FicNTON — Physiography and Glacial Geology of 8. Patagonia. 207 



with sandy earth ; the latter also exhibits false bedding in thin layers. This 

 arrangement of the terrace shingle gives one the impression of a great, fairly 

 rapidly running river carrying with it a large amount of silt. The terrace 

 which we are now considering, with its fellow on the opposite side, would 

 make a river valley perhaps five or six miles in breadth; and when we 

 consider that the whole of this horizon