Hendrix Ranch Geology

The ranch lies on an interface between two major geologic formations. The lower formation, the Pennsylvanian, is described as the tops of eroded, buried ancient mountains. The Ouachita Mountains rose about 300 million years ago as a result of the collision of North America and other continents to form the super continent of Pangaea. As Pangaea pulled apart into the smaller continents during the early Cretaceous period about 144 million years ago, a shallow ocean covered the tilted limestone and shale layers of the Pennsylvanian period (locally identified as the Marble Falls Limestone) and newer Cretaceous limestone and sandstone deposits were laid over the top of the sunken mountains. These younger layers of limestone are not tilted and are generally more porous than the Pennsylvanian limestone. The oldest rocks on the ranch (from the Marble Falls Limestone) are found closest to the Pedernales River and the Park, the youngest near the front entrance gate, with intermediate Cretaceous deposits in between.

The striking geology on the property is made most evident by the rapids in the Pedernales River caused by the dikes and pools created as the water runs over the hard, tilted limestone. These falls are unique to this portion of the river and are famous throughout the area. The broad boulder farm along the bank of the river north of the pool is also a manifestation of the tilted ancient tops of the buried Ouachita Mountains.

Most of the Marble Falls Limestone is made out of the remains of little animals called crinoids, and out of worms. The crinoids look like flowers. Once you know what to look for, you can see tiny crinoids in lots of the rocks. The cross sections of crinoids can look like donuts. The flower has five sides, and we have seen fossils with five circles in a pentagon arrangement. Those are the center of the "flower." These mostly show up at the ranch as white markings on the gray stone. On the rocks just upstream from the gazebo, there are funny globs on top of the rocks. Those are the mounds of material left by worms digging. If you sliced one of the rocks vertically, you could see the shafts of the worm holes.

Sometimes near the river you see gray rocks with brown rock stuck on the top. The brown stuff is chert, the parent mineral of flint. Chert was used for tools and weapons by indigenous people, and many chert and flint shards have been found around the ranch as evidence of human occupation on the ranch thousands of years ago. A few nice arrowheads have been found as well.

Cretaceous limestone is noted for its tendency to form slump caves and dissolution caverns. The interface between the level layers of the Cretaceous Limestone overlaying the tilted Pennsylvanian is often marked by large open fractures. An excellent example of this is found in the Burren. Open fractures also allow for the underground dissolution of the substrate and create slumped areas, several small examples of these are found along the road paralleling the river. There are about three caves on the ranch big enough for skinny people to go spelunking, including the Burren and the bat cave.

In the very youngest rocks on the ranch, the limestone found near the entrance gate, you can find fist-sized fossils of early clams. They do not exist in the Marble Falls Limestone by the river, because clams and oysters had not evolved yet when that rock was formed.