National Geographic : 1896 Jun
198 THE SEINE, THE MEUSE, AND THE MOSELLE In this valley it receives two branches on the southern side, to which special attention should be given. The first is the Sur melin, whose head is found in the upland near its eastern pre cipitous margin; but, curiously enough, although this stream of course diminishes toward its source near Montmort, the valley that it occupies maintains an almost constant width some six miles farther, nearly to the escarpment of the upland. The second branch is the Petit Morin. This, like the Marne, heads in the lowland east of the upland, and also, like the Marne, escapes by a deep and narrow valley through the upland. The lowland area that it drains is, however, very small, and for about ten miles from its head there is an extended marsh, known as the Marais de St. Gond, lying partly on the lowlands and partly in the entrance to the narrow valley in the upland. In searching for a reason for this arrangement of the Marne and its two branches, it is important to notice that if the branches were prolonged eastward they would both lead to streams, the Soude and the Somme,* flowing for some distance on the low land toward the heads of the branches, but then turning north ward and entering the Marne directly. The beheading of the Surmelin and the Petit Morin.-In explana tion of all these facts let it be supposed that the two pairs, Soude Surmelin and Somme-Morin, were once actually continuous streams at a time before the lowland was eroded on the weak rocks east of the upland, and let the verity of the supposition be tested by the likelihood of a natural, spontaneous change from that condition to the present. When the paired streams flowed westward, they, like the Marne, must have run in the direction of the dip of the strata; hence they may all be called consequent streams. They must all have passed from the weak Cretaceous strata to the resistant Tertiary strata. The Marne is much the largest of these three streams, and its valley must have been deepened rapidly, while the other valleys must have been deepened slowly. As the valleys were deepened they progressively widened, but the widening must have been much more rapid on the weak than on the resistant strata; and the deep valley of the Marne must have widened in the weaker strata much more rapidly than the neighboring shallow valleys of the Soude-Surmelin and the Somme-Morin. Now the question arises, will the divides between these three valleys shift in such a manner as to alter the assumed original * Not to be confused with the river Somme in northwestern France.