Elsewhere the karst river may emerge from its subterranean course in a broader depressed area bounded by vertical cliffs, from which it later disappears beneath the limestone wall. Such depressions of the karst are known as poljen, and appear in most cases to be above the downthrown blocks in the intricate fault mosaic of the region. Some of these steeply walled inclosures have an area of several hundred square miles, and especially at the time of the spring snow melting they are flooded with water and so transformed into seasonal lakes ([Fig. 199] and [p. 422]). It appears that at such times the cave galleries of the region with their local narrows are not able to carry off all the water which is conducted to them; and in consequence there is a temporary impounding of the flood waters in those portions of the river’s course which are open to the sky and more extended. The rush of water at such times may bring the red clay into the subterranean channels in sufficient quantity to clog the passages. The Zirknitz Lake usually has high water two or three times a year, and exceptionally the flooding has continued for a number of years. It has thus in some districts been necessary to afford relief to the population through the construction of expensive drainage tunnels.
Fig. 199.—The Zirknitz seasonal lake within a polje of the Karst (after Berghaus).
The conditions which are typified in the Karst area to the east of the Adriatic Sea are encountered also in many other lands; as, for example, in the Vorarlberg and Swiss Alps, in Lebanon, and in Sicily.
The return of the water to the surface.—Water which has descended from the surface and been there held between impervious layers, may be under the pressure of its own weight or “head”; and will later find its way upward, it may be to the surface or higher, where a perforation is discovered in its otherwise impervious cover. Such local perforations are produced naturally by lines of fracture or faulting (widened at their intersections), and artificially through the sinking of deep wells. The water, which at ordinary times reaches the surface upon fissures, is usually concentrated locally at the intersections of the fracture network, where it issues in lines of fissure springs ([Fig. 200]); but at the time of earthquakes the water may rise above the surface in lines of fountains ([p. 83]), or occasionally as sheets of water which may mount some tens of feet into the air.
Fig. 200.—Fissure springs arranged upon lines of rock fracture at intersections, Pomperaug valley, Connecticut.
In contrast to the flow of surface springs, which varies with the season through wide ranges both in its volume and in temperature of the water, the volume of fissure springs is but slightly affected by the seasonal precipitation, and the water temperature is maintained relatively constant. Rock is but a poor heat conductor, and the seasonal temperature changes descend a few feet only into the ground. Thus water which rises from depths of a few hundred feet only is apt to be icy cold, while from greater depths the effect of the earth’s internal heat is apparent in a uniform but relatively higher temperature of the water. Such “warm” or thermal springs are apt to contain considerable mineral matter in solution, both because the water is far traveled and because its higher temperature has considerably increased its solvent properties.
It has long been recognized that lines of junction of different rock formations at the base of mountain ranges are localities favorable for the occurrence of thermal springs. These junction lines are usually within zones where by movement upon fractures the widest openings in the rock have formed, and the catchment area of the neighboring mountain highland has supplied head for the ground water. A map of the hot springs within the Great Basin of the western United States would present in the main a map of its principal faults.