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.