Jl. Walck et al., SANDSTONE ROCKHOUSES OF THE EASTERN UNITED-STATES, WITH PARTICULAR REFERENCE TO THE ECOLOGY AND EVOLUTION OF THE ENDEMIC PLANT TAXA, The Botanical review, 62(4), 1996, pp. 311-362
Rockhouses are semicircular recesses extending far back under cliff ov
erhangs that are large enough to provide shelter for humans. The large
st sandstone rockhouses in the eastern United States are at the heads
of gorges, and they are in stream valleys cut during the Pleistocene;
most are formed in Mississippian- and Pennsylvanian-age rocks. Compare
d to the surrounding environment, the interior of rockhouses is shaded
, is warmer during winter and cooler during summer, and has lower evap
oration rates and higher humidities. Water enters rockhouses primarily
by groundwater seepage and by dripping from the ceiling. Soil consist
s mostly of sand with low pH, but high levels of some nutrients are as
sociated with saltpeter earth and with ecofactual and artifactual rema
ins left by human occupants during prehistoric time. Most plant taxa i
n sandstone rockhouses in eastern United States are native C-3 phanero
phytes or hemicryptophytes, and similarities in species composition am
ong rockhouses are low. Eleven plant taxa belonging to eight families
of flowering plants and ferns are endemic or nearly endemic to sandsto
ne rockhouses in eastern United States. Three endemics are restricted
to the gorges of a single river, and only one taxon ranges far north o
f the Wisconsinan Glacial Boundary. The endemic ferns are Tertiary rel
icts derived from tropical taxa. The majority of endemic flowering pla
nts are derived from temperate taxa that grow in habitats in the vicin
ity of rockhouses; their relative age ranges from Late Tertiary to the
Recent. All the endemic taxa are perennial; two ferns occur as indepe
ndent gametophytes. The endemic taxa of rockhouses are threatened prim
arily by disturbances associated with recreation.