Occoneechee Mountain is a State Natural Area managed by the Eno River State Park. Follow all park rules, including NOT collecting rock and mineral samples. This mountain, on the eastern edge of the Piedmont, is surprisingly similar to the topography of the foothills of the North Carolina mountains. The western summit of Occoneechee Mountain rises more than 300 feet above the nearby Eno River, and is the highest point in Orange County with an elevation of approximately 867 feet above mean sea level (ft msl). Occoneechee Mountain, along with a series of other hills in this area, are called monadnocks. A monadnock is a hill or mountain of resistant (hard) rock that has remained in place as the surrounding rock has eroded away. These monadnocks are part of a larger northeast-trending dissected plateau.
Use the marked trails to find your way to the EarthCache location. Refer to park maps for assistance. When you reach the EarthCache GPS coordinates you will be standing in the abandoned quarry of Occoneechee Mountain. Care must be taken because of tripping hazards and loose rock. Look around the quarry; notice the color of the rocks, the amount and sizes of the rock debris, and the orientation of the ridge that is Occoneechee Mountain. The pile of rocks at the bottom of the quarry is the result of a rockslide that occurred in February 2001. Part of the quarry wall collapsed, carrying nearly 5,100 tons of rock down 200 feet in a wide path of destruction.
The quarry was reportedly opened before the Civil War and used for fill material during construction of nearby railroad tracks and in the Hillsborough area. Active quarrying ceased sometime around 1908. In 1906 the quarry was operating under the name of the Southern Broken Stone Company. An active pyrophyllite quarry, in operation since the 1960s, is present to the northeast on Occoneechee Mountain – the active mine is not open to the public.
To begin to solve the mystery of why Occoneechee Mountain resisted erosion and yet contains one of the softest minerals, pyrophyllite, we must look at a process in geology called hydrothermal alteration (a change caused by hot water). About 630 million years ago, the Hillsborough area was home to many active volcanoes that were fed by hot magma (molten rock). This hot magma heated groundwater and caused it to permeate through layers of volcanic ash. When the heated groundwater rose to the surface, it formed hot springs and geysers similar to those of Yellowstone National Park. The heated groundwater associated with these hot springs changed the composition of the volcanic ash by adding, removing or redistributing chemical elements. The heated groundwater essentially dissolved out silica (SiO2) from some of the rocks and deposited silica in other rocks. The resulting rock is called hydrothermally-altered rock. The hydrothermal alteration changed the chemical makeup of the rocks. The rocks whose silica was dissolved out formed deposits of kaolinite clay and the mineral sericite (very fine-grained white muscovite mica). When the silica was deposited, it formed a very resistant rock composed almost entirely of quartz.
With time, the magma that provided the heat source cooled and solidified. Later, the rocks were folded and metamorphosed (changed by heat and pressure). The heat and pressure of the metamorphism again changed the chemical makeup of the rocks. The kaolinite clay was altered to deposits of pyrophyllite, and the sericite deposits were transformed into sericite phyllite (the majority of the white rock in the abandoned quarry). The red and orange staining on the rock is from the oxidation of iron minerals (the iron is rusting). The silica rich rock was transformed into areas of resistant quartz rock. The quartz rock in the central spine of the mountain is what is “holding up” Occoneechee Mountain. The softer rock composed of the minerals sericite and pyrophyllite are located on the flanks of the mountain.
The information for this EarthCache location is from a soon-to-be-published book (available June 2007) - The Geology of the Eno River. A more detailed description of the geology of Occoneechee Mountain and the Eno River will be provided in the publication along with interpretive trail guides from the Eno River area. Contact the North Carolina Geological Survey for availability at NCGS and 919-733-2423.
***Do not log this cache until you already sent in the answers, your log will be deleted otherwise without question.***
To log this EarthCache:
1. Find an example of the mineral pyrophyllite and have your picture taken with it (check out the background photo)!
2. Email me a popular use of Pyrophyllite that we use every day.