Septembee 9, 1910] 



SCIENCE 



325 



quently occur on the sides as well as at the 

 heads of valleys ; such, for instance, as the 

 two in the massif of the Uri Rothstock on 

 the way to the Surenen Pass and the Fer 

 a Cheval above Sixt. The Lago di Ritom 

 lies between the mouth of a hanging valley 

 and a well-defined step, and just above that 

 is the Lago di Cadagno in a large, steep- 

 walled corrie, which opens laterally into 

 the Val Piora, as that of the Lago di Tre- 

 morgio does into the southern side of the 

 Val Bedretto. Cirques may also be found 

 where glaciers have had a comparatively 

 brief existence, as the Creux des Vents on 

 the Jura ; or have never been formed, as on 

 the slopes of Salina, one of the Pipari Is- 

 lands, or in the limestone desert of Lower 

 Egypt." I have seen a miniature stepped 

 valley carved by a rainstorm on a slope of 

 Hampstead Heath; a cirque, about a yard 

 in height and breadth, similarly excavated 

 in the vertical wall of a gravel pit; and a 

 corrie, measured by feet instead of fur- 

 longs, at the foot of one of the Binns near 

 Burntisland, or, on a much reduced scale, 

 in a bank of earth. On all these the same 

 agent, plunging water, has left its marks 

 — runlets of rain for the smaller, streams 

 for the larger; convergent at first, per- 

 haps, by accident, afterwards inevitably 

 combined as the hollow widened and deep- 

 ened. Each of the great cirques is still a 

 "land of streams," and they are kept 

 permanent for the greater part of the year 

 by beds of snow on the ledges above its 

 walls. 



The "sapping and plucking" process 

 presents another difSeulty — the steps al- 

 ready mentioned in the floors of valleys. 

 These are supposed to indicate stages at 

 which the excavating glacier transferred 

 its operations to a higher level. But, if so, 

 the outermost one must be the oldest, or 

 the glacier must have been first formed in 



'A. J. Jukes-Browne, Geol. Mag., 1877, p. 477. 



the lowest part of the incipient valley. 

 Yet, with a falling temperature, the reverse 

 would happen, for otherwise the snow must 

 act as a protective mantle to the mature 

 pre-glacial surface almost down to its base. 

 However much age might have smoothed 

 away youthful angulal-ities, it would be 

 strange if no receptacles had been left 

 higher up to initiate the process ; and even 

 if sapping had only modified the form of 

 an older valley, it could not have cut the 

 steps unless it had begun its work on the 

 lowest one. Thus, in the case of the Creux 

 de Champ, if we hesitate to assume that the 

 sapping process began at the mouth of the 

 valley of the Grande Eau above Aigle, we 

 must suppose it to have started somewhere 

 near Ormont Dessus and to have excavated 

 that gigantic hollow, the floor of which 

 lies full 6,000 feet below the culminating 

 crags of the Diablerets. 



But even if "sapping and plucking" 

 were assigned a comparatively unimportant 

 position in the cutting out of cirques and 

 corries, it might still be maintained that 

 the glaciers of the ice age had greatly 

 deepened the valleys of mountain regions. 

 That view is adopted by Professors Penck 

 and Briickner in their work on the glacia- 

 tion of the Alps,^° the value of which even 

 those who can not accept some of their con- 

 clusions will thankfully admit. On one 

 point all parties agree — that a valley cut 

 by a fairly rapid stream in a durable rock 

 is V-like in section. With an increase of 

 speed the walls become more vertical; with 

 a diminution the valley widens and has a 

 flatter bed, over which the river, as the 

 base-line is approached, may at last me- 

 ander. Lateral streams will plough into 

 the slopes, and may be numerous enough 

 to convert them into alternating ridges and 

 furrows. If a valley has been evcavated 

 in thick horizontal beds of rock varying in 



'""Die Alpen in Eiszeitalter," 1909. 



