The highway skirts the outer edge of the twelve-mile wide caldera so there is no one place that one can see the full scale of the feature. The meadows in the picture above are only 2 miles across.We'll have to depend on radar imagery to get a full sense of the shape of things (below).
|Image of the Valles Caldera courtesy of the New Mexico Bureau of Geology and Mineral Resources.|
It has been around 60,000 years since the last volcanic activity at the caldera, but magma still simmers in the heart of the volcano. Heat flow under the caldera is high, and hot springs can be found around the margins. We found one of them while driving down San Diego Canyon, something so out of place that I hit the brakes and came to an unexpected and unannounced stop (to the chagrin of the five vans following me...). It has to do with the "inside-out" cavern of today's post title. We had happened upon Soda Dam, an outstanding example of a travertine bridge.
The hot springs have been active for nearly a million years, as evidenced by travertine deposits high on the canyon walls. Other deposits nearly the valley floor are around 60,000 years old, giving an idea of how rapidly San Diego Canyon was carved. The Soda Dam itself is about 7,000 years old.
A nice source on Soda Dam and the many other geologic features of northern New Mexico can be found in The Geology of Northern New Mexico's Parks, Monuments, and Public Lands from the New Mexico Bureau of Geology and Mineral Resources.