Key terms:
Duricrust - hardpan near top of soils
Caliche - type of duricrust made of lime
Gypcrust - type of duricrust made of gypsum
Capillary action - movement of water up in a very thin tube
Be sure to take a look at the adjacent park.
In order to claim the cache, please use geocaching.com webmail to tealeaf in order to answer one question, it is about the park
You are looking for boulders with lots white crust on them, any one you like is fine
There are a lot boulders strewn around ground zero. Look closely at the white coating you see on most of them. The collective name for hardened whitish layers, found close to the soil surface, is duricrust. We also call these layers hard pan.
In the Rio Grande Valley two kinds of duricrust can be found easily. One kind is caliche, the white stuff on your newly found boulder buddy. The other kind is gypcrust. We can find this gypcrust in the Rio Grande Valley in old playas. One example is the gypsum mine site, East of I-25 in the Galisteo wash, just before you begin ascending La Bajada.
Caliche is calcium carbonate. Like limestone, lime, and chalk.
Gypcrust is calcium sulfate. Like the gypsum you use to improve clayey soils and in the sheetrock on the inside walls of your house.
Both of these are common in arid areas. Formation of the two kinds of duricrust is different. You can think of caliche as forming from from the bottom up, and gypcrust forming from the top down.
Caliche depends on subsurface moisture which moves up by capillary action and evaporates leaving behind salts it picked up along the way up.
Gypcrust forms from water flowing downhill on the surface and then puddling on a flat surface, like on a playa. The water already comes into the puddle with tiny amounts of calcium sulfate dissolved in it. It evaporates and leaves salts behind, too. Since a lot of the sediments here in the valley have at least a tiny amount of calcium, the soils that live on or near those sediments may have either duricrust form. Caliche is more common.
Note the boulders have been moved, and dumped unceremoniously, probably during construction of drains and erosion control walls (those stairstep things North of you) for the wash. So they are not like the way they were in the ground
You will see that there is a bare surface on the boulders - can be hard to see because of the present orientation of the boulder. Most of the boulders have caliche on several "sides" and the bottom. Because the caliche formed "from the bottom up", so the top exposed top surface never got any white stuff on it. Pretty good indication we have caliche!
Questions:
1. If a duricrust were to form in the soil in the park, what type is most likely?
Answer is (Please pick just one): caliche, gypcrust
Thanks for taking a look at a really cool feature of the Rio Grande Valley