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San Ysidro Anticline: Where the Colorado Plateau Meets the Rio Grande Rift

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We're about to hike along the south rim of what geologists call the San Ysidro anticline, or alternatively the Tierra Amarilla anticline. Trail bikers call it the White Ridge . Millions of years ago, things looked quite different here: we're peering into a valley that was a ridge, not a low point. The peak of the ridge followed the center of the valley you see here - so the highest part of that ridge is now the lowest part of today's valley. Geologists call this "inverted topography": the highest parts of the land have been transformed into the lowest parts. How did that happen? The rock layer that covered the top of the ridge was made of shale, limestone, and gypsum. You can see the white gypsum on the trail at the edges of this photo (geologists call it the Tonque Arroyo Member). This hard rock cover was eroded off the top of the ridge by wind, precipitation, and freezing and thawing. As it wore away, much softer sandstones, mudstones, and siltstones beneath
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The Earliest Evidence of Basketmakers in the Southwest: Falls Creek Valley

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Looking across the Animas River Valley, north of Durango, Colorado, to the white, banded cliffs that are above the Falls Creek Valley. In the background are the La Plata mountains with early Spring snow.  These cliffs are designated the Falls Creek Archaeological Area because some of the first dwellings built by Basketmaker people in the Southwest were discovered here. There were dwellings inside rock overhangs at the base of those cliffs and on the slopes below. The rock shelters and caves in these cliffs were first lived in some 2,700 years ago, at the end of the Archaic and beginning of the Basketmaker period. They remained occupied until this region was vacated by Indigenous people around the year 825. People may have preferred to live high up on the side of the Falls Creek Valley because it was warmer there than on the valley bottoms, which collected colder air that tended to sink. These Basketmakers were among the first people in the Mesa Verde region to cultivate maize, squash a
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Crossing Over the Rim into the Valles Caldera

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We hiked up into the lovely Cañada Bonito from the base of the Pajarito Ski Area. It is unique because it is one of the very few high mountain grasslands in the area that has not been subjected to livestock grazing since 1943, when the Los Alamos National Laboratory restricted land use. We reached the rim of the Valles Caldera at the head of Cañada Bonito. The trail turns north and follows the top of the rim through stands of aspen trees, reaching an elevation of 9,600 feet. This fireweed was growing alongside the Pipeline Trail that drops steeply down into the bowl of the Valles Caldera. The Pipeline Road delivered us to the Valle De Los Posos, a misspelling of pozos, meaning spring or well. The circular bowl of the caldera, which is 45 miles in diameter, began to open up in front of us. Thunderstorms from the previous day left puddles, foreshadowing weather conditions later in the day. The domed mountains in the background are the Cerros de los Posos. They were
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