The Far Arroyo

[I originally wrote this in October 2011, but never posted this series on the local geology. The pictures are still worth seeing.]

I live between two arroyos, each about a quarter mile from the house. The one to the north I think of as the near arroyo because I cross it every time I drive into town. The other, to the south, I think of as the far arroyo because I have to walk across pueblo land to get to it.

The far arroyo changes its character every fifty or a hundred feet, partly because of humans. At the time the USGS map was revised for this quadrangle, the local ditch emptied into the near arroyo.

At some later time, a pipe was installed to carry water across the arroyo and out to the land downhill from me. A neighbor told me the land under his house and mine were once part of a ranch, perhaps the same one that survives beyond the arroyo. The acequia extension dumps a few hundred feet after it reaches pueblo ground.

 The water has cut its own path to the far arroyo. From a distance it can be followed by the trees that grow along those banks, including those yellow cottonwoods pictured in the previous entry.

Thursday I walked down the arroyo to the point where the acequia feeder enters the arroyo. At that point the water has cut a path not much more than a foot deep.
 

The water flows immediately to the right where it has cut through the soft bottom land. The sand and clay wash away, leaving a path of gravel, the generic Santa Fe Conglomerate that covered this area before the rift opened.

 

The soft soil absorbs water from both the air and the arroyo, which makes it more likely to crumble.
 

The water apparently eats back through the soft soil under harder upper layers, until they collapse from lack of support. The water line under the rock can be traced by the appearance of Russian thistles that find enough sustenance to germinate in late summer.

 

The arroyo widens and deepens where some previous geological event created the layers of hard rock.

Then, when the hard rock ends, the wall abruptly stops and the arroyo returns to bottomland vegetation.

That’s the point I turned back Thursday.