Popular Science Monthly/Volume 22/January 1883/Playas and Playa-Lakes
|PLAYAS AND PLAYA-LAKES.|
OF the many characteristic features of the arid region of the far West known as the Great Basin, none attract the attention of the traveler more forcibly than the desert mud-plains that have been left by the evaporation of former lakes.
These areas are known locally as mud-flats, salt-flats, salt-marshes, borax-flats, alkali-flats, deserts, sinks, etc., the name usually indicating some peculiar feature of the valley to which it is applied. As these desert regions have an almost identical history, the Spanish word playa—meaning shore or strand—has been adopted by geologists as a generic term under which the various desiccated lake-basins may be grouped. Valleys more absolutely desolate than the playas of the Great Basin can not be found, even in the midst of Sahara. They occur as mud-plains, occupying the lowest portion of arid valleys, and form a horizontal, even floor that is soft and perhaps covered by a shallow lake during the winter, but in the summer and fall becomes hard and dry, and crossed by innumerable shrinkage-cracks that give the whole broad surface the appearance of a tessellated pavement of cream-colored marble. At other times, after the water has evaporated, the salts contained in the mud of the playa are brought to the surface in solution by the action of capillary attraction, and a saline incrustation is formed on the surface of the desert when the water that held the salts in solution evaporates. In such instances the playa appears as white and dazzling as if covered with drifting snow. A journey across such a plain during the heat of summer, when the mirage renders even the most familiar land-marks uncertain, becomes the most weary and trying that the explorer is called upon to make.
Examples of playas of broad extent are furnished by the desert region that borders Great Salt Lake on the west, which was left as a vast mud-plain by the evaporation of Lake Bonneville. This is an absolute desert, more than a hundred miles long by thirty or forty miles broad, composed principally of fine, tenacious, greenish clay. Other areas that now exist as great playas occur on the Carson Desert and on the Black Rock and Smoke Creek Deserts of Northwestern Nevada: these are portions of the bottom of Lake Lahontan that have been laid bare by the desiccation of the former lake. Playas of smaller extent, but which are yet typical examples of the deserts left by the withdrawal or evaporation of Quaternary lakes, are found in Diamond Valley, White-Pine Valley, Gabb's Valley, and Osobb Valley. All of these examples are in Nevada; but hundreds of others, of greater or less extent, might be enumerated that are scattered throughout the length and breadth of the Great Basin. As already mentioned, many of these playas are covered with water during the rainy season. Others exist as lakes, excepting when the season is unusually dry. They then become mud-flats that can not be distinguished from the playas that become dry and hard every summer.
The lakes that cover playas at certain times, and appear and disappear as the wet and dry seasons alternate, and are sometimes born of a single shower, and become many square miles in area during a single night, may with convenience be designated as playa-lakes, as they have many features peculiar to themselves. These lakes are without outlets, are seldom more than a few feet deep, and usually hold no more than a few inches of water; they are commonly alkaline or brackish, and are always, so far as the observations of the writer extend, of a peculiar yellowish or greenish-yellow color. The characteristic tint is due to the extremely fine mud, and probably chemical precipitates, that the waters hold in suspension. Owing to the extreme shallowness of these lakes, the fine mud at the bottom is agitated by every breeze, and thus the clearing of the water by the subsidence of the material held in suspension is prevented.
Playa-lakes, that form in the wet season and vanish again during the summer months, occur in a great number of the desert valleys and small inclosed basins to be found in the arid region between the Sierra Nevadas and the Wahsatch range. Some of these annual lakes are of considerable dimensions. On the northern part of the Black Rock Desert, Nevada, where Quin's River enters the desert from the northeast, a shallow lake is formed during the rainy season that is reported to be from ten to fifteen miles broad and as much as forty or fifty miles in length. When the dry season comes on, this lake evaporates, and the river that formed it shrinks back seventy-five or a hundred miles, leaving its channel a dry water-course, with perhaps a few water-holes to indicate its former extent.
