Gneiss boulder along Ryan Mountain Trail

This slab of banded gneiss shows a series of en-echelon fractures (filled with aplite, an intrusive rock composed of quartz and feldspar minerals). The cell phone is 4 inches long for scale. This slab is along the Ryan Mountain Trail. Rocks like this form at pressures and temperatures in the mid crustal depths (possibly 8 miles or more) (Barth and others, 2008). The fact that they are exposed on the surface of a mountain peak is a testament to the ongoing tectonic uplift and erosion forces changing the landscape.