Israel Cook Russell

“ Besides the kinds of lakes which we have enumerated, there are others, which are of rare occurrence and exceptional in their mode of formation; such is the beautiful little lake in Switzerland known as the Märjelen-See, which is formed by the glacier of the Aletsch blocking up the mouth of a tributary valley, and thus forming a wall of ice above which the waters accumulate. This ice-dam breaks away every few years, and allows the complete and rapid drainage of the lake, which often causes great inundations of the valley below. ”

“ As the great glacier retreated northward, it left the country covered with beds of bowlder-clay and strewed with huge erratics from northern regions, which together with other débris form the surface material of all our northern country, where it has not since been swept away or covered by other and more recent deposits. It is often well exposed along our lines of railroads, and may be known at a glance by the great number of worn and rounded stones of all sizes that are scattered promiscuously through it. ”

“ A clearer idea of the manner in which a flowing glacier wears out a rock-basin can be gathered, perhaps, from the accompanying diagram, where the rock R R is shown, over which passes the glacier G,
which wears its bottom less at the lower end, not only for the reason that the ice is continually wasting away, and growing thinner in the lower portion, but also because the material carried down on the surface of the glacier is deposited at its extremity M, in the form of a terminal moraine, and thus protects the rock beneath from further waste. ”

“ Nor are the lakes which fill glacier-worn rock-basins confined to our own continent, but they form the most common and grandest lakes of temperate latitudes, which might be called the lake latitudes, so completely are the lakes of the world confined to these regions. ”

“ 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. ”

“ 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. ”

“ 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 well-known Salt Lake of Utah is another example of a lake filling an area of depression, and was of far greater extent in past time, as is very plainly shown by the lines of ancient terraces which are so sharply drawn between Ogden and Salt Lake City, nearly a thousand feet above the present level of the lake. ”

“ 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. ”

“ 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 ”

“ 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 ”

“ As facts accumulated, however, it was noticed that the sides and bottoms of such lakes are smoothed, in many cases polished, and almost always covered with grooves and scratches; and also that in their vicinity beds of clay are usually found, intermixed with pebbles and large bowlders which, like the rocky basins, are also smoothed and frequently scratched. It was noticed, too, that the rock from which these bowlders and pebbles had been formed commonly differed from the rocks in place on the shores of the lakes. ”

“ In our own country we sometimes find lakes which owe their existence to the industry of the beavers, who often build their dams in our streams, and sometimes form shallow lakes of considerable extent. ”

“ Those filling glacier-worn rock-basins; 2. Those confined by banks of sand, gravel, bowlders, etc., or, in one word, by moraines; 3. Those formed by a subsidence of their bottoms, or by the elevation of the country surrounding them, commonly by the secular changes of level to which the crust of our globe is subject; 4. Lakes filling basins formed by volcanic action.
1. Lakes which fill rock-basins are such as are confined on all sides by the common rock of the country, so that in some cases a person can walk entirely around them without stepping off the solid rock ”