Examination of the central Virginia Blue Ridge shows that both regional and local factors affected the development of 1,107 debris avalanches during Hurricane Camille in 1969. Major factors influencing the triggering of debris avalanches were rainfall, topographic relief, and contrasting lithologic associations between the Pedlar massif with its metavolcanic cover and the Lovingston massif. Within areas of high rainfall and high chute density, secondary factors affect the susceptibility of individual rock units and chute orientations within those rock units. Units that have high to moderate susceptibility generally have a prominent biotite or amphibole foliation and/or multiple rock fabrics (compositional layering and foliation). Units showing a moderate to low susceptibility are generally more massive and/or lack a well-defined biotite foliation. Rock fabrics influenced preferred chute orientations in most rock types. Chute azimuths are moderately concentrated within a 75° arc between N60°E and S45°E, subparallel to the direction of dip of the regional foliation. These Camille results suggest that most of the Pedlar massif has a moderate susceptibility to shallow-seated landsliding. By contrast, the topographically rugged areas of the Lovingston massif have high to moderate susceptibility.

Abstract Debris fans in low-order Appalachian Mountain drainage basins can be used to estimate the return periods between catastrophic debris flow events such as the Hurricane Camille storm of 1969 in Virginia. Debris fans in Davis Creek, Virginia, have been the sites of repeated debris flow deposition at least three times during the last 11,000 years. Debris flow frequency estimates are possible if individual events can be recognized in the fan stratigraphy. Discrimination of events is based on the recognition of paleosols, and on abrupt changes in sediment texture and in matrix composition at suspected event boundaries. Major controls on slope stability appear to include the orientation of the slope, bedrock structure, and presence of colluvial hollows at the sites prior to slope failures. Hollows are sites of between-event accumulation of colluvium, and are areas of subsurface water concentration during heavy rains. Tropical air masses seem to have been a factor in most historical Appalachian debris flows. The early Holocene initiation of debris flow activity on the central Virginia fans appears to coincide with paleoclimatic data, indicating the commencement of conditions that permitted the invasion of tropical moisture into the region at the close of Pleistocene time.

The Roseland district of Nelson and Amherst Counties, Virginia, is a typical Grenvillian terrane, analogous to similar terranes of eastern Canada. The oldest rocks in the Roseland district are layered granulites, quartz mangerites, and quartzo-feldspathic gneisses. Ages on zircon from the Shaeffer Hollow Granite, a leucocratic granite related to these oldest rock types, are discordant but apparently pre-Grenvillian. The Roseland Anorthosite intrudes these older rocks and consists of andesine antiperthite megacrysts and blue quartz in a finer grained oligoclase-K feldspar matrix. The Roseland Anorthosite is more alkalic and silicic than massif anorthosites elsewhere. Pyroxene megacrysts and rutile are found in its border areas. After the emplacement of the anorthosite, the Roses Mill and Turkey Mountain ferrodiorite-charnockite plutons (of the Roses Mill Plutonic Suite) were intruded. Layered diorite and nelsonite, an ilmenite-apatite rock, are found near the bases of these plutons. These rocks may have formed, in part, by liquid immiscibility. Ages determined on the Roses Mill Pluton are about 970 m.y. The largest part of the plutons is altered to biotitic granitic augen gneiss. The Rockfish Valley deformation zone crosses the district from northeast to southwest. Northwest of the deformation zone are ferrodioritic charnockitic rocks of the Pedlar massif, which are slightly older than the Roses Mill Pluton. These ages may have been modified by metamorphism. The Mobley Mountain Granite is a fine- to medium-grained, subsolvus two mica granite and has been dated at 650 m.y. Various mafic and ultramafic dikes, of different ages, are present throughout the district. The major structure of the district is the Roseland dome, cored by the Roseland Anorthosite, and trending northeast for at least 22 km. Three periods of deformation are evident; one is possibly pre-Grenvillian and is seen only in the oldest rocks. The Roses Mill Plutonic Suite was deformed and underwent retrograde metamorphism to lower amphibolite assemblages in Proterozoic Z time. Paleozoic deformation was responsible for a reactivation of the Rockfish Valley deformation zone, which originated as a Precambrian feature and a selective overprinting of retrograde greenschist facies metamorphism on the granulite- to upper amphibolite-facies assemblages of the country rock. The principal resources of the district are rutile and ilmenite. Both are present as hard rock and saprolite deposits. The rutile formed at the anorthosite border, and ilmenite is contained largely in nelsonite and mafic ferrodiorite bodies.