Jun 15, 2010

Rockhouse flowers

The Cumberland Plateau is home to many unusual plants. A seafloor when the Appalachian mountains were younger and taller, the plateau's geology is distinct, but its forests are similar to Appalachian forests. Where the plateau's geological youth is most apparent -- at the eroding rim of the gorge and in the dynamic river channel -- the unique conditions posed an evolutionary opportunity.

Plants that took advantage of that opportunity diverged from their Appalachian ancestors. Numerous species are known only from the Cumberlands. Some of these are found on the river shoals, others in the rockhouses along the rim, like the pictured catchfly. A close relative of fire pink, roundleaf catchfly (Silene rotundifolia) grows only on rock faces in the Cumberlands.

A rockhouse is a cliff that has eroded beyond vertical, creating an overhang and a sheltered space below. These sheltered areas may be large enough for a camp. Some like Hazard Cave in Pickett State Park are large enough for a village. The floor of a rockhouse is sandy, offering a dry, soft place to sleep, and the sloping rock face carries smoke from a fire up and away. Native Americans used rockhouses for shelter, but they considered the Cumberlands dangerous and cursed, so these camps were temporary stopovers during hunting or trading excursions and never as elaborate as cliff dwellings in southwestern states.

To protect endemic plants and animals, camping in rockhouses is now forbidden. One of the most vulnerable endemics is the Cumberland sandwort (Minuartia cumberlandensis), a lacy plant that lines the edges of rockhouse floors, living off what little moisture flows down the rock or blows in during rains. Roundleaf catchfly grows higher up in nooks etched into the sandstone, so it can not be trampled, but heat and smoke from a fire could harm it. Mountain spleenwort (Asplenium montanum), a fern, likes the same sort of nooks as roundleaf catchfly.

In the drip zone along the mouth of a rockhouse you can find lady fern, sedges and rockhouse featherbells (Stenanthium diffusum), a rare lily only recently recognized as a species. University of Tennessee botanist Gene Wofford realized the plant he kept finding in the Cumberland plateau was different than eastern featherbells (Stenanthium gramineum). He published his findings and a new endemic was born to science.

These plants grow happily in a rockhouse's unusual habitat, but how they get there in the first place is a bit of a puzzle. Sandstone in a rockhouse often appears crusted, with a harder layer above eroding more slowly than the rock below. These crusts are curved and irregular, not stratified like typical rock layers.

On my most recent visit to the Cumberlands, a botanist explained how those crusts form. It is a present-day phenomenon, not a trace of an ancient process. Just as tree roots stabilize a riverbank, mosses and lichens protect sandstone from erosion. In particular they slow the impact of winds. On a still day it is hard to imagine what a wind could do when it hits a rockhouse, but if you get caught in one during a storm or just put your imagination to work, you can see the sand at the base of the rock wall kicking up or rain and debris blowing in to knock grains from exposed sandstone.

Those same winds can cast a small seed onto a ledge evolution made its ideal home. Those same winds are what carves a rockhouse from a cliff in the first place. Where winds in the gorge tend to collect and swirl is where rockhouses form, so what looks like shelter from a storm can actually concentrate its power. No wonder native Americans preferred to live elsewhere.

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