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Modern and Ancient Alluvial Systems
Geomorphology and Sedimentology of Humid-Temperate Alluvial Fans, Central Virginia
Alluvial fans in mountainous humid-temperate climatic regions are typically formed by mass wasting processes generated by infrequent catastrophic rainfall events. Coarse gravel alluvial fans are the dominant depositional landforms along the slopes of the Blue Ridge Mountains in central Virginia. Humid-temperate alluvial fans such as those in central Virginia are markedly different in process, geomorphology and sedimentology than alluvial fans described in arid, humid-glacial humid-tropical regions and humidperiglacial regions.
Fans along the western slope of the Blue Ridge contain clast-supported sandy gravels presumed to have been deposited by braided streams. These gravels are subrounded, well-imbricated, contain channel fills, and show occasional large-scale planar cross-bedding.
Fans east of the Blue Ridge, in Nelson County, are composed of cobbles and boulders in a fine-grained matrix and result from infrequent debris avalanche and debris flow processes. Radiocarbon dating of fan stratigraphy indicates that periods of inactivity range from 3,000 to 6,000 years. These fans appear to have been active throughout the Quaternary and most recently in 1969 due to the intense rainfall from Hurricane Camille. Recognition of individual events was based on identification of buried soil horizons, changes in matrix mineralogy, bedding character, and clast weathering characteristics. Discrete depositional events were correlated between alluvial fans in the Nelson County area by radiocarbon dating of buried soil horizons formed on fan paleosurfaces.
The Virginia alluvial fans are small, have steep segmented profiles and are less than 100 m thick. Most of the fans observed were less than 20 m thick. Soil horizons are well-developed on fans which have been inactive for thousands of years, while inceptisols occur on fans that have received recent sedimentation.
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