Eric Butler - NPS Photo
The oldest rocks in
If you look closely at these rocks, you will see a variety of minerals that can be distinguished with the naked eye. Feldspar generally appears white-pink and forms angular, blocky crystals with flat surfaces that reflect light. There are several types of feldspar, including plagioclase and orthoclase, but these are not always easy to distinguish without magnification. Quartz is usually clear-white and appears somewhat glassy or waxy. While these two minerals make up most of the rock, you may also see darker minerals in small amounts. Pyroxene appears as little black, blocky crystals, while biotite mica forms thin, shiny flakes. Garnets may also be present, looking like small, dark red dots within the rock. You may also notice a layered, or banded, texture to these rocks, composed of alternating zones of different minerals. This is a structure called “foliation”, which is a result of very strong deformation at high temperatures and pressures.
So how did these rocks form, and what stories do these features tell? These rocks are mostly igneous in origin, meaning the crystals grew together as molten magma cooled deep underground. During the formation of an ancient mountain range, over one billion years ago, these rocks were deformed by the incredible pressures generated by mountain building, a similar process to the formation of today’s
This ancient mountain range vanished long before the current
Excellent examples of this rock can be found at Old Rag Mt, Hogback Mt, Mary’s Rock, Hazel Mt Overlook (milepost 33), and Bacon Hollow Overlook (milepost 69).