BEAUCATCHER MOUNTAIN ROAD CUT, ASHEVILL NC
by Dorothy Hussey
[Editor's Note: The following article was submitted by Dorothy Hussey, who undertook the challenge of converting a scientific article into "lay reader's" language. Dorothy's contribution has been reviewed and approved by Dr. William Miller, Jr., Associate Professor of Geology at the University of North Carolina at Asheville, whose original scientific article appeared in Southern Geology, November, 1996 issue.]
It is always a thrill to drive west through the Beaucatcher Cut and suddenly see the panorama of a breathtaking sunset or the sharp outline of the beautiful mountains. It is even more exciting to learn from Dr. Bill Miller of UNC-A about the many kinds of minerals in the cut. Completed in 1978 on Interstate 240 in Asheville, it is the largest excavation of its kind in the state. One thousand three hundred ninety-four (1,394) feet long and 279 feet high, it is a spectacular exposure of rocks in the Ashe Metamorphic Suite (AMS). The dominant rock is gneiss with strong foliation defined by mica and bands of light and dark minerals. The amphibolites are approximately 50 percent hornblende and 50 percent plagioclase. Rocks in lesser amounts include, garnet, granofels and actinolite schist. The joining of sillimanite and muscovite indicates amphibolite facies metamorphism. Foliation is moderately to steeply northwest-dipping and defines part of a northeast trending fold.
The texture of the gneiss at the cut reveals several variations in the exposure of the cut. It is schistose, but shows some irregular banding of feldspar and quartz in varying thickness which parallel the foliation. Gneiss (usually bluish gray quartz-feldsparmuscovite schist) in a near-vertical shear zone is annealed completely. Minerals of the gneiss are mostly plagioclase (olioclase) feldspar and quartz with some biotite. Almandine garnet is partly altered to chlorite., There are minor amounts of chlorite, muscovite, zircon, apatite and calcite.
Garnet makes up more than twenty-five percent of one amphibolite in the northeast side of the cut. In one area garnet is rimmed by sillmanite in thin layers. Kyanite is rimmed by sillimanite which indicates that sillimanite crystallized at a later stage than kyanite did. The garnet-biotite schist has five percent microcline and contains small amounts of sphene, apatite and calcite. Muscovite biotite garnet schist occurs sporadically. Biotite muscovite schist is widespread in thin layers throughout the cut. Actinolite schist is found in only one layer, a few meters thick, on both sides of the cut. It contains chlorite and much pyrite with some crystals as large as a centimeter. Pegmatite layers of crystalline white feldspar and quartz are found throughout the cut.
Ilmenite is the most common oxide. Rutile is widespread, but abundant only in the garnet granofels and garnet-biotite schist. Ilmenite, rutile and small traces of magnetite concentrate in biotite and chlorite, but are also found in some other minerals. Hematite and geothite appear in trace amounts.
The time of peak metamorphism of the cut is estimated to be 320 to 430 million years ago at a temperature between 600 and 700 degrees C. If the amphibolites have retained their relative crystallinity, the finer grained amphibolites at the cut represent lava flows and mafic tuffs. The coarser grained amphibolites represent intrusive rocks.
The cut exposes the Ashe Metamorphic Suite, which is dominately metagraywacke represented by gneisses, and metabasalt and metagabbro represented by amphibolite and garnet granofels. Metamorphism reached a peak here during the Taconic Orogeny. Many deformational events are indicated by isoclinal folds within the gneissic foliation which developed a large shear zone. Shear deformation continued as textures of the other mylonite rocks are not annealed. Most original sedimentary and igneous features were obliterated.
Reprinted from Mountain Mineral Monthly, Vol.66 Number 8, August 1998. Used by permission.