646 



SCIENCE. 



[N. S. Vol. V. No. 121. 



MM. de la Noe and de Margerie have shown 

 similar forms in eastern France north of 

 Besangon.* The combination of blind val- 

 leys and sinks gives various forms ; the 

 greater the number of subterranean passages 

 for the water, the greater will be the irregu- 

 larity of the surface. The simplest type of 

 a blind valley is found where a single val- 

 ley gently descends on a continuous grade to 

 a flat depression of little or no greater width, 

 under which is the subterranean outlet for 

 the water. The method of formation of the 

 passages below the surface of the Chalk is 

 discussed in the chapter on subterranean 

 water in Mantel's Les Cevennes (Paris, 

 1890) and in his Les Abimes. 



MAKAIS DE SAINT GOND. 



Map of France, 1 : 80,000, sheet 50. 

 Chalons, S. W. 



Upon a recent trip up the valley of the 

 Petit Morin, toward the open Champagne, 

 it was observed by the writer that the iioor 

 of the valley that trenches the Tertiary up- 

 land was aggraded for the whole distance 

 from a point a few miles west of Montmirail 

 (sheet 40) up to the head of the St. Gond 

 marsh. There are places where the valley 

 sides approach each other more closely, 

 leaving a narrower aggraded bed, thus in- 

 dicating more resistent layers in the Lower 

 Tertiary or Upper Cretaceous strata, and 

 hence harder work for the Petit Morin- 

 Somme-Vaure when it was cutting the val- 

 ley, now aggraded, before the capture of the 

 headwaters by the Soude.f Professor Davis 

 has shown that the diminished volume of 

 water in the Petit Morin would necessitate 

 aggradation. The smaller amount of water 

 is not able to carry off the same amount of 

 detritus which is still washed down from 

 the same slopes. The soil creeps down, the 

 storms wash much fine detritus from the 



*See fig. 1 in Les Formes du Terrain. 

 fSee the Seine, the Meuae and the Moselle, by 

 "W. M. Davis, Nat. Geog. Mag., VII., 1896, 197-202. 



slopes into the valley bottom, and the small 

 side streams, w^hich now are as able to do 

 the work given them as before the capture 

 of the headwaters of the Petit Morin, also 

 carry much waste into the valley. At the 

 western end of the marsh, near St. Prix, the 

 little side stream entering here from the 

 north has brought in considerable detritus, 

 but this is only one of the minor factors in 

 the production of the Marais de Saint Gond. 

 The Petit Morin has lost the greater 

 part of its drainage area. It had developed 

 a good-sized adolescent valley, particularly 

 broad east of the hard rocks which form 

 the great Tertiary escarpment of the Paris 

 Basin. Since the loss of its headwaters it 

 has been compelled to aggrade throughout 

 the greatest portion of its course, thus 

 causing many small swamps in the lower 

 narrow valley and a broad marsh, le Marais 

 de Saint Gond, at the upper limit of the 

 beheaded Petit Morin. The present condi- 

 tion is one of unstable equilibrium. The 

 small stream at the elbow of capture of 

 the head waters of the Petit Morin by the 

 Somme-Soude, in the small village of Ecury- 

 le-Repos, will soon cut through the low 

 divide, on account of the steeper grade of 

 the Somme, and will drain the Marais de 

 Saint Gond near the village of Morains.* 



THE AGGRADING BAE. 



The little wriggling bar staggering blindly 

 along in a broad meandering valley is like 

 a small boy attempting to fill his grand- 

 father's boots. The waste supplied from 

 the sides of the adolescent valley, cut by 

 the ancestor of the present stream, is much 

 too great a load for a little brook. Beneath 

 the recent deposits of the bar, Professor de 

 Lapparent has found, by means of excava- 

 tions lately made, a deposit of argUlaceous 

 green sand, which must have been trans- 

 ported from the basin of the Aire when 



*See Atlas Cantonal, Department de la Marne, 1 : 

 50,000, sheet No 5, Canton de Vertus. 



