National Park Service Hovenweep U.S. Department of the Interior Hovenweep National Monument

Geology

This pothole, worn into the Dakota Sandstone on the canyon rim, was formed by a combination of water and wind erosion. Potholes collect and hold water thus providing a unique microenvironment for communities of algae, mosses, fairy shrimp, and insects.

History Hovenweep National Monument is located on After the Burro Canyon Formation was Cajon Mesa in the northwest quadrant of the deposited there was a fairly long period of San Juan River Basin. Mesa means ‘table’ in erosion lasting almost until the end of Mid- Spanish and generally implies a highland that Cretaceous times. In geology this is known is fairly fl at on top. Cajon Mesa is tilted to the as an “unconformity” because one or more southwest starting at 6,800 feet near Cutthroat layers of rock are missing from the geologic Castle and ending up at 4,950 feet southwest record - usually due to erosion. of Cajon ruin. Deposited during late Cretaceous times Despite the many deep and revealing canyons (30-70 million years ago) and representing a on Cajon Mesa, only two geologic formations transition from river to swamp to marginal are easily visible at Hovenweep. The earlier of marine conditions, porous Dakota Sandstone these to be deposited was the Burro Canyon is the caprock on Cajon Mesa and the other Formation laid down between 100 and 136 layer visible from within the monument. It million years ago in the early Cretaceous comprises the open slickrock on the canyon period by a river and fl oodplain complex rims and was utilized as a building material containing occasional small brackish ponds. and for manos and metates, implements used It is composed of white conglomerate, green to grind corn in ancestral Puebloan times. In shale, mudrock, and sandstone layers with addition to this yellow/gray sandstone, the interspersed pebbles and cobbles of chert, Dakota Formation is composed of mudstones silicifi ed limestone, and quartzite. The and a few thin beds of coal. Look for coal Burro Canyon Conglomerate is easily seen deposits along the trail to Twin Towers and in at Cutthroat Castle and along the canyon the canyon crossing. crossing section of the Little Ruin Canyon trail. This conglomerate was an important Together, these two geologic strata are source of material for tool production during responsible for the canyonhead seeps that ancestral Puebloan times. Also look for the were so important to Hovenweep settlement green shale that can be seen along the upper patterns. Seeps occur where the porous Burro Canyon Conglomerate along portion of the Holly Trail. Dakota sandstone meets the relatively the Little Ruin Canyon Trail. impermeable Burro Canyon shale. Water Sediments deposited on Cajon Mesa continued percolates slowly down through the Dakota to erode through the late Pleistocene up to Sandstone and is channeled horizontally arly Recent times (6-10 thousand years ago). along the top of the Burro Canyon Shale to It was around this time that early people, the the nearest available surface opening. When Archaic, wandered this area in search of game. the seep appears in a canyon wall, as is the They utilized caves and overhangs for shelter, case with all of Hovenweep’s seeps, they often and shaped metamorphic and igneous rocks form small caves or alcoves. to make points and hammers. By A.D. 700 the bow and arrow was being utilized to more While only two geologic strata are easily visible eff ectively hunt smaller game as the people within the monument, the landscape features in this region settled into a more agricultural that make up the horizon have an interesting lifestyle. The majority of the reddish-brown geologic story. During the Miocene epoch, 10- loess soils that are predominant in this area 25 million years ago, masses of molten trachyte are from the fi ne grained soils of Monument (fi ne-grained, light colored, igneous rock) Valley and the lower reaches of the San Juan intruded all the way up into the Mancos Shale River. They were deposited here by south- (a layer of rock which lies atop the Dakota southwesterly winds in Recent times (10,000 sandstone and has since been eroded away in years ago to present). This soil allowed the this area). This molten magma never reached ancestral Pueblo people to grow crops of the surface and hence cooled slowly forming a corn, beans, and squash for hundreds of years. mounded bed of crystalline igneous rocks. In Mostly composed of fi ne-grained sandstone the Pliocene epoch the overlying Cretaceous with a reddish coating of iron oxide, this soil and Tertiary beds of rock were eroded away supported what was once a densely populated exposing what are now known as Sleeping Ute area. Today it still provides fertile cropland for Mountain to the east and the Abajo Mountains alfalfa, bean, wheat and saffl ower farmers in to the west. In geologic terms these formations nearby Montezuma Valley communities. are known as “laccoliths.”

Protect the past Your help is needed to preserve Hovenweep • Take photographs; do not take artifacts. Any for the future National Monument. There are several person who excavates, removes, damages, things you can do to preserve and protect alters, or defaces any archeological resource Hovenweep for future generations: on federal lands is subject to arrest and felony prosecution as dictated by the Archeological • Stay on designated trails and away from fallen Resources Protection Act of 1979. walls and mounds of stone. Walking on or near walls and structures weakens them, • Eat at designated picnic tables. Do not eat accelerating their deterioration. or leave trash at archaeological sites. Food attracts rodents that will burrow and build • Areas behind chain barriers are closed to nests in fragile structures. protect fragile sites. Do not cross these barriers. • Pets are allowed on trails but must be on a six foot leash at all times. • Look but do not touch. The oils from you hands permanently stain rock surfaces.

Protect Yourself • Bring enough water for yourself and your • Respect the wildlife. Venomous insects and pets. snakes live in the Southwest.

• Wear sunscreen and hat; rest often. • Do not eat plants. Some may be poisonous.

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