The document discusses different types of structural geological features found in Big Bend National Park, including normal faults, thrust faults, fault block mountains, synclines, and anticlines. Normal faults occur due to tensional stress and can be seen in cliffs and canyons. Thrust faults result from compressional forces pushing rock layers on one side of the fault up onto the other side. Fault block mountains form from numerous normal faults that cause rock layers to drop down between the faults. Synclines and anticlines are folds in rock layers caused by compressional forces, with synclines bending layers downward and anticlines bending them upward. Joints are fractures across rock layers without displacement.

1. When massive tectonic plates beneath the Earth's surface shift, slide, and collide. These
powerful effects can cause many different deformations of the Earth's crust. Faulting, folding,
and jointing are the three different kinds of common structural features that can be found
within the park.
Normal Faults -- Faults are fractures in the Earth's crust in which a reasonable amount of
displacement has occurred. Normal faults occur as a result of tensional stress (pulling apart) in
the Earth's crust. Many of the cliffs in the Big Bend area are actually the result of faulting that
occurred in the park shortly after the mountain building episodes that created the Rocky
Mountains.
Boquillas Canyon -- Located in the southeastern corner of Big Bend, Boquillas Canyon contains a
perfect example of a normal fault. In the picture to the right, a fault cuts across beds of Santa
Elena limestone from the top right to the bottom left. The layers of rock on the left hand side
are in a vertically lower position than those on the right. Many different faults can be found
scattered around the Boquillas area, all aligned to the north/south, meaning that there was a
east/west tensional force on the Earth's crust at some time in Big Bend's geologic history.
Thrust Fault -- Thrust faults are a result of compressional forces in the Earth's crust.
When the crust is being pushed from opposing sides, compressional forces are released by
faulting. If the resulting faults are at a near horizontal (typically less than 20 degrees) angle,
continued compressional forces can sometimes thrust the layers of rock on one side of the fault
up onto the corresponding layers of rock on the other side of the fault.
One of the mountains that stands guard over the north entrance of Big Bend National Park is a
really good example of a thrust fault. The red line drawn onto the mountain indicates the
position of the fault. The composition of the mountain to the right of the red line is Glen Rose
formation. The rock layers to the left of the red line are (starting from the bottom to the top)
Glen Rose formation, the Maravillas, and Tesnus formations.

2. Fault Block Mountains -- When numerous normal faults occur and a layer of rock slides
down between them, the "mountains" created are called fault block mountains. The layers of
rocks that slide down are known as grabens and the layers that remain elevated are called
horsts. The basin and range regions that stretch to the west side of Rocky Mountains were
formed by these forces. Many of the fault block ranges in Big Bend are in fact extensions of the
basin and range.
Juniper Canyon is an example of a fault block structure. The walls of the canyon are horsts and
the floor of the canyon is a graben. Notice how other fault block structures can be seen
extending out toward the horizon.
Syncline -- A syncline is a feature that is formed by the folding of the Earth's crust,
usually associate with compressional tectonic forces. Synclines are features created when those
compressional forces cause layers of rock to bend down. Synclines can have a dip of a few
meters or several kilometers.
If you look closely at the picture of the mountain to the right, you will notice that the right side
of the mountain is a good example of a syncline. This mountain is located along Ross Maxwell
drive on the east side of the road. It lies directly to the west of Ward Mountain, one of the
principle peaks of the Chisos Mountains.
Anticline -- An anticline is another deformational folding feature akin to synclines.
Instead of the layer of rock bending in a downward arc, the layers of rock are bent in an upward
arc. Like synclines, anticlines form as a result from compressional forces. These compressional
forces can be the result of wide scale plate tectonic activity or a regional intrusion of lava that
folds the overlying beds. Can you spot the anticline in the picture above?
Jointing -- Joints, much like faults, occur when a fracture cuts across layers of rock. However,

3. unlike faults, no displacement occurs on either side of the fracture. Sometimes these fractures
are filled with superheated solutions during volcanic activity. The joint pictured to the left has
been filled in by calcite crystals. This joint can be found on the Boquillas Canyon hike.