SEDIMENTOLOGY ACROSS THE NORTHERN VIRGINIA FALL LINE

Construction exposed unconsolidated sediments at two sites separated by 1.5km in northern Virginia, each on opposite sides of the Fall Line. At the Piedmont site, the vertical section is approximately 35m from top of upland gravel (el 66m) to top of saprolite (el 31m). At the Coastal Plain site, the vertical section is estimated at 19m, from top of upland gravel (el 31m) to slate outcrop in Pohick Cr. Unconsolidated sediments in this area are mapped as Cretaceous Potomac fm and Tertiary upland gravel, but no fossils contemporary with deposition have yet been found at either site. The Piedmont upland gravel consists mainly of quartz and quartzitic sandstone pebbles, but weathered chert pebbles occur with fragments of spiriferoid and strophemenoid brachiopods characteristic of Lower Devonian faunas in the central Appalachians (J. T. Dutro, Jr.). Such rocks now outcrop at least 100km away. Pebbles on the Piedmont also include good Skolithos tubes characteristic of Cambrian sandstones in the same Appalachians. The upland gravel at the Coastal Plain site consisted almost entirely of quartz pebbles, the few chert and sandstone pebbles having no fossils. Low in the section at both sites, there are pebbles of bleached slate. At both sites, sands have locally steep bedding with reversely inclined pebbles (pebbles dip downstream more steeply than bedding). At both sites, the most common sediment is muddy sand, both massive and thin bedded, and the Coastal Plain site has expansive clay. Just before crossing the Fall Line, Pohick Cr goes through a complete meander wavelength, incised 24m into bedrock granite, and its main tributary enters Pohick Cr perpendicular to the Pohick within that meander. Sedimentologic and geomorphic data are consistent with uplift on the Piedmont side of the Fall Line.