Fig. 437.—Diagrams to illustrate the characteristics of crater lakes. The Roman Campagna is a plain formed of volcanic ash, with the crater lakes of Bracciano, Vico, and Bolseno arranged on a line traversing it.
Coulée lakes.—Far more important as lakes are those volcanic basins which arise from the flow of a stream of lava across the valley of a river so as to impound its waters ([Fig. 438]).
At the time of the great eruption under Skaptár Jökull in 1783 the river Skaptár and many of its tributaries were blocked by the flow of lava, which it is estimated exceeded in bulk the mass of Mont Blanc.
Fig. 438.—View of Snag Lake, a coulée lake with lava dam shown in middle distance (after Fairbanks).
Morainal lakes.—As we have learned, the obstruction of drainage, due to the distribution of rock débris by continental glaciers, has yielded lakes in almost countless numbers. Probably ninety per cent or more of the known lakes have had this origin, and the type is so common within the once glaciated regions that it forms perhaps the best distinguishing mark of former glaciation. The hummocky surface of morainal deposits is so characteristic that the lakes of this type are never very large and are correspondingly irregular in outline. They have often numerous islands, and their banks are formed of the combination of rock flour and ice-worn materials known as till ([Fig. 439]). The smallest of the morainal lakes are mere kettles on the marginal moraine, and these rapidly become replaced by peat bogs. In contrast with pit lakes, morainal lakes lack the steep surrounding slopes and the encircling plain.
Fig. 439.—Diagrams to illustrate the characteristics of morainal lakes, and a sample map of such lakes from the glaciated region of North America.
Pit lakes.—The so-called pit lakes have their origin in continental glaciation, and are found in groups within broad plains of glacial outwash (mainly sand and gravel), which are for this reason described as “pitted plains” (see [p. 314]). Those areas which lay between neighboring lobes of the ice sheet were subject to particularly heavy deposits of outwash material, and are, in consequence, particularly likely to be occupied by pit lakes. As has been pointed out in an earlier section, the water derived from surface melting within the marginal portions of a continental glacier descends to the bottom in the crevasses and thereafter flows in an ice tunnel under the same conditions as water flowing in a pipe. Having in most cases a considerable head at the outer margin of the ice, this water may rise and issue well above the lower ice layers and so cover a portion of the ice margin beneath sand and gravel ([Fig. 440]). Separated blocks, often of massive proportions, are thus buried beneath nonconducting materials by which they are long protected from further melting. Eventually, however, with the approach of still milder climates they disappear, thus causing the overlying sand and gravel to descend and form a pit of steep walls similar to the sawdust pits over melted ice blocks within our storehouses.