Nearly every year my family and I drive up to a family camp in the Sierra Nevada Mountains just west of Yosemite National Park. (This was a tradition I inherited from my wife's family, who are from California.) Along route 120 there is a scenic turnout with a broad view. The turnout claims this is the "Rim of the World". Well, as much as I love the area where we camp each year... sorry, but if there is a rim of the world in the continental U.S., and it's not the South Rim of the Grand Canyon, it would have to be the South Rim in Big Bend National Park.

When you first arrive at the South Rim after a hike of several hours from the Basin (taking the western route), you come across a view like that below, toward the west. Unfortunately, we were there on a day when the visibility was especially poor. The degradation of the air in the park is a tragedy. If you look at photographs taken twenty years ago and compare the visibility then with what it usually is these days, it's heartbreaking. I don't know if desert power plants or desert development to the west or something else is to blame, but what should be a clear view deep into Mexico is now a challenge to make out the Rio Grande.

It is still an awesome view, however. You are standing at about 7000 feet in elevation - one mile above the Rio Grande; in the distance is Mesa de Anguila, composed of Cretaceous limestone. If you look closely you will see what looks like, from your distance of over 20 miles, a tiny slot in the limestone. This is the entrance to Santa Elena Canyon. Hard to believe from this distance that the walls of the canyon are 1500 feet high. To the right of the canyon entrance (north) and this side of the Rio Grande is Texas; to the left and beyond the Rio Grande is Mexico.

Santa Elena Canyon was carved by the Rio Grande (and the Rio Conchos before it) as it ate down through softer rock into the harder limestones uplifted along the Terlingua fault, whose trace is marked by the dashed line. (Ironically, the topographic high that is the Chisos Mountains is actually a down-faulted block called a graben. The reason the Chisos stand high is due to the resistant South Rim volcanic rocks, which form a "caprock" protecting less resistant rocks below.) In addition to forming the "cap" of the high Chisos, South Rim rocks are present as erosional remnants in the Sierra Quemada ("burned mountains") and, until recently, were thought to exist as remnants on Goat and Kit Mountains (in the distance). However, now it is thought the volcanic rocks on these mountains have a different origin. The Sierra Quemada largely consist of the mostly volcanic Chisos Group (formerly known as the Chisos Formation) - more on "group" vs "formation" later). Also visible is a volcanic dike that stretches from Blue Creek Canyon across the mountains on the extreme right (above image). Dikes are remnants of sheet-like intrusions of magma that result in fissure volcanic eruptions if they reach the surface. As this is one of the series of east-west trending dikes in the Park, it is likely a feeder dike for the top-most unit of the South Rim Formation, the Emory Peak Rhyolite.

The above view is a bit farther along the trail. You can see some of the features that appeared in the first image from a somewhat different perspective. The drainage on the left of the center peak is Fresno Creek.

The telescopic view below is farther along the trail yet, looking just west of south.

Landmarks in this photo include the famous Mule Ear Peaks and Punta de la Sierra. A sheet-like body of solidified magma called a sill is seen intruding the Chisos Group. A sill is formed when magma seeps along a weakness in the pre-existing structure - in this case between two beds of the Chisos. Note the light bands outcropping on the side of mountain above the sill. These are beds of consolidated ash called tuff. The "cap rock" of the South Rim appears on the right, a volcanic rock formed from lava rich in silicon and oxygen (silica) called rhyolite. Formerly, it was called the Lost Mine Rhyolite (and still is in the image), but is now part of the Boot Rock Member of the South Rim Formation. The South Rim Formation appears to have been erupted from a small caldera (a circular collapsed volcanic feature) called the Pine Canyon Caldera around 32 million years ago. These volcanic rocks are unusually high in potassium and other alkali metals.

I should point something out that may escape those not used to thinking in geological terms. The current relief does not mean the South Rim Formation was erupted on top of a mountain, as it might appear at first. These volcanic rocks covered a much larger area than at present and flowed over the ground as it then existed (on top of the Chisos Group). They have since been lifted up by geological forces, including those involved in the emplacement of the igneous intrusions exposed in the high Chisos, and largely eroded away, with the remnants forming the "cap rock" of the Chisos mountains.

The South Rim Trail takes you on an arc with views of the Big Bend region gradually more toward the east via the south. The view below is looking almost due south into Mexico.

In this photograph you see the South Rim Formation caprock and several mountains five to six miles away. These mountains are igneous intrusions emplaced circa 30 million years ago that have been exposed by - guess what! - erosion. Note that they have pushed up into the Chisos Group and are therefore younger by the geological "principle of cross-cutting relations". Elephant Tusk is a particularly prominent landmark. A telescopic view of these igneous bodies appears below.

To the west and south of the igneous intrusions is the strangely ragged Punta de la Sierra. The ragged appearance is not due to the presence of the South Rim Formation as you might first suppose. Whatever rocks were originally above have been removed by erosion exposing the youngest unit of the Chisos Group: the 32 million-year-old Tule Mountain Trachyandesite.

I realize "trachyandesite" is quite a mouthful. An andesite is a volcanic rock that is moderately rich in silica; richer than basalt but not as rich as rhyolite. The prefix "trachy" has to do with the greater than average abundance of alkali metals in the rock. Other rock types that make up the Chisos Group include several episodes of basalt (iron-rich volcanic rock) flows between 47 and 30 million years ago, the younger ones being interbedded with the 33 million-year-old Mule Ear Spring Tuff (hardened volcanic ash) and the aforementioned trachyandesite. Also included in the Chisos Group are other volcanic rocks and ash beds, and sedimentary rocks derived largely from the erosion of volcanic rocks. Note another one of the ubiquitous Big Bend igneous intrusions in the Sierra Quemada between the camera and Punta de la Sierra.

To the east and south of the igneous intrusions, you see below (if I can reasonably use the word "see" here, due to the bad visibility) the entrance to Mariscal Canyon and the Mariscal Mountain anticline.

Mariscal Canyon (along with Boquillas Canyon farther east) was formed the same time as Santa Elena Canyon. All three canyons are due to the Rio Grande cutting down into uplifted Cretaceous limestones, and all three are spectacular for their shear cliffs, although Santa Elena is the best of the three in this regard. Also "visible" in the image is Mariscal Mountain, formed by the up-warping of rock into what is called an anticline. The geologic symbol for anticline marks its location. The Mariscal Mountain anticline is one of a series of N-S trending anticlines formed in SW Texas during a mountain building period, the Laramide orogeny, that deformed the Earth's crust throughout western North America during the Late Cretaceous and Early Tertiary.

The rock exposure at the South Rim is a layer cake of volcanic rocks, and it's time to have a slice. Complicating the discussion is a change in nomenclature that I'm assuming either is official or soon will be. I've already mentioned that the Chisos Formation is now referred to as the Chisos Group. The formation is the basic mappable unit of geologists. A group is a collection of related formations. Often, a formation will consist of a series of distinctive beds which are designated as members. So you have this heirarchy: Groups contain formations and formations contain members. Members themselves may be divided up into smaller beds.

In the picture below the traditional names of the rock units are used, mainly because I created the image before becoming aware of the changes, and I'm too lazy to go back and re-create the image. You see me gazing out over the South Rim, but, more importantly, you see the layers of the layer cake.

The youngest member of the South Rim Formation is missing in the cliff: the Emory Peak Rhyolite (formerly known as the Burro Mountain Rhyolite). The Wasp Spring Flow Breccia and the Lost Mine Rhyolite have become the Boot Rock Member of the South Rim Formation, whereas the Brown Rhyolite is now called the Pine Canyon Rhyolite. These name changes reflect a new understanding of the origin of these rocks.

The evidence now is that the South Rim Formation was erupted in conjunction with a caldera called the Pine Canyon Caldera 32 million years ago. (Pine Canyon is in the northeastern part of the Chisos Mountains.) The Pine Canyon Rhyolite was erupted at the formation of the caldera and consists of rock formed by glowing ash flows (called ignimbrite) and volcanic glass (obsidian). The Boot Rock Member includes rhyolite and ignimbrite erupted from volcanic vents after the caldera was formed.

Below are four views toward the southeast and east from the South Rim.

In the first view (above) you again see Mariscal Mountain, formed as Cretaceous limestone was folded upward in an anticlinal fold during crustal compression associated with the Laramide mountain building event. Note the arrow on the line marking the axis ("center") of the fold. This indicates the anticline disappears under the surface in the direction the arrow points, making it what is called a plunging fold. A new feature is yet another igneous intrusion, this one a sill serving as the cap rock of Talley Mountain. The Cretaceous Ahuga Formation, a continental (as opposed to a marine) deposit that contains fossil dinosaur bones near Talley mountain, is exposed on the flanks of the mountain. And, of course, the mostly volcanic Chisos Group rocks constitute the foreground.

This second view exhibits the San Vicente Mountains in Mexico, which were formed by a plunging anticlinal fold during the same crustal compression that created the Mariscal Mountain anticline. In the distance you can just make out the Rio Grande; the ground between the Chisos rocks and the Rio Grande to the left of Talley Mountain is mostly alluvium, sediment deposited by running water - in this case by Tornillo Creek, which, when it rains, flows from the northern areas of the national park into the Rio Grande.

Thirdly, you look in the direction of Chilicotal Mountain, which is yet another igneous intrusion, a sill that may be the same one that tops Talley Mountain, separated from the latter by erosion. However, there is a geological bonus here in that a fault has offset Chilicotal's sill. If you look carefully, there is a horizontal line of dark rocks, broken and moved relatively upward on the left. The fault actually runs from left to right in the photograph and is up on the far side. The magma that forms Chilicotal Mountain intruded the Cretaceous Javelina Formation, a unit of mostly clay and some sand that, like the Ahuga, sometimes contains dinosaur fossils. In the distance the Sierra del Carmen in Mexico loom into view. These mountains consist of nearly horizontal layers of Cretaceous limestone.

This final view is almost due east. You see Nugent Mountain and Hayes Ridge, both formed by intrusive igneous rock. Hayes Ridge appears to be part of a system of volcanic fissures that surrounded the Pine Canyon Caldera. The South Rim layer cake is again in the picture, with alluvial sediment and talus draped over the Chisos Group rocks below.

The last image of this section is one I got off the Internet from the US and Mexico International Park Association, dedicated to the creation of an international park involving land on both sides of the border. This is a worthy cause, if for no other reason than to make it again possible to cross over (by boat and burro) to the village of Boquillas, Mexico, shop, and have a cerveza or two. As it is, I hear the village is now practically a ghost town.

Getting back to the this image, which is an aerial shot of the South Rim, the role of the South Rim Formation as a cap rock is clearly seen. The high Chisos Mountains are an erosional remnant, protected for the moment by the resistant volcanic rock. However, geologically speaking, the Chisos are not long for this world. The greatest threat is the erosion by mass wasting working from around the edges of the South Rim Formation. (Mass wasting in this case results from falls as rock weakened by weathering gives way and plummets down the cliffs.)

The Chisos are a microcosm of sorts of what happens to islands and continents, which are both worn away, not just from on top, but also by ocean waves eating away at the rock mass. No ocean here, of course, but mass wasting performs the same function.

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