Examples of playa-lakes that reach desiccation only during exceptionally dry seasons are furnished by Eagle Lake, Worth Carson Lake ("Carson and Humboldt Sink"), and Yellow-water Lake, in Nevada, and by Honey Lake and the lakes of Surprise Valley in California.
We have spoken of playas as being formed by the annual evaporation of small lakes; others by the evaporation of lakes after a term of years when the season was unusually dry; there are also other playas that are only covered with water during exceptionally wet seasons. One might perhaps include in the list of playa-lakes the great lakes of the Quaternary, whose fluctuations extended through geological periods, and whose desiccation has left the largest of all the playas. Lake Bonneville and Lake Lahontan, however, can be called playa-lakes only during the closing chapters of their history; in the earlier portions of their existence, they were fresh-water lakes of great depth, and one of them, at least, overflowed.
When we examine the material composing playas more critically, we find that they are formed of at least two varieties of sediments. In the broad, open playas, like Great Salt Lake Desert and the two desert regions of the Lahontan Basin in Nevada, the surface is composed of soft, fine, greenish, saline clay, that is commonly saturated with alkaline water at a depth of a few feet, and becomes tenacious and difficult to handle. These clays are, without question, simply lake-beds that were deposited by sedimentation at the bottom of the great lakes that once occupied the valleys where they are found.
The second variety of playa-beds occurs in restricted basins and in valleys that are without outlets. These are found very commonly behind shore-bars of Lake Bonneville and Lake Lahontan, and in valleys and canons the mouths of which have been crossed by the embankments of gravel built along the shores of these Quaternary lakes. The material forming playas of this nature is always of a light yellowish color, becoming almost white when dry; is extremely fine, and readily crumbles into dust between the fingers; near the surface, the beds are full of small globular vesicles that were apparently once filled with gas or water. These characteristics hold good even when the playas are surrounded on all sides by dark basalts, from the disintegration of which the playa-beds must have been formed.
True playa-beds, composed of light-colored material as described above, have been penetrated to the depth of five to six feet without revealing any change in the composition of the deposit. The thickness that may be reached by these slowly accumulating beds depends on the nature of the basin in which they are deposited; in some cases they can not be less than twenty or thirty feet in thickness. The coarse material swept down the sides of these inclosed basins by the infrequent rains is invariably left at the edge of the playa, and in a section of the beds appears as thin wedges of gravel and angular fragments, that thin out and become lost as we trace them away from the shore. Playa-beds may become covered with lake-beds, thus forming a peculiar light-colored stratum, in reality a fossil playa, that bears record of a time of desiccation; and, when lake-beds occur below, it is evidence that a dry period has intervened between two periods of more abundant precipitation.
The salts that form on the surfaces of playas are composed principally of the chloride, sulphate, and carbonate of soda, but sometimes contain borate of soda, sulphate of potash, sulphate of magnesia, and other salts in smaller proportion. In places the surface of a playa is sometimes formed of sulphate of soda several feet in thickness, as near the Buffalo Salt-Works on the Smoke Creek Desert, Nevada. Again, crystals of sulphate of lime (selenite), forming a bed more than six feet thick, cover hundreds of acres of the playa-surface, as on the eastern border of the Sevier Desert in Utah. Sometimes a playa for many square miles in extent is covered by a layer of salt a few inches in thickness, as was the case when Sevier Lake in Utah evaporated to dryness a few years since, and as is shown also by the large salt-field in Osobb Valley, Nevada. At other times the beds composing the playa contain brine beneath the surface, which yields large quantities of nearly pure salt upon evaporation; the supply of salt from this source in Nevada is practically without limit.
When by a change of climate a playa is no longer flooded, the subaërial gravels that are constantly moving down toward the bottom of a valley eventually overflow the entire surface of the playa, and the valley acquires a rounded instead of a horizontal floor. The same action tends to obliterate the beach-marks that a lake makes along its shores, so that in time all records that a lake has once occupied a valley become buried and erased: where once a broad, clear lake existed in which glacial covered peaks were reflected, there now stretches an arid desert, bearing only a scanty growth of artemisia. This, in brief, is the history of a large number of the valleys of the Great Basin.