Postcards from the Field

Geologic mapping in the Bell Mountain 7.5-minute quadrangle

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Caption: The white line in this annotated photograph shows the approximate
paleosurface that existed right before the basalt erupted. On the right side of the
photograph, you can see that the line cuts down and forms a steep slope. The rock that
is above the white line is basalt. The small dome on the left side of the photo is a small
basalt cone and is likely where this basalt flow erupted from. As such, you can imagine
how the basalt might have flowed down the slight gradient on the paleosurface and filled
up this old paleovalley.
(click for a larger version)

2026 Photo by Becca Goughnour

This is a zoomed in view of the paleovalley, now filled with a thick package of basalt. It is hard to see in this photo, but the basalt here has columnar jointing (columns indicated by short white lines). Columnar joints form as a lava flow cools and they are usually parallel to the direction of heat loss. On a flat lava flow, these columns would likely be straight and vertical. In this example, however, the columnar joints are slanted and sometimes curved. This is likely because the basalt flow cooled on the slope of this paleovalley, rather than on a flat surface.
(click for a larger version)

2026 Photo by Becca Goughnour

Here are some close-up examples of columnar jointed basalt.
(click for a larger version)

2026 Photo by Becca Goughnour

By Becca Goughnour, Field Geologist
NMBGMR

Bell Mountain 7.5-minute quadrangle
August 21, 2026

Last spring, I mapped geology in the area just north of Hillsboro. The rocks in this area have been very exciting to map because they include ~1.6 billion year old basement rocks, ~500 to 300 million year old limestones and shales, ~70 million year old intrusions, 40 to 28 million year old volcanic rocks, ~20 to 5 million year old basin-fill deposits (Sante Fe Group), and terrace deposits that are up to 700,000 years old. Last time I was in the field, I noticed some really interesting basalt flows. These basalts are around 4.8 to 4.5 million years old and are related to Rio Grande rifting. Usually, these basalts form the tops of broad, flat mesas in this area; however, we noticed a place where the basalt seems to “dive down” and fill an old paleovalley.