Friday, January 13, 2012

Black Rock Trail White Tank Mountains: Touring a Bajada

Lower alluvial fan, furthest away from the mountains, of the bajada on Black Rock Trail in the White Tank Mountains.  Cacti and bursage, shallow rooted plants characterize this part of the bajada indicating some caliche development in the soil.  Creosote, a deeply rooted plant shows that this caliche is not well developed and still allows for deep roots to penetrate it.

As for easily accessible trails, the Black Rock trail in the White Tank Mountains Park is a good one.  Crowded, but not as crowded as the nearby Waterfall Trail and the landscape is not nearly as trampled.  This particular trail has two loops, a short loop with a smooth and wide path, and a long loop which is narrower and slightly rocky.  The short loop is half a mile while the long loop which branches off and returns to the small loop is 1.3 miles.  While most hikers may find such a short and easily accessible hike sort of boring, I believe a little scientific knowledge of the area can give you some things to look for, making the hike quite interesting.  I personally would have considered this hike sort of boring if I wasn’t taking the time to examine landscape features.  But taking a little time to smell the roses, or rather examine the dirt, rocks, plants and so on, can make this boring hike quite interesting.  Looking closely, this loop is an excellent example of a classic Sonoran Desert bajada, specifically the upper to mid portions of a bajada.
First of all, a bajada is simply an aggregation of several alluvial fans.  And if you are wondering what an alluvial fan is, it is simply the sediments or dirt that is deposited on the ground by water.   Alluvial fans often make a fan-shaped landform when viewed from the air.  Alluvial fans are deposited adjacent to mountains when sediments wash off the mountain.  Several alluvial fans deposited in succession together form a bajada, and in the Sonoran Desert the further a fan is away from a mountain the younger it is.  The Black Rock Trail has a slight slope up towards the mountains.  Hiking along this slope you are hiking over very old alluvial fans which sediments were once part of the White Tank Mountains.

In the case of Black Rock Trail, there are three alluvial fans, each with is distinct soil type.  Starting off at the trail head, furthest away from the mountain is the youngest fan.  The plants indicate there is a weakly formed layer of caliche here.  Creosote is most common in this area of the trail being its deep roots do not tolerate well developed caliche layers, and therefore indicates weak caliche or an absence of it.  Triangle-leaf bursage also common to the area however requires some caliche development in the soil.  So put together bursage and creosote indicate a weak caliche horizon. 
Middle alluvial fan on the Black Rock Trail.  Characterized by shallow rooted plants such as cacti and triangle leaf bursage.  The presence of caliche in the soil favors these shallow rooted plants.
Hiking up the slight incline a short distance there is an increase in cacti, bursage, and palo verde, and a sharp decrease in creosote.  Palo verde, bursage, and cacti all seem to prefer stronger caliche development while creosote doesn’t tolerate it well.  This is the second alluvial fan, and plants with shallow roots that can accumulate above the caliche dominate this area.  Deeper rooted plants such as creosote are not able to penetrate the caliche and therefore don’t grow in abundance.

If it were just for the bajada, this trail might be at least a little boring.  But in the beginning to mid portions of the trail you pass some striking out-crops of granite.  What is striking about these out-crops is not simply that they are granite, but rather that the rocks are out of place in the bajada and that their coloration is pitch black in places.  These out-crops are sort of islands of rock surrounded by the dirt and sediments that form the bajada they pierce through.  As the bajada sediments were deposited off of the mountain these rocks remained in place, largely unmoved.  The alluvial fans and bajada was simply deposited around the rocks.  The rock formations are therefore very old, much older than the surrounding bajada.  The old age of the rock formation is indicated by the presence of the thick and very dark black desert varnish layers that color these rocks.
Granite outcropping along the Black Rock Tail.
Hiking beyond the short loop and onto the long loop the trail becomes a little rougher.  You will notice that the ground becomes increasingly rocky and more uneven.  This indicated the transition to the third and oldest alluvial fan closest to the mountain.  Here, there still may be some triangle-leaf bursage indicating caliche, but the further you go there is defiantly a shift towards more brittle bush.  Brittle bush prefers very rocky and often unstable soil surfaces, both common to this area.  There are still on this third fan a lot of cacti as well as palo verde and some ironwood trees.  The Ironwood trees seem to increase the closer you get to the Waterfall Trail, possibly a result of their deep roots accessing water that sinks deep into the sediments after if flows over the waterfall.
The upper alluvial fan of the bajada.  If you were able to see the ground you would notice that it is much  rockier and rougher than the lower two alluvial fans.  This type of soil favors brittle bush which is the shrub layer you see covering the ground here.  Cacti are still common here indicating some caliche in the soil.
As the long loop of the Black Rock Trail loops back and starts heading away from the mountains you begin your return down the bajada.  The whole trip requires at most 45 minutes or so if you go slow, and in that short period of time you can get a great tour of a Sonoran Desert bajada and the plant ecology.  Really, tours of this type don’t get any easier.  After this hike I am at about six miles of my 150 mile hiking goal for 2012.  So far so good.  I am hoping to do some longer hikes in the near future and hopefully take some big chunks out of my goal pretty quickly.