BIG BEND RANCH STATE PARK 4 at the Crossroads

By Blaine R. Hall Blaine R. Hall Crossroads: Intersection, Road, and the Oso Loop passes Junction, Gathering Place. Big directly through it. Bend Ranch State Park (BBRSP) has been all of these for at least Basin and Range Trend. the last 11,000 years as Native 4 The fourth geological event Americans, ranchers, miners, s n i a t is marked by development of the freighters, travelers, and now park n Basin Rocky u and Mountains o Basin and Range Province. The visitors have lived and traveled M Range n ia here. The scenery is magnificent h map illustrates how the Basin ac Ouachita Mountains al pp and Range is characterized by and the landscape varies from river A lowlands, through deep canyons, Trans-Pecos north-trending mountains and Volcanic Field Buried across high plateaus, and up steep intervening sediment-filled valleys Marathon Llano Basin Uplift mountains. Ultimately all of this Buried or basins, which began forming is controlled by the character and about 25 Ma. These uplifted variety of the underlying geology ranges and down-dropped basins and the processes that created it are separated by parallel faults. over millions of years. This deformation is due to the E D 1 N 3 A Adapted by Blaine R. Hall But why a crossroads of geology? R G west-to-east stretching of the

O A look at the geological map above I Big Bend R earth’s crust that may have 2 Trends of four major geological can answer that question just by Ranch been caused by relaxation of the State Park Big Bend events converge at the park. following the variously colored areas 4 National Colored numbers indicate where compressive force that had built to their convergence in the Big Bend Park each can be viewed. the Rocky Mountains. In the region. The trends illustrated here park, the Basin and Range trend represent the four major events that evident in the white band of rocks is evident along the , have shaped North America over the that bends and contorts along the where the river flows through a past 500+ million years, and all of hillside and across the saddle. 3 them are represented in the park. So, series of elongated basins bounded get in your car, mount up your horse, by long, continuous faults. Photo hop on your bike or strike up a hike 4 shows one of the best-developed and visit the Crossroads of Geology! 2 examples along the north edge of Colorado Mesa, where the

bounding fault extends from right

Blaine R. Hall to left beyond the photo margin. 1 The shot was taken just west of 47 Ma. and 18 Ma. throughout Closed Canyon. Here the road western North America. The

Blaine R. Hall parallels the fault for about 2.5 large Trans-Pecos Volcanic miles east–one of the longest Field was created at this time straight stretches of F.M. 170 in Rocky Mountain Trend. by the eruption of molten rock, 2 the park. About 200 million years called magma, which originated later, the park area was again As you travel through BBRSP, Blaine R. Hall from a slab of oceanic crust covered by an ocean basin, driven from the west deep watch for evidence of all four but this time the rocks were Appalachian/Ouachita/ underground and then melted. geological events–for folds and deposited in a shallow, near-shore 1 Marathon/Solitario Trend. This volcanism had the most faults, and tuffs. Take with environment and consisted Around 520 Ma. (million years direct and profound effect of the you a fuller appreciation for the ago) sandstone, conglomerate, mostly of thick-bedded limestone, four events at BBRSP and is the geological enormity of time and limestone, shale, and chert formed shale, and sandstone. The second most completely represented. scale and the resulting variety of in an ocean basin offshore from major geological event, building The Bofecillos Mountains make landscape and scenic beauty here ancient North America. By 300 Ma. the Rocky Mountains, was up the high, central part of the at the Crossroads. this old ocean was closed up and completed about 50 Ma. here park and were formed at 27 Ma. Additional Information. For a the rocks were strongly deformed in . This time the by the eruption of very extensive general geological map and trail and uplifted as what is now Africa folding, faulting, and uplift was flows and abundant volcanic side geology of the Park pick up collided with eastern North driven by compression originating ash that forms rock called . a copy of Down to Earth at Big America to form the Appalachian at the western margin of North The interior of the Bofecillos Bend Ranch State Park, Texas, Mountains. Concurrently, South America. Photo 2 shows how this Mountains is characterized by and for a more complete and America crashed into the southern event caused once-horizontal high rugged peaks marking old detailed report try Geology of Big edge of North America to form reddish and grey rocks to bend the Ouachita Mountains. This eruption sites and level plateaus Bend Ranch State Park, Texas. and fold from left to right. This same event closed the old ocean where lava flows accumulated, Both are available at any of our one-sided fold marks what is further to the south, extending the while the edges of the mountains visitor centers. known as the Fresno-Terlingua mountain trend into West Texas. are incised by deep canyon The strongly deformed rocks that Monocline, located along the drainages. Photo 3 provides a view southeastern edge of the park in © 2013 Texas Parks and Wildlife Department PWD LF P4501-152P (11/13) are present today in the Marathon of the central Bofecillos vent area, In accordance with Texas State Depository Law, this publication is available at the Texas State Publications Clearinghouse and/or Texas Depository Libraries. Basin can also be seen at BBRSP. the Contrabando area. a source for most of the lavas and TPWD receives federal assistance from the U.S. Fish and Wildlife Service Photo 1 documents this mountain- Trans-Pecos Volcanic tuffs comprising the mountains. and other federal agencies. TPWD is therefore subject to Title VI of the Civil Rights Act of 1964, Section 504 of the Rehabilitation Act of 1973, Title II of the building event, where highly folded 3 Field. Undoubtedly the On the left is Oso Mountain, the Americans with Disabilities Act of 1990, the Age Discrimination Act of 1975, and faulted rocks are exposed in the Title IX of the Education Amendments of 1972, in addition to state anti-dis- most dramatic geological event highest peak in the Park at 5135 crimination laws. TPWD will comply with state and federal laws prohibiting interior walls of the Solitario. The affecting the area was strong feet. The vent area is readily discrimination based on race, color, national origin, age, sex or disability. If strong deformation is particularly you believe that you have been discriminated against in any TPWD program, volcanism that occurred between accessible from the Main Park activity or event, you may contact the U.S. Fish and Wildlife Service, Division of Federal Assistance, 4401 N. Fairfax Drive, Mail Stop: MBSP-4020, Arlington, VA 22203, Attention: Civil Rights Coordinator for Public Access.