Geological Survey Bulletin 1265 Ancient Lavas in Shenandoah National Park Near Luray, Virginia

The Blue Ridge is the southeasternmost of the great series of parallel ridges that form the Central part of the Appalachian mountain system. All the other ridges of the Appalachians are carved from folded sedimentary rocks of Paleozoic age that were originally deposited in regular horizontal layers on the sea bottom. Near the end of the Paleozoic era these sedimentary layers were warped into a series of enormous elongate folds, some of them several miles across and tens or even hundreds of miles long. Subsequent erosion has sculptured the present topography by wearing away the limestone and softer layers of shale. The intervening harder, more resistant layers of sandstone, conglomerate, and shale were left as the long, parallel, even-topped ridges so characteristic of this part of the Appalachians.

The Blue Ridge, on the other hand, is more varied in both topography and geology. It is not really a single ridge at all, but an irregular chain of hills and ragged peaks, in some places only a few hundred feet high, but locally rising more than 3,000 feet above the floor of the Shenandoah Valley to the northwest (fig. 1) and the Piedmont plateau to the southeast. The Blue Ridge marks the northwest side of an enormous wrinkled upfold in the earth's crust known as the Catoctin Mountain-Blue Ridge anticlinorium. The anticlinorium extends for more than 250 miles from southern Pennsylvania southwest at least as far as Lynchburg, Va. (fig. 2). The southeastern flank of the upfold is marked by Catoctin Mountain in Maryland and by the Bull Run Mountains, Southwest Mountains, and other low ridges in Virginia.

Figure 1. View of the Blue Ridge from the west. The flat fertile valley floor (foreground) is underlain by limestone of Paleozoic age capped by terrace gravel. The low wooded foothills (middle distance) are underlain by steeply dipping quartzite and shale of the Chilhowee Group. The main mass of the Blue Ridge is composed of granitic basement rocks, but the highest part of the ridge is capped by greenstone of the Catoctin Formation. National Park Service photograph.

Figure 2. Geologic map of the Catoctin Mountain-Blue Ridge anticlinorium in Virginia. Generalized from the geologic map of Virginia (Virginia Division of Mineral Resources, 1963) (click on image for an enlargement in a new window).

The core of the anticlinorium brings up some of the oldest rocks exposed anywhere in the eastern United States—granite and various granitelike gneisses formed at least 1.1 billion years ago. Because of their great age, their position beneath all the other geologic units, and their enigmatic character, these rocks are commonly called basement rocks and are referred to collectively as the basement complex.

Along the southeast side of the anticlinorium the basement complex is overlain by hundreds or thousands of feet of younger rocks that were originally dark shales, sandstones, and conglomerates, which have been metamorphosed to phyllite, gneiss, and mica schist. These sedimentary rocks are called the Lynchburg Formation. In the Blue Ridge, on the northwest side of the anticlinorium, the basement is overlain by the Catoctin Formation, a thick sequence that is composed mainly of greenstone derived from metamorphism of basaltic lavas and that also contains many thin and discontinuous layers of metamorphosed sandstone, shale, and volcanic ash. The Catoctin Formation can also be recognized along the southeast flank of the anticlinorium where it rests on top of the Lynchburg Formation. The Lynchburg Formation, on the other hand, is represented on the northwest flank of the anticlinorium only by a discontinuous layer of sedimentary rock, generally less than 150 feet thick, at the base of the Catoctin.

Along the northwest flank of the anticlinorium, the Catoctin Formation is overlain by slightly metamorphosed sandstone and shale of the Chilhowee Group, the basal deposit of the thick sequence of Paleozoic sedimentary rocks that form the Appalachians northwest of the Blue Ridge. In a few places the uppermost beds of the Chilhowee Group contained fossils that are clearly of Early Cambrian age. The lower part of the Chilhowee Group contains abundant worm borings, but no fossils have yet been found that would definitely establish its age. It is also generally assumed to be of Early Cambrian age (King, 1949).

The Chilhowee Group also overlies the Catoctin Formation on the east side of the anticlinorium in Maryland and can be traced southward through the Bull Run Mountains as far as Warrenton, Va. South of Warrenton, the Catoctin is overlain by the Evington Group, composed of rocks that somewhat resemble those of the Chilhowee but whose age and correlation are not yet definitely established.

One of the greatest puzzles in the geology of the Appalachians is the question of the age of the vast terrane of schists and gneisses that underlies the Piedmont plateau southeast of the Catoctin Mountain-Blue Ridge anticlinorium. Many of these rocks were deposited as sediments, chiefly shale and sandstone, but most of them have been so deformed and altered by metamorphism that their original sequence is difficult to decipher. The character of the Piedmont rocks before metamorphism was clearly quite different from that of the Paleozoic sedimentary rocks northwest of the anticlinorium, but part or all of them may be also of Paleozoic age. It is also possible that many of the Piedmont rocks are approximately equivalent in age to the Catoctin and Lynchburg Formations and are, therefore, older than any of the rocks of the folded Appalachians. The problem of the age of the Piedmont rocks must be resolved before we can satisfactorily reconstruct the geologic history of this part of the Appalachian mountain system.

The Catoctin Formation is of special interest in this connection because it is one of the few geologic units that can definitely be recognized on both sides of the anticlinorium. Thus it furnishes a tenuous bridge between the folded Paleozoic sedimentary rocks on one side and the Piedmont rocks on the other. It is also of interest because of its position between the Precambrian basement rocks and the oldest fossil-bearing Paleozoic sedimentary rocks and, because it records the only extensive volcanic eruptions in this part of the Appalachians.

This paper describes the Catoctin Formation in a small area in the Shenandoah National Park near Luray, Va., where the formation is particularly well exposed. It is a slightly abridged and modified version of a previous paper by Reed (1955).

The principal objectives of the study were the determination of the original nature of the Catoctin lavas, the manner in which they were erupted, and their relations to the overlying and underlying rocks. The paper is reprinted here in the hope that in spite of its technical nature it will be of interest to many visitors to Shenandoah National Park and that it will serve to increase their appreciation of the landscape and geologic problems of the park.

Many of the geographic features referred to in the text are shown on the index map (fig. 3). These features can also be located on the U.S. Geological Survey topographic maps of the Luray, Thornton Gap, Big Meadows, and Old Rag Mountain 7-1/2—minute quadrangles and on the trail maps of the northern and central sections of Shenandoah National Park published by the Potomac Appalachian Trail Club. Since publication of the original paper, part of the area has been remapped by Allen (1963); some readers may wish to refer to his excellent paper for more detailed descriptions of the rocks overlying and underlying the Catoctin Formation.

Figure 3. Area described in this report. (click on image for an enlargement in a new window)