Fig. 357.—Portion of the Herman quadrangle of Minnesota, showing the position of the Herman beach on the shore of the former Lake Agassiz. The lake basin is to the left, and the pitted morainal deposits appear to the right (U. S. G. S.).
Just as the “parallel roads” of Glen Roy, roads in name only, are the beaches of earlier glacial lake stages, so in Lake Agassiz we have parallel beaches of the barrier type which are often roads in fact as well as in name, and which mark the stages of successive lakes within this vast basin. The Herman beach, corresponding to the highest level of the lake, is thus a sharp topographic boundary between lake deposits and morainal accumulations, and is further itself a well-marked topographic feature composed of wave-washed and hence well-drained materials ([Fig. 357]). Farmers of the district have been quick to realize that these level and slightly elevated ridges lack the clay which would render them muddy in the wet seasons, and are thus ideally adapted for roads. They have in many sections been thus used over long stretches and are known as the “ridge roads.”
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.
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).
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]).