Showing posts from January, 2021

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

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

Arroyo Negro Pueblo: In My Backyard

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

Hiking to the Mouth of the Gila Box

We watched the Gila River flow at The Nature Conservancy's (TNC) Lichty Ecological Research Center, four miles south of where it emerges from the Mogollon Range (in the background), and the rugged Gila Wilderness. The TNC's Gila Riparian Preserve here protects more than 1,200 acres of fragile Southwest riparian habitat and verdant gallery woodland along the Gila River, the last of the Southwest's major free-flowing rivers. The Gila is a rare example of a river with a natural pattern of flows that are unimpeded by dams. They range from winter floods to snowmelt run-off and low flows in spring. A variety of native plants and animals have adapted to these seasonal pulses of flooding and drought, including a number of endangered species.  Two-and-a-half miles upstream from the Lichty Center, we set out to hike to the mouth of the box canyon where the Gila River exits the mountainous Gila Wilderness: the world's first designated wilderness area and New Mexico's largest.