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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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Arroyo Negro Pueblo: In My Backyard

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For several years, I tried to pin down the location of the site of a 1,000-year-old Pueblo village that was supposed to be near my home. Eventually I found it, just 600 feet from my backdoor, on top of the ridge that you see in the background of this photo. This view of the ridge that climbs about 100 feet above my house is looking from the rooftop of my house, and my neighbors' houses. This pueblo was first excavated by archaeologists in the 1920s, who named it the Arroyo Negro Pueblo. It is considered perhaps the best mapped, although least described, of the Santa Fe River pueblo sites from the Late Development period. Wood cores were taken from structural support timbers of the buildings in 1934 to do tree ring dating. The Ancestral Puebloans spent 50 years constructing seven buildings up there, from the years 1050 to 1100. The buildings consisted of anywhere from 2 to 25 rooms. Alongside most of the buildings, on the side facing the Santa Fe River, was a round, free-stand
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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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