[DirtMcGirk]

I was thinking today sitting here at work about the places I've ridden as its too damn hot to be outside today, and I miss riding my DH bike.

I began to notice that the further south I've gone, the more loose, dusty, and generally "sh!t show" the trails have become. Starting from Whistler, with its buffed runs, and rocks that are slick, to the east coast, with its mud and roots, down through Oregon with its ferns and total lack of rocks, into Northern Nevada with its rocks, some dust, and a lot of granite, down through Utah with its slick rock, forests and high mountains, down here into Arizona with its busted up sand stone, sand and red clay.

I never took geology in college. Turns out one nutrition course and a dabble in anthropology and I had covered my requirements to get a Political Science degree. Can someone clearly smarter than I explain why it is that the further south I've gone, the looser its gotten? Is it lack of water? Is it the fact that its a blast furnace the further you go? Teach me something, 'cause I'm clearly not gonna learn **** at work today.

 

[johnbryanpeters]

John McPhee, Basin and Range

 

[sanjuro]

 

[buildyourown]

Whistler is hard cause it was impacted by millions of years of glaciers. There is only 30" max of topsoil and then you hit really dense clay/rock. The small rocks have been tumbled by glaciers, so they are round vs sharp.

Oregon is dusty because of the volcanos.


Geology is more micro than you are making it out to be. There is a lot of different stuff going on in a 100 sq mile area.

I never took geology, but I married one.

 

[conor]

you married a rock? that must be hard.

 

[DirtMcGirk]

you married a rock? that must be hard.

 

[Chumble spuzz]

Geology is more micro than you are making it out to be. There is a lot of different stuff going on in a 100 sq mile area.

In my case the trails are totally different in about four miles. The trails on the Vermont side of the Connecticut River are buffed, dirt, smooth, and fast. Very little in the way of rock, but what rock is present is typically shale and quartz. The trails on the New Hampshire side of the river are granite. There is hardly any dirt or roots - just rock, slow and technical.

So here's confirmation that the micro-geological phenomena are significant - in my area anyway.

 

[Pesqueeb]

I'm pretty sure your deteriorating trail conditions coincide nicely with regional average rain fall amounts. Not necessarily geologic conditions to a given area.

 

[mudgirl]

http://www.soiltypes.org/

Fellow political scientist here, but I used to study geology as a "hobby" when I was younger.

 

[Skookum]

i went for a ride at Anacortes Community Forest Lands yesterday. It's on Fidalgo Island on the Puget Sound. Pretty much part of the San Juan Islands, which were it's own micro-continent floating in the Pacific, until it collided into North America and the crust tilted. Then it got carved by the glaciers. So basically if you were a geologist you would probably have a hard time riding the trails as you would have to stop at every trail and check out all the variation of rock. i think there is around 10 different shelves of rock, each one exposed is millions of years of separation.

Quick google about the island geology to confuse us all but to validate what i'm saying. A sequence of Jurassic rocks on Fidalgo Island, Washington, is interpreted to be ophiolite. The order of rock types, from the base upward, is serpentinite, layered gabbro, a dike complex made up mostly of plagiogranite, volcanic rocks that are dominantly keratophyre, coarse breccia with clasts of keratophyre and plagiogranite, pelagic argillite, and siltstone-sandstone turbidites.

 

[DirtMcGirk]

a dyke complex

I've seen a dyke complex once or twice too...

 

[buildyourown]

I thought this was a dyke complex?

 

[DirtMcGirk]

So here's my next question:

With the exception of dealing with rain differences, as that's a whole other topic, is there something specific inherent to trail/freeride/dirt jump building in each geological area?

For example, in Whistler you can almost pack and forget things.

Reno, it hardens up but you still gotta water it.

 

[buildyourown]

So here's my next question:

With the exception of dealing with rain differences, as that's a whole other topic, is there something specific inherent to trail/freeride/dirt jump building in each geological area?

For example, in Whistler you can almost pack and forget things.

Reno, it hardens up but you still gotta water it.

You can't really separate them. A little moisture makes a big difference on how much you can compact soil. That's why they have to water dirt as they compact it for a road bed.
Whistler also has the compacted clay with zero organic matter that stands up very well to erosion. A little too well in fact. Rain water runoff is a big deal around here.

 

[TRON]

Arizona soils have less clay content than Whistler, so they will not hold moisture as well. The hotter and drier climate does not help either. If you go to the southeast, you will be at the same latitude as Arizona and the trails will be different.

 

[Gex]

Tron your actually wrong... Arizona soils have a much higher silt content then those found in Whistler. Whistler soil is pretty "raw" in terms of grain size due to the massive glaciations that shaped the area last 30,000 years ago but also all through the Holocene.

