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Desert Jewel
| Article
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16498 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
5 / 1989 |
1,599 Words |
| Author
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Stanley Welsh
Stanley Welsh is a plant taxonomist and professor of botany at
Brigham Young University, Provo, Utah. He is the author of 10
books and dozens of scientific articles dealing with plants of
Utah and the Arctic (especially Alaska and the Yukon). His
most recent book is A Utah Flora (Great Basin Naturalist
Memoir 9). |
In 1895, Alice Eastwood accepted an offer from Alfred Wetherill, who assisted in the discovery of the great cliff houses on Mesa Verde, to visit Utah's San Juan region. Nearing Bluff City, a Mormon outpost, they paused to admire great sandstone cliffs. There she saw her first hanging garden--an oasis in the desert.
"About midway on these cliffs, the character of the rocks changes, and the change is marked from a distance by a line of green. The water from the mesa sinks to the underlying strata, and there on a narrow bench, not more than a yard wide, it constantly oozes out. Here is a most strange vegetation, a boreal oasis in the midst of a Sonoran desert."
Eastwood asked the question that each succeeding visiting botanist has asked: "How did these waifs reach that isolated bench, with nothing in the surrounding country in the slightest degree allied?"
The constant cutting by the waters of the Colorado River and its tributaries, with the resultant removal of stone, created the stepped landscape of canyon-lands. The river has opened the book of geological time. The harder sandstone formations now form the magnificent stripped cliffs, whose margins can fall away due to erosion of softer rock formations beneath them. The softer and more porous formations, such as Navajo sandstone, are great water bearers, and the erosion at the cliff's edge allows water trapped in the stone for uncounted years to escape, turning the cliff face dark and glistening.
Moisture in an arid land--and much of the canyonlands of Utah qualify as arid--is at a premium. The paucity of water is attested to by the scarcity of green fields adjacent to the towns that cluster about the region's few perennial streams. Therefore, no wet place is destined to be unvegetated for long. Spores of ferns, mosses, algae, and dust-size seeds of flowering plants are carried by the wind to an exposed wet site. They adhere to the film of water and begin to sprout. The roots of seed plants and ferns grow tightly pressed to the stone, penetrating microscopic depressions and anchoring the plants, albeit tenuously, even on vertical or overhanging walls.
Two processes interact to help form hanging gardens. The water-laden sandstone, exposed to the air, is less resistant to erosion because the material that bonds the grains of sandstone is dissolved by the flowing water. Also, as plants grow on the wet surface, they yield weak acids that hasten the removal of the cementing
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