[Illustration: FIG. 355.—Narrows of the Warren River below Big Stone Lake, where it passed between jaws of hard granite and gneiss (after Upham).] [Illustration: FIG. 356.—Map of the valley of the Warren River in the vicinity of Minneapolis, with the young valley of the Mississippi entering it at Fort Snelling (after Sardeson).] It is interesting to follow this ancient waterway and to discover that, like our normal, present-day streams, it was held up in narrows wherever outcroppings of harder rock had constricted its channel (Fig. 355). The upper end of the Warren River valley is now occupied by the long and relatively narrow Lakes Traverse and Big Stone, each the result of blocking by delta deposits where a tributary stream has emerged into the valley, but this gigantic channel continues down to and beyond Minneapolis, occupied as far as Fort Snelling by the Minnesota River—a mere pygmy compared to its predecessor. To the earnest student of glacial geology there can be few sights more impressive than are obtained by standing at Fort Snelling, just above the confluence of the Minnesota and the Mississippi rivers, and surveying first the steep and narrow valley of the Mississippi above the junction,—a stream fitted to its valley for the simple reason that it has carved it,—and then gazing up and down that broad valley in which the great Warren River once flowed majestically to the sea, now the bed of the Minnesota above the Fort and of the Mississippi below it (Fig. 356). [Illustration: 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.”