Arizona actually has predominately sedimentary rocks which are the result of erosion and weathering of the continental crust that has seen little to no geological activity (tectonically speaking) for a long long time (AZ has some of the best precambrian (500+million years) rocks in the US).

The soils in AZ are renowned for their super nasty expanding clays, these things are the bane of geotechnical engineers down there.

-And yes I am a geologist!

Edit: The clays in Whistler are of the non expanding variety. Same kind we have down here in Washington.
See

http://www.azgs.state.az.us/HomeOwners-OCR/HG4_problemsoils.pdf

http://southwest.library.arizona.edu/azso/back.1_div.10.html

Clay minerals - Wikipedia, the free encyclopedia@@AMEPARAM@@/wiki/File:OxfordClay_Weymouth.JPG" class="image"><img alt="" src="http://upload.wikimedia.org/wikipedia/commons/thumb/b/bd/OxfordClay_Weymouth.JPG/220px-OxfordClay_Weymouth.JPG"@@AMEPARAM@@commons/thumb/b/bd/OxfordClay_Weymouth.JPG/220px-OxfordClay_Weymouth.JPG

Montmorillonite - Wikipedia, the free encyclopedia@@AMEPARAM@@/wiki/File:Mineraly.sk_-_montmor.jpg" class="image"><img alt="" src="http://upload.wikimedia.org/wikipedia/commons/thumb/1/1d/Mineraly.sk_-_montmor.jpg/240px-Mineraly.sk_-_montmor.jpg"@@AMEPARAM@@commons/thumb/1/1d/Mineraly.sk_-_montmor.jpg/240px-Mineraly.sk_-_montmor.jpg

 

[sanjuro]

BTW, I got 5 positive rep for this stupid vid posting.

 

[Ciaran]

BTW, I got 5 positive rep for this stupid vid posting.

You're lucky I still need to spread it around otherwise you would be getting a neg. I disapprove of spreading teh juggalo.

 

[TRON]

Gex - The trails I have been to in Arizona were far sandier than Whistler. I agree that Arizona trails will have more silt because silt is a grain size larger than clay. Being that it's a grain size it is irrelevant to your expansive clay information. Your "raw" grain size at whistler would suggest that it's a larger grain size, but I really don't remember it being sandy. Also, I don't understand how you are relating clay type to clay content. If the clay minerals in Arizona are of a shrink-swell variety, that does not mean the soil has more clay by volume. As for the lack of geologic activity in Arizona, take a look at the Grand Canyon.

 

[SkaredShtles]

<snip>

The soils in AZ are renowned for their super nasty expanding clays, these things are the bane of geotechnical engineers down there.

Mmmmm... bentonite...

 

[Gex]

Glacial tills... Read chapter 10 in your Compton Geology in the Field. You'll learn what you want about the soils in whistler and az.

The Grand Canyon as we see it today was shaped by the Colorado River, wind erosion, and freezing/thawing. The primary rock types seen in it are limestone and arenites. They're numerous marine fossils which show that these rocks were formed in the empiric sea that was once in the middle of the north american continent. The basement rock is indeed granite and schist which are igneous and metamorphic rocks respectively.

Look it up G.

 

[jdcamb]

Rocks are hard and heavy. If a Rock falls on you from a great height it will crush you dead. Even a small one.

 

[AngryMetalsmith]

Rocks are hard and heavy. If a Rock falls on you from a great height it will crush you dead. Even a small one.

Yeah, but squirrels throwing acorns can be just as deadly. Little bastards.

 

[boostindoubles]

Moisture plays a huge role in why you see so much variation in the trails your talking about. Whistler is on the Pacific Coast. It gets way more precipitation than the Southwest. With moisture comes compaction, like has been said, but it also brings plant life. The plants are continually renewing themselves laying down a thick layer of organic matter (OM) that mixes in with the mineral content of the soil. This mixing is done by microbes, worms, rain, etc... What you get with all this OM amending the soil is super fine, tacky, even spongy feeling trails. (Whenever you see black soil, it generally will have high OM content)

Arizona and the rest of the Southwest, save certain micro-climates, sees way less rainfall. Theres no deposition of OM which hinders the development of those tacky, spongy trails that the wet Northwest has.

An example could be archeology. In the Southwest, if you know what to look for you can find prehistoric artifacts laying on the surface of the Earth. They have been there for hundreds, if not thousands of years and nothing has covered them up. In the wet environments of the Northwest, or back east, or anywhere that sees lush growth and rainfall, an object laying on the surface is buried in OM in a short time.

Basically, the Southwest is an erosional landscape, where the surface of the Earth is being removed by wind, water, etc... faster than its deposited. Whistler and similar areas are the opposite.

So I didn't answer your question but I hope I shed some light on why you see such changes in environment.