Geology for Dummies*

* Well, not exactly dummies - but for those of us born into an earlier era, there comes a time when it’s necessary to step back and try to grasp what any modern child might learn today in grade school.

When I was young in glacier dominated Michigan there were simply seven continents, and some rather mysterious drawings of folds, faults and subduction zones. Since nothing really mattered before the Pleistocene, I ignored those drawings.

Since then, scientists have agreed on the existence of plate tectonics and continents are no longer givens, but the results of processes. Here in the Española Valley we’re on the boundaries of plate activity that I’ve spent the past few hours trying, once again, to understand.

The Wikipedia entry on the Wyoming Craton has a useful schematic showing the elements that coalesced or were absorbed into the Laurentian plate that became North America. This part of New Mexico was somewhere on the boundary of the collision of a southeast facing section of that continent with Yavapai-Mazatzal that resulted in lines of weakness that were subsequently buried under layers of sedimentary shale, sandstone and limestone.

Laurentia later overrode the Farallon plate in the Pacific. As the last, western most section of Farallon was swallowed, undissolved bits on the eastern side tipped up as part of the mountain building that produced the Sangre de Cristo.

When the Farallon plate finally did disappear, one section of the North American plate began rotating clockwise to drift into the newly vacated area. As it shifted, the great rift began to open as one part of the land shifted with it and another stayed in place. As the rift expanded, blocks of land roughly defined by those old Yavapai-Mazatzal collision lines dropped, some facing east, some facing west and magma welled through the Jemez Lineament.

The Emdudo faults are the southwest-northeast northern frontier of the Española valley and the La Bajada faults part of the southwest-northeast running southern one. Almost no displacements occur without some tilting. Those visible at the Cochiti exit are mild, those a few miles away at the Garden of the Gods are extreme.

The layers everywhere are probably the same, but what’s visible is probably as much a function of road building as natural forces. At La Bajada the lower red stones are more visible than the lighter colored stones above that either are covered or have washed away. At the Garden of the Gods, the limestone has lasted longer than the softer red sediments.

Above picture from Garden of the Gods on State Road 14; the others from La Bajada at the Cochiti exit.

Between Dixon and Picuris

Now that I have a new camera I have to test it.

My usual method for day tripping has been to look at the map, find something that looks interesting, load the ice chest, and start driving.

There is nothing that can repeat the absolute surprise you feel when you come upon something like the Abiquiu Dam when you’re not expecting it and nothing you want to do to deaden that heart-stopping awe, that sense of unity with all the explorers from the stone age to the present who’ve been there before.

When I get home, I start to read about the place I’ve just been. Then when I go back, I have some better ideas what to look for and some idea about where it’s safe to pull off the road.

Saturday, I took the new camera back to that road beyond Dixon where I had problems with my tires earlier this summer.

After NM 75 leaves Dixon, it feels very much like a typical mountain road with the Rio Embudo on the right side, an occasional gorge coming down from the left, and continually opening V’s as the road climbs towards the Sangre de Cristo south of the Picuris mountains. The trees are all still juniper, but closer together than they were below.

Then I came to a section where raw rock was exposed, standing on edge. It looked like shale at the Rio Grande end, then layers of sandstone. However, they seemed so weathered they had no distinct colors, and I’m not sure I even know what shale looks like, other than it’s gray.

My first thought was, why on earth did anyone go to the expense of blasting a road through here. This is New Mexico, where they didn’t widen the road to Los Alamos until after I moved here. Route 75 was described as “a graded dirt road” in the 1930's WPA Guide when the road that came up from Chimayó through Truches and Las Trampas could get you to Picurus pueblo and Peñasco.

I spent yesterday learning absolutely nothing.

The Española Basin, which tilts west down to the Rio Grande, ends at Velarde. The San Luis Basin, which tilts east, begins farther north. Between the two lies a diagonal belt of disruption that goes from the Picuris mountains to the northeast through the Embudo Fault across the peninsula between the Chama and Rio Grande rivers toward the opening into Santa Clara Canyon.

I presume I was somewhere on the edge of that fault, but can get no confirmation. Is this what a fault looks like?