K :: C V/ LEDGE 



•■November 1, 1S92. 



Between E and F the glacier des Tetes Eousses falls 

 rapidly by a steep incline of something like 45°. 



Fi(j. 4. — Sketch Map of Glacier des Tfites Rousses and Surroundings. A. Glacier 

 de Bionnassay; B, Glacier des Tetes Bousses ; c, Glacier de la Gria ; D, lower 

 earitT ; E. F, Rocks of the Tetes Bousses. 



On the plateau of the Totes Rousses glacier, near where 

 the letter B appears on the plan, there was formed in the 

 glacier a great cavity filled with water, a glacial lake : it 

 was probably entirely sub-glacial, i.e., no part of the water 

 was visible on the surface. Whether anyone had passed 

 over or near this part of the glacier this year, before the 

 catastrophe, seems doubtful, but last year no one had 

 noticed the existence there of any lake. This cavity was 

 about 260 feet by 130 feet in superficial extent, and had 

 a vertical depth variously estimated from 130 to 160 feet. 

 This cavity communicated by a passage through the ice 

 with another cavity situated in the sharply inclined part 

 of the glacier, and at or near the spot marked D on the 

 map (Fig. 4). 



Neither of the cavities disclosed the rocky floor of the 

 glacier ; the floor both of the passage and of the upper 

 cavity was formed of broken masses of ice which had fallen 

 in. Whether the solid ice of the glacier or the rock lay 

 immediately beneath these broken bits was not apparent. 

 M. Tairraz believed the entire system of cavities to be in 

 the substance of the ice. 



M. Vallot describes the lower hole as having a diameter 

 of 40 metres, or about 130 feet, and a depth of 20 metres, 

 or about 65 feet. M. Tairraz described it as much 

 shallower. M. Vallot describes the passage as having a 

 length a little over 50 metres, or from 160 to 170 feet, 

 whilst M. Tairraz estimated its length at nearly twice as M 

 much. 



M. Tairraz, who visited the spot the day after the acci- 

 dent, and M. Vallot and his party, who visited it on the 

 19th of July, all passed from one hole to the other 

 through the passage, though not without difliculty from 

 the masses of ice which had fallen in from the roof. 



Fig. 5 shows the upper of the two cavities. It will be 

 observed that the roof fell in without any disturbance of 

 the surrounding glacier. 



Fig. 6 shows the lower of the two cavities, the cavity 

 on the incline. From this cavity ramifying passages were 

 observed by M. Vallot, and may have led to other reser- 

 voirs of water. Near this cavity the glacier has been 

 strangely changed by the event. The ancient level of the 

 glacier was continuous with the upper line on the horizon ; 

 the event has destroyed this continuity, and has set free 

 a vast mass of ice which lias slidden away and disappeared, 

 leaving behind the great ice cliff which appears in the 

 drawing, which is some 130 feet in height, and which 



stretches in a crescent form the whole way across the 

 glacier between the points E and F on our last map. 

 Fresh snow fell almost immediately after the 

 event, and is seen on the lower part of the 

 picture. It wiped out all traces of the dis- 

 ruption on the new surface of the glacier. But 

 both in the cavity itself and on the newly- 

 formed ice cliffs are apparent the lines which 

 indicate the divisions between the layers of ice 

 formed in successive years. 



Towards the bottom of the hole will be 

 observed a cave-like hollow, which is the 

 commencement of the sub-glacial passage to 

 the upper cavity. 



The imaginary section which forms Fig. 

 7 will perhaps help further to explain the 

 situation. A is the upper ca\'ity with its 

 roof yet intact, B is the sub-glacial passage, 

 and C the lower cavity. The dotted line 

 will indicate and bound the mass of ice 

 which was detached by the accident. The 

 horizontal lines indicate water, and the 

 oblique lines rock. 



What seems to have happened was this : that on the 

 morning in question the roof of the upper cavity fell 

 in with a momentum which was sufficient to drive the 

 water violently through the passage and into the lower 

 cavity. In this cavity the upward pressure of the water 

 was sufficient to detach the superincumbent mass of ice 

 and to set it rolling rapidly down the incline. The roof 

 of the upper cavity must have operated like the plunger of 

 a piston and produced a sudden and intense pressure 

 throughout the whole water cavities. The ice, being less 

 thick above the second cavity than above the passage, 

 yielded, and in its turn operated by suction on the water 

 in the cavity and drew it after it. I suppose that the 

 sudden slipping away of a great mass of ice would tend to 

 produce a vacuum, which operating on the lower hole drew 



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Fig. 7. — Imaginary section of the two cavities, with connecting 

 passage. A, upper cavity ; b, passage ; c, lower cavity. 



the whole contents out of it and of the adjoining cavities. 

 In this way only can I account for the vast mass of water 

 which formed part of the descending mass, and for the 

 fact that on the day after the accident the two ca\'ities 

 appear to have been empty of water. It seems to me 

 probable, as I shall hereafter mention, that the ice above 

 the lower cavity was more or less loosened by crevasses 

 running across the line of motion of the glacier and there- 

 fore in the same line as the cliff' left by the departed mass 

 of glacier. 



The mass of ice and water thus set free slid over the 

 steep glacier or neve to the west, and then turned in a 



