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Lifelong Hike – Occoneechee Mountain

October 7, 2011

On October 5, Martha Fisk and I took a group of the “Lifelong Hikers” to Occoneechee Mountain State Natural Area near Hillsborough, NC. It was a beautiful day for a hike with cool weather and a hint of fall color on the trees.

At 867 feet, Occoneechee Mountain is the highest point in Orange County, due in large part to its interesting geologic history. Millions of years ago, this area was part of a volcanically active island arc. Like in present day Yellowstone National Park, water seeping into the ground from rain and snowmelt was heated by the underlying magma chamber, rose to the surface, and formed thermal features like geysers, hot springs, mudpots, and fumeroles (steam vents). As the water traveled through the bedrock, it dissolved silica from the rocks, leaving behind rocks that had been hydrothermally altered to form kaolinite clay and sericite mica. At the surface, the silica was redeposited as sinter when the water cooled.

Goethite

The mineral goethite gets its rainbow coloration from water bound up in its chemical structure.

Much later, when the island arc collided with another landmass, the rocks were deformed. The kaolinite was transformed into the mineral pyrophyllite, the sericite into sericite phyllite, and the siliceous sinter was transformed into quartz rock. Quartz is a very common mineral, so you might already know that it is quite hard and resistant to weathering. The quartz rock forms the spine of Occoneechee Mountain. Pyrophyllite is a very soft mineral – as soft as talc – and historically was mined in a quarry on the north side of Occoneechee Mountain.

A geologic highlight for the group was seeing the mineral goethite in the area of the old quarry. The rainbow coloration is caused by the water bound up in its chemical structure.

Occoneechee Mountain also hosts a diverse community of plants and animals. Some species found on its ridge and north slope are more commonly seen in the NC mountains, including galax, mountain laurel, and Catawba rhododendron. As our group hiked along the north side of the mountain, we noticed the slightly skunky smell of galax in the air – a smell that is reminiscent of hikes in the North Carolina mountains. The dark green leaves of galax grow close to the ground, but in spring and early summer their tall white flower stalks are impressive. Huge chestnut oaks dominate the plant community along the ridge of the mountain and provide acorns for hungry squirrels, deer, and turkeys. From our observations of large quantities of chestnut oak acorns on the ground, we suspect that this will be a very good mast year!

A final highlight was seeing a red-spotted purple butterfly egg and caterpillar in the picnic area as we were eating lunch!

Red-spotted Purple Caterpillar

The caterpillar of the red-spotted purple butterfly feeds on wild cherry trees.

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