The "Basin" is a bit of a misnomer, but when you first arrive there you might think it well-named. You are surrounded by tall mountains, some of which are populated by huge Easter-Island-like rock formations high on their slopes. As on the Grapevine Hills Trail, you may get the feeling you are being observed. These pillars of rock are due to jointing patterns like those at Grapevine Hills.

A basin is a place where water and/or sediment collect. This is not exactly true for this remarkable geologic feature. However, it's almost a basin in the sense there is but one avenue of escape for water that runs down the mountainsides after a desert storm: the "Window". Facing west-northwest, the Window overlook is a favorite gathering spot at sunset for lovers and photographers. (See the second image below.)

The water spills down what is called a "pouroff" (more about these when we get to Burro Mesa) as a waterfall into Oak Creek through Oak Canyon, but this only happens infrequently, usually a few times during the so-called monsoon season. In the view above (pirated from the Basin Webcam on a particularly clear day) you see Amon Carter (left) and Vernon Bailey Peaks framing the Window. You are looking over the pouroff to Oak Canyon, over Burro Mesa, toward Study (pronounced "stoo-dy") Butte. The two peaks framing the Window are due to an episode of igneous intrusion that occurred approximately 30 million years ago. The lonely-looking dark mountain in the middle distance is appropriately named Maverick Mountain and is another intrusion of igneous rock. Behind Maverick Mountain is Black Mesa. The town of Lajitas and Lajitas Mesa are out of view off to the left. This picture is foreshortened by about a factor of two due to the 2× magnification of the webcam, so that all objects are twice as far away as they appear. Maverick Mountain is about 12 miles distant, and Black Mesa is over twice as far at around 27 miles.

It's morning and time to hit the trail to the South Rim. This is a rather lengthy hike that many like to turn into a two-day event. The trail takes you up the east side of Ward Mountain. Everywhere, there is igneous intrusive rock. So as you make your way along the trail enjoying the scenery, you are taken aback when you look down and see, of all things, Cretaceous fossils! (See below.)

Had I examined the geological maps before we took off on our hike, I would have seen the green color-coded Cretaceous rock on the map along our route. On the other hand, this made for a delightful surprise discovery: Cretacous Inoceramus clams exposed on the trail. This was like unexpectedly running into a dear friend at a foreign tourist spot. I was familiar with these fossils from their presence in the Cretaceous Anona Chalk of northeast Texas and southwest Arkansas. Here, they are found in the Boquillas limestone. (Note that absolutely no samples of fossils or rocks were taken by us from the park. Not only is this unlawful, but it is an act of great selfishness.)

Why Cretaceous rocks and fossils in this igneous setting? The image below helps explain.

In this picture you see the reddish igneous rocks of the Oligocene Ward Mountain intrusion but also light and dark gray rocks. These are remnants of the older Cretaceous rocks the magma that formed the Ward Mountain intrusion squeezed into and between. Present are formations familiar from other parts of the national park: the Boquillas, Pen, and Ahuga formations; except here they have been forced up 2000 feet or more by the intruded magma.

Farther up the trail you begin to gain some altitude and get more of an overview of the Basin. The photograph below was taken as we ascended toward the heights occupied by the South Rim Formation of volcanic origin.

You see another intrusion of the Oligocene igneous activity, Pullium Peak. Due to its resistance to erosion, it, like Ward Mountain, has become a topographic high. On the right, behind the west flank of Casa Grande Peak, is Green Gulch, which is followed by the road that accesses the Basin. Casa Grande is topped by the South Rim Formation - in this case volcanic rhyolite that forms a erosion-resistant cap. In fact, as will be seen later along the South Rim Trail, the Chisos Mountains owe their existence to the protective cover afforded by the South Rim Formation. The Basin was formed where the South Rim was breached by erosion, allowing erosive forces to hollow out an expansive bowl in the exposed, less resistant rock. Finally, note the cute little igneous erosional remnant near the center of the photograph, which my brother fondly refers to as the "Nubbin". The Nubbin is in the vicinity of the park development in the Basin.

FORWARD to the South Rim Trail, South Rim

BACKWARD to the Grapevine Hills Trail

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