The rugged landscape of the Wichita Mountains Wildlife Refuge is the result of over half a billion years of geologic history. The first known geologic event in this region took place 550 to 600 million years ago. The land sank forming a basin which filled with sea water. Rivers carried clay, sand and gravel into this sea and thick layers of sediments were formed. Sometime after the deposition of the sediments, a hot molten lava formed deep below the surface and was forced up into the overlying sediments. The lava cooled slowly, forming a dark gray to black igneous rock known as gabbro. There are several large outcrops of this rock in the Wichitas. After the solidification of the gabbro, the area was uplifted thousands of feet.
The next major geologic event took place 500 to 550 million years ago. A hot granitic lava developed deep below the surface and was forced upwards into the overlying gabbro and some reached the surface through volcanic vents. The cooling of this hot liquid at depth formed a red granite which is the most extensive rock in the refuge. The granite intruded the gabbro as a sheet a few hundred feet thick. The granite exposed in the southern part of the refuge is more coarsely crystalline than the granite of Mt. Scott. Rhyolite may also be seen in the refuge. Rhyolite, a rock resembling granite, but finer grained, was formed when hot lava reached the surface and poured out and cooled quickly. It is finely grained, composed of sodium and potassium-rich feldspar, which gives it a red color and quartz crystals which give it a smoky color.
The mountains of the region were formed some 300 million years ago. They were created by a tremendous uplift accompanied by large folds and faults. Some of the faults were miles in length and produced large blocks. Some of these settled with respect to others, and the large flat areas bordering the Scenic Highway in the central part of the refuge probably originated by faulting. The mountains originally were considerably higher than the present ones as erosion has worn away the upper parts and deposited this material in the flats, reducing the relief. Most of this erosion took place in Permian time (some 250 million years ago) and produced extensive gravel deposits. Granite is the dominant rock of the refuge, and the gravels formed by erosion are mainly composed of granite fragments.
The main geologic event that has taken place since is the erosion of most of the Permian gravels which once covered this area. In recent times, some newer gravels have been deposited along the small streams and in the flats. The natural force of erosion continues. Rocks crack and expand as water freezes and thaws. Lichens, plants, wind and water over time slowly wear away the rock.
To log this earth-cache, please send me an email with answers to the following questions: (Do not post your answers in the computer log!)
1. Describe the rock that makes up Heart Rock. (What does it feel like? Is it rough or smooth? What color or colors are in it? Is is fine-grained or coarsely-grained?)
2. Based on what you have just described and what you have learned here: of what type of rock do you think Heart Rock is composed?
3. There are several examples of recent forces of nature observable in the rock surrounding Heart Rock. Look for evidence of them and tell me what you see....please name at least two.
Heart Rock is a beautiful and unique feature of the Wichita Mountains Wildlife Refuge. Enjoy your visit here and feel free to post a photo with your log (this is optional).
Reference:
Geological excerpts from C. A. Merritt, School of Geology, University of Oklahoma and University of Texas, Dallas, from the Wichita Mountains Wildlife Refuge website www.fws.gov/southwest/refuges/oklahoma/wichitamountains/index.htmlLink text