Few people of the many hundred thousand living on or crossing the Llano Estacado (the Staked Plains of West Texas) realize that the Llano is a gigantic mesa.
The Llano was formed in several stages, it was originally part of the High Plains that stretched from southern South Dakota to the Rio Grande. Its surface was formed by erosion of the Rocky Mountains and the deposition of the eroded material east of the mountains. This detritus was spread in a vast sheet that filled existing valleys and spread over the area until a flat surface was formed.
Much of this was stream gravels washed from the ancestral Rocky Mountains, and became the great Ogalalla Aquifer, now being rapidly depleted by irrigated farming.
After this surface was established, 10 to 30 feet or more of caliche accumulated to form a "cap-rock." Caliche is rock that is formed in arid soils as calcium carbonates are collected from the soil and then come out of solution at the average depth of penetration of rainfall.
The Llano was then formed by the action of several rivers. The Canadian River cut its valley from its present headwaters in the Sangre de Cristo Mountains in northern New Mexico to its junction with the Arkansas River far to the east. This cut the Llano off from the High Plains to the north.
At the same time the Pecos River lengthened its valley from the Rio Grande north to its present headwaters also in the Sangre de Cristos and by doing so separated the Llano from the mountains to the west.
Simultaneously the east side of the Llano was eroded westerly by the Colorado, Brazos and Red Rivers and their tributaries. This erosion produced the high escarpment along the east side of the Llano. At the present time none of these streams on the east side cut through the Llano, although there are a few draws that almost reach the west side.
The surface of the Llano is marked by thousands of irregularly shaped and sized depressions that are very conspicuous after periods of heavy rainfall. These intermittent lakes, called "playas," provide resting places for thousands of ducks and shorebirds on their long journey to and from their summer homes in Canada.
Forty two salinas, or salt lakes, are also found on top of the Llano Estacado, providing roosting places for over 400,000 sandhill cranes during the winter.
The only other relief on the Llano is provided by a few "draws" which were originally streambeds formed during a period of much higher rainfall. These draws provide sufficient moisture to permit growth of small groves of trees. The rest of the landscape is either pasture that has become thicker with mesquite when occurrences of prairie dogs and fire were greatly diminished after settlement, or cottonfields and grainfields.
This is the Llano: a mesa of vast size, with draws and depressions breaking the monotony of its flat surface and providing a fascinating variety of flora and fauna not to be expected on such a table-land.
Nature Notes is sponsored by the Dixon Water Foundation and is produced by KRTS Marfa Public Radio in cooperation with the Sibley Nature Center in Midland, Texas. This episode was written by Burr Williams of the Sibley Nature Center.