FIG. 347.—Broadly terraced valleys outside the divide of the St. Lawrence basin, which remain to mark the floods that issued from the latest continental glacier during its retreat (after Leverett).] Wherever the relief was small there occurred in the immediate vicinity of the ice front a temporary diversion of the streams by the parallel moraines, so that the currents tended to parallel the ice front. This temporary diversion known as “border drainage” was brought to a close when the partially impounded waters had, by cutting their way through the moraines, established more permanent valleys (Fig. 348). =Temporary lakes due to ice blocking.=—Whenever, on the contrary, the advancing ice front entered a valley from the direction of its mouth, or a _retreating ice front retired down the valley_, quite different results followed, since the waters were now impounded by the ice front serving as a dam. Though the histories of such blocking of rivers are often quite complex, the principles which underlie them are in reality simple enough. Of the lakes formed during advancing hemicycles of glaciation, and of all save the latest receding hemicycle, no satisfactory records are preserved, for the reason that the lake beaches and the lake deposits were later disturbed and buried by the overriding ice sheets. We have, however, every reason to suppose that the histories of each of these hemicycles were in every way as complex and interesting as that of the one which we are permitted to study. [Illustration: FIG. 348.—Border drainage about the retreating ice front south of Lake Erie. The stippled areas are the morainal ridges and the hachured bands the valleys of border drainage (after Leverett).] As an introduction to the study of the ice-blocked lakes of North America, and to set forth as clearly as may be the fundamental principles upon which such lakes are dependent, we shall consider in some detail the late glacial history of certain of the Scottish glens, since their area is so small and the relief so strong that relationships are more easily seen; it is, so to speak, a pocket edition of the history of the more extended glacial lakes. [Illustration: FIG. 349.—The “parallel roads” of Glen Roy in the southern highlands of Scotland (after Jamieson).]