flint, chert and
jasper are among the cryptocrystalline forms of quartz. Quartz is a constituent
mineral of other rocks, including granite, is very hard and resists weathering.
This quartz was probably introduced into cracks in the existing rock as an
aqueous solution. That rock has weathered and eroded away, leaving the hard,
more resistant quartz exposed.
As you continue on the old road you pass the Pomona Mine
trail on your right. The Pomona Mine produced scheelite, an ore of tungsten.
(There is a student paper on mines in the Huachuca Mountains.) In about .4
miles, you reach a concrete trough which usually contains water. The water is
not fit to drink. Brown Canyon Road comes in from the left. (You can take this
road to Ramsey Canyon Road and walk down to the parking area if you don’t want
to go further up the canyon or return through Brown Canyon.)
The trail now enters the Miller Peak Wilderness Area and becomes more steep and narrow. This is a beautiful riparian area with hardwood trees. There is another example of jointing in granite shortly after entering the wilderness area. Photo 19. This granite has smaller crystals, no phenocrysts, and has not weathered as much as the granite in other parts of the canyon. Smaller crystals in granite usually mean the granite cooled more quickly which does not allow for the formation of larger crystals.
photo 19
The trail takes a 90 degree turn and passes in front of a
rock-walled cistern. If you go up the creek bed for about 200 feet, you will
come to the Brown Canyon Box. This requires some rock scrambling. There is a
circular pool carved out by water tumbling over an 18 foot drop from a small
gorge above. The rocks above the right side of the gorge have been deformed by
tectonic forces into an “s” shape. You can view the gorge and the rocks if you
continue past the cistern and go up a series of switchbacks. At the top of the
switchbacks you have a magnificent view of lower Brown Canyon and the San Pedro
Valley. You can also see the rocks as you look across the gorge. Photos 20 and
21.
photo 20
photo 21
In the gorge you can see a tree growing between layers of the rock, Photo
22, and a second tree, Photo 23, growing within pieces of rock. These are
examples of mechanical weathering. The trees are loosening or breaking the rock
as they grow larger. The rock on the far side of the gorge was probably twisted
into this shape as a result of thrust faulting or folding. The constituent rock
appears to be a siltstone or quartzite, both metamorphic rocks. The rocks end
abruptly at the box. At some time in the distant past, this unit of rock likely
extended further eastward.
photo 22
photo 23
Weathering has broken down and erosion has
transported this rock. It is now down the canyon and out in the valley in front
of you. Photo 24.
photo 24
The trail continues along the stream bed and then climbs
to a ridge which divides Brown and Ramsey Canyon. You pass through the remains
of a gate and begin dropping into Ramsey Canyon. You have several options at
this point. To stay in Brown Canyon you must retrace your steps. You can hike
out on the Brown Canyon Road to Ramsey Canyon Road at the trough or continue
down the canyon. Remember at the ruins you can take the trail on the right for
some different views on the return trip. You may also continue into Ramsey
Canyon. This route will give you several options including the west end of
Ramsey Canyon Road. The get to Ramsey Canyon Road you will need to go through
the Ramsey Canyon Nature Preserve. It is approximately 1.5 miles to the Preserve
and another 1.7 miles down Ramsey Canyon Road to the parking area.
A hike in Brown Canyon is well worth the time and effort. The scenery in the canyon is beautiful and contains many interesting geological features. The hike is only moderately difficult and most of the trail is shaded. Don’t be satisfied to sit in front of your computer and take a “virtual” hike—go hike the canyon!
References:
Taylor, Hiker’s Guide to the Huachuca Mountains, 1991.
General References:
http://skywalker.cochise.edu/wellerr/aawellerweb.htm