Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 459.—Diagrams to illustrate the characteristics of barrier lakes, with an example from the southern coast of the Island of Nantucket.

Barrier lakes.—The Salton Sink illustrates a type of lake which is formed at the border of the sea through the erection of some kind of barrier which captures a small area of the ocean’s surface. Though such lakes may be properly described as strand lakes, it is usually at the mouth of a river that the process becomes effective. The common type of barrier lakes is found developed on most ragged coast lines where the shore currents have formed first bars and later barriers at the mouths of the estuaries ([Fig. 459]). Such embankments are usually gently curving or crescent shaped and are composed of sand or shingle which presents a steep landward and a gradual seaward slope.

Fig. 460.—Dune lakes on the coast of France (after Berghaus).

Dune lakes.—Within the narrow strips of shore in which all the fine soil that could be available for plant life has been washed away by the waves, beach sand is exposed to the direct action of the winds. In time of storm the sand is picked up and after drifting in the wind is collected in long ridges parallel to the shore. Constantly traveling along shore, these dunes block the mouths of rivers and thus produce a series of lakes such as are indicated in [Fig. 460].

Sink lakes.—Another class of lakes are due either directly or indirectly to the work of underground waters. In districts which are underlain by limestone, the surface water descending along the joints of the limestone may widen these passageways through solution of the rock and at lower levels flow on the floors of caverns eaten out by the same process on bedding planes of the formation. At the intersections of joints, more or less circular shafts known as “swallow-holes” go down to the caves from the surface. Locally, also the cavern roofs give way so as to choke the galleries with rubble and leave a basin at the surface which has an irregular but generally a more or less oval outline. If sufficiently clogged at the bottom by finer rock débris, these basins become occupied by small lakes which are known as sinks, and constitute one of the best surface indications of a limestone country.

Fig. 461.—Sink lakes in Florida, with a schematic diagram to illustrate the manner of their formation (map from U. S. G. S.).

Karst lakes—poljen.—In the limestone country to the north and east of the Adriatic Sea—the so-called Karst region—there are many interesting features which are directly traceable to the solution of the country rock. Here all the surface water descends in certain districts along the widened joint planes so that the drainage is largely subterranean. The so-called dolines or sinks of very regular and symmetrical forms resembling deep bowls cover a large part of the surface.

The entire country is, moreover, faulted in the most intricate fashion into many rift valleys. The drainage being so largely subterranean, these downthrown blocks of crust, the so-called poljen, become flooded at certain seasons of the year when the subterranean passages become choked or are too small to carry away all the water. A seasonal lake of this character is the Zirknitz Lake ([p. 189]).