Go back

Synonyms

Cumberland Riverscour (NatureServe 2015)

Boulder (cobble) bar (Bailey and Coe 2001)

Boulder beds (Shaw 2003)

Distribution

Northern Tennessee (Scott County, eastern Fentress and northern Morgan counties), restricted to river margins of the upper main stem of the Big South and the lower sections of it’s tributaries, Clear Creek and the New River.

Vegetation Description

Cumberland Plateau Escarpment Sandstone Boulder Bar communities occur in small, linear patches and are dominated by perennial grasses and forbs and occasionally shrub thickets not exceeding 1.5 meters in height and are notably absent of a developed woody overstory.  Boulder bars occur among similarly situated cobble and sand bar communities.  Surrounding habitat in the incised river gorges of the Big South Fork and lower sections of the Clear Creek and New River are forested alluvial terraces and slopes.

Physical Characterization

The Cumberland Plateau Escarpment Sandstone Boulder Bar community type is confined to the dissected gorge bottoms of the Cumberland Plateau and the eroded rim of the Cumberland Escarpment, as delineated by the EPA’s level IV ecoregion classification map.  Occuring at an elevational range between 250 to 400 meters in Tennessee on a slightly pitched (south to north) but essentially flat-aspect.  Boulder bars are created by the mass wasting of the plateau’s sandstone cliffs (Shaw 2003) and the subsequent transport of substrate during violent flooding events.  The boulders themselves are blocks of Pennsylvanian aged Fentress, Rockcastle Conglomerate, Gizzard, Crooked Fork and Crab Orchard Mountain Group sandstone originating from the cap of the Cumberland Plateau and can be anywhere from head-sized to as big as a modest house.  Both the New River and the Clear Fork river gorges still run through the relatively impermeable sandstone plateau top, while along the Big South Fork, the river bed has eroded through the Pennsylvanian mantle into older strata of Pennsylvanian and Mississippian Pennington formations of limestones, shales and coal seems that are exposed along the Escarpment’s eroding edge .  The Cumberland boulder bars belong to a class of riverscour communities whose floristic composition is, to a large degree, shaped by periodic and seasonal disturbance in the form of unchecked flooding events.  Soils of boulder bar communities are typically devoid of organic matter and are mostly sand.

Natural Processes

Occasional seasonal floods maintain this community type by abating the encroachment of woody vegetation on to the boulder bars.

Dominant Plants