Ranch Road

Yesterday I walked out the ranch road towards the Black Mesa. I went as far as the pueblo boundary where it branches, with one prong going through a gate to the ranch and the other through an arroyo and on toward the mesa.

The road looks and feels like the arroyo bottom, but the neighboring soil is the usual erodible mix of clay and sand. When you get to the Y, the road on pueblo land becomes two tracks of uncovered dirt that would be difficult for a car to negotiate in wet weather.

As I walked back on compacted soil veneered with fine gravel, I wondered if the ranch owners had dredged the bottom of the far arroyo to protect their road, and if others had also done that in the past here and in the other arroyos.

If that were true, I wondered if the arroyo bottom had once been filled with gravel, had been bare like it is now, or had once supported vegetation like the arroyo on pueblo land. I also wondered if their mining had widened or flattened the bottom, or if the general shape had been carved by nature and only slightly been modified by them.

Bottom photograph shows the land to the side of the road, taken when I stopped to take the middle picture of the road where I kicked down to expose the base. Top shows the unimproved road on pueblo land.

Black Mesa

It seems, wherever you go in this part of the world, you find a Black Mesa. It shouldn’t be surprising, really, when you consider the Rio Grande crosses an oblique line of volcanoes.

There are two in this area. North of town, the ten-mile long finger of land between the Rios Grande and Chama has been called the Black Mesa by geologists since George Wheeler named it so in 1876. Down on the San Ildefonso grant near the road that people use to commute between the valley and Los Alamos there’s another, rising 500' from the surrounding flat lands that slope to the river.

When I first moved here, I was told or read the San Ildefonso mesa was the cap of the Valles Grande volcano that had landed there when it erupted. I’ve also been told it was the last retreat of the pueblo peoples fighting Diego de Vargas in the entrada and that there’s a pair of peregrine falcons nesting there.

Ted Galusha and John Blick resolved the ambiguous naming problem by calling the San Ildefonso mesa the Round Mountain. I prefer to find another name for the peninsula of land, perhaps Wheeler’s Black Mesa or the Chamita Black Mesa.

They also dispelled any romantic notions about its origins.

It’s an independent cinder cone built around a volcanic neck that was formed some 4.4 million years ago in the early Pliocene, possibly as part of the Cerros del Rio volcanic field southeast of the Otowi bridge. According to Daniel Koning, one of its lava flows has been dated to about the time the Rio Grande was becoming a perennial river, long before the Toledo and Valle Grande eruptions.

The base is dark gray to black basalt. The top is covered with river cobbles and pebbles laid down about 1.5 million years ago, long before the lakes described by Steven Reneau and David Dethier.

Cinder cones are the simplest type of volcanos, central vents surrounded by fans of erupted debris. Volcanic necks are formed when the magma hardens within the volcano. Red cinders found on the southwest side suggest an eruption was through that side.

When water washes away the softer materials that surrounds a volcano, the basaltic blocks drop into a dense mass around the base that eventually prevents further erosion. If indeed this volcano was standing in 500' of water in the early Pleistocene, there was a great deal of water available to consolidate the current formation into a fortress which has since survived those lakes and, perhaps, created the fans where juniper now grow.

Top: Black mesa behind cottonwoods along the Rio Grande taken from the west side along route 30.
Bottom: Mesa taken from northeast; the ranch road wanders towards it after leaving the far arroyo.

The Cone

This afternoon I went back to the cone, determined not to be fooled again by its wiles. This time I took the simple path, the one from the near wash. As I walked up the easy grade, I realized the cone might just be the tip of the hill I live on, the one that had caused so many water problems in the past.

As soon as I got close I saw that it wasn’t bare because of erosion caused by the ATV rider. It was actually stone and not some soft soil. Daniel Koning had said it was tertiary, not the more recent quaternary. The two were there to see on the slope where the grass couldn’t hold its own.

I started climbing the cone. The more I climbed, the more the top receded into a face of gravel.

I felt no overwhelming urge to make it to the top and yodel. My knees were chanting "Jack and Jill went up the hill." The view of the black mesa was quite spectacular where I stood.

On the way back down I picked up some pieces of the grey, rough textured stone for the Rock Queen.

Hopefully she can tell me what it really is, something more useful than tertiary side of the geological change.