Fort Valley Trail system

rockman said:

Thats why I'm not a big fan of the steep, freeride skidder trails being put in on Elden (ie., Red Onion and Wasabi). They quickly become deep ruts and basically turn into a rocky wash with a few hard rains. IMO.

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Not really, private reserve and steel reserve do not become deep ruts/rocky washes. Wasabi has a a few deep ruts at the top, just because it stays so dry in the summer, in fact it's just as bad in the hobbit forest, so it's not like those trails stand out for any particular reason.

I'm sorry, I have to completely disagree, private reserve is one of the best trails I've ever ridden, I don't mean it's the most challenging, or that it has the biggest jumps, but it all flows together, is well constructed so it doesn't change much, and it simply had a lot of care put into it so that erosion doesn't affect it. During the monsoon season it was fine. If you wan't to see crappy fall-line trails that have become huge washes, just come to prescott and I can show you a few (but we have plenty that are not like that too). What you are describing is a problem in some place, but not on the trails you are describing. Upper Oldham has some big washes where the trail takes the fall line/water path and consequently there's big ruts and rock washes.

I'm not sure what you mean by "skidder" trails, you must be referring to things like Oldham and Shultz Creek, at least that's where I see a lot of "skidders".

FishMan473 said:

I thought most of those 'mountains' in the Phoenix area were made of granite, since they were the cores of ancient mountains/volcanos. I haven't ridden all the trails in the area, but South Mountain, Estralla, and McDowell all seem to have pretty course soil that does well after rain.

Now that the thread has been completely hijacked, so what makes Dacite different from granite? Is it just the color, granite is more pale with more pink feldspar? Are there any other significant differences? I've never really understood the way igneous rocks are classified.

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I mainly study rivers and the impacts of dams but I will try to answer your questions and continue to thread hijack.

Igneous rocks are subdivided by: 1) the size of the mineral grains and 2) the chemistry or mineralogy of the rock. Mineral grain size depends on how quickly the magma cools and crystallizes. Thus, granites, diorites, and gabbros are rocks the formed deep and cooled slowly. Rhyolites, Dacites, Andesites, and Basalts are fine-grained because they were exposed to air on the earth's surface. If a magma is cooled very quickly , particularly a rhyolite magma, it may not have a chance to crystallize at all and instead will form a natural glass, obsidian (that is there is no organized crystalline structure).

Then there is chemistry which direclty affects the types of minerals that will crystallize. At the mafic end of the spectrum (ie. basalt) the minerals are likely to be olivine and pyroxene, minerals rich in iron and magnesium, and a form of plagioclase feldspar. Hence the rock is dark colored (basalts and gabbros). At the felsic end, the rock is likely to contain feldspars rich in calcium and sodium, and quartz. These rocks are light colored (rhyolites and granites). Of course there are exceptions to this crude scheme but in general it holds true. So, you are correct granite has more pink feldspar than diorite which contains less quartz and has more mafic minerals. It is also finer grained because it ventured forth onto the surface as a lava flows Granites are often associated with batholiths, huge bodies of intrusive material that in many places form the cores or root of mountain ranges. Dacite on the other hand is more often associated with volcanism, like the SF Peaks, a stratovolcano, and the lava domes of Dry Lake Hills and Elden Mountain.

So the types of soils formed from weathering igneous rocks (which applies to sedimentary and metamorphic rocks as well) depend on the stability of the minerals present, the bonds holding them together, and the percentage of quartz (very stable). Since feldspar is a key mineral in most rock types it weathers into one kind of clay and other silcate minerals into other types of clay with varying abilities to soak up water (clays are "sheet" silcates so you can think of them as pages of a book and water spreads the pages apart and makes them cleavable-slippery).

As for the mountains of Phoenix there is a lot of variety. The fables Superstition Mountains are an enormous caldera complex that erupted violently some 40 million years ago. Lots of dacitic ash-flow tuffs. The Mazatzal Mountains are way old Precambrian sedimentary rocks that got uplifted and thrust faulted to the surface during the Mazatzal orogoeny (1.6 billion years ago and are possibly the oldest rocks in AZ). Orogeny means mtn building and plutonism was also associated with this such as the granites around Payson and Prescott, the McDowell Mountains and Sierra Estrella. South Mountain and the White Tank Mountains are metamorphic core complexes composed of intrusive or metamorphic rocks which rose to the surface as the region got pulled apart during development of the basin and range province 10-25 million years ago (hence many of the ranges are parallel with large basins separating them). These ranges have lots of foliation (the rocks got stretched and hosed over) and the rocks are gneisses and mylonites. They have more gentle peaks than the jagged peaks elsewhere in the phoenix basin. Camelback Mountain is composed of Tertiary sedimentary rocks overlying Precambrian granite.

AZ is indeed a cool state geologically speaking.

Well, I love rocks so its easy for me to pontificate about them although spelling was never my forte.

I believe the rock staining is probably iron and magnesium that has been dissolved as part of chemical and physical weathering from the nearby host rocks that then when exposed to the surface and air, precipitates onto the surface of whatever the surface water may be flowing over.

JM-I'll have to agree to disagree. I guess my rock/soil and associated trail thing should be considered a hypothesis but I will be interested to see how some of the steep social trails on Elden will hold up in the next year or two. Only the expert riders can descend Wasabi without skidding their way down. An arid climate with violent hard rain during the late summer monsoon months begs for careful trail building. Again, in my opinion. Moto is a perfect example. It originally arose as an equestrian trail that mtn bikers discovered and improved on in 93/94 but there was no thought to switchbacks and runoff diversions. Within a couple of years it had turned from buff singletrack to a fairly rocky trail, albeit still pretty dang fun. It will be interesting to see if the new sections of the Fort Valley Trail and the additions/improvements/rerouting of Moto will hold up. You can't go as fast because it is more circuitous but I suspect that in the long run it will fare better than Wasabi and the trail you mention Private Reserve (i'm not familiar with that name). My 2 cents.

Well, you refered to DH and skidding "trails", but is your only specimen the upper wasabi trail? As I said, the top of it has a few ruts, characteristic of the angles it's built at and the kind of bikes used, and in the summer it does get dry a few times and it gets dusty and a little rutted, just like sections of the hobbit forest. Perhaps if I get a better DH fork for the front of my new frame I can show you some of the other trails up there. The upper section of wasabi is short though and obviously the water doesn't run down the entire trail, there are big dropoffs, rocks, and logs to keep it from doing so, so what you are seeing is a result of the angle of the trail and conditions. How many people are skidding, widening, and loosening rocks from the other trails? Lots.

I'l give you the fact that upper wasabi is not the best constructed and is fairly extreme, to build a trail such as wasabi it is going to be hard to keep some ruts/skidding from becomming a problem. I think of DH trails at ski resorts and I see the same things often. I have to disagree because upper wasabi is not indicitive of the "downhill trails" in flag. Other trails have much better flow, much less erosion, much better construction, etc.

BTW, sometimes I've wished I became a geologist/vucanologist, those two subjects intrique me a lot and I've read a few college texts on them. I especially like vocanic fields and the areas around Mono Lake/Mammoth Lakes. Just fascinating.