Carolina Environmental Diversity Explorations
Roan Mountain Highlands · By Dirk Frankenberg and Jennifer Godwin-Wyer
The oldest rock in North Carolina
Figure 4. Seams of quartz are visible in this gneiss, which is the oldest rock in North Carolina. (Photograph by Dirk Frankenberg. More about the photograph)
Figure 4 is a close-up of the oldest rock in North Carolina. This gneiss is definitely older than all other rocks that have been analyzed in the state, and its age has been dated in one test at 1.8 billion years. Single tests are rarely considered the final word in science, so most geologists call this rock “clearly older than other dated rock from the state,” but do not stake their reputations on the measurement of 1.8 billion years. There is general agreement that this metamorphic gneiss was formed during the mountain building event that preceded that which formed the Appalachians. The earlier event is called the Grenville orogeny. It produced ancient mountains as high as the Himalayas.
The rocks in Figure 4 were formed deep in the earth’s crust during this period. They were thrust upwards during formation of both the Grenville and the Appalachian mountains, and were finally exposed here after the Appalachians underwent erosion for more than 200 million years. If you look closely at Figure 4 you can see vertical white stripes running through it. These are seams of quartz formed when sand or sandstone melted within the crust and the quartz minerals organized themselves into distinct layers between sections of dark gray feldspar minerals. This layering shows up more clearly in recently quarried samples of the material shown in Figure 5.
