=Episodes of the glacial lake history within the St. Lawrence valley.=—Within this great drainage basin it has apparently been possible to read the records of each stage in the latest lake history—complex as this has been. We have only to recall the lake stages cited from the Scottish glens and remember that each new stage was begun in a retirement of the glacier front which unblocked an outlet of lower level than the last. This sequence might, however, have been varied by a temporary readvance of the ice, as indeed once occurred in the Huron-Erie lobe of the great North American glacier. [Illustration: FIG. 358.—The continental glacier of North America in an early stage of its recession, when it covered the entire St. Lawrence drainage basin. The dashed line is the approximate position of the divide (based on a map by Goldthwait).] [Illustration: FIG. 359.—Outline map of the early Lake Maumee, with the bordering moraine and the water-laid moraine remaining on the site of the former ice cliff.] =The crescentic lakes of the earlier stages.=—So long as the glacier covered the entire drainage basin of the St. Lawrence River system, all water was freely drained away by streams which flowed _away from_ the ice front (Fig. 358). So soon, however, as at any point the front had retired behind the divide, impounding of the waters must locally have occurred. Lakes of this type are to-day to be seen in Greenland and in the southern Andes; and though upon a diminutive scale, some idea of their aspect may be obtained from the appearance of the Märjelen Lake of Switzerland, here blocked by a mountain glacier (Fig. 446, p. 411). Within all areas of small relief, such as the prairie country surrounding the present Laurentian lakes, the earlier and smaller stages of such ice-blocked lakes are generally crescentic in outline. This is because a moraine in most cases forms the land margin of the lake, and because the ice cliff upon the opposite border, although somewhat straightened, as a consequence of wave-cutting and iceberg formation, still retains the convex outlines characteristic of ice lobes (Fig. 359). [Illustration: FIG. 360.—Map to show the first stages of the ice-dammed lakes within the St. Lawrence basin (after Leverett and Taylor).] Within each of the Great Lake basins a crescentic lake early appeared at that end of the depression which was first uncovered by the glacier: Lake Duluth in the Superior basin, Lake Chicago in the Michigan basin, and Lake Maumee in the Huron-Erie basin (Fig. 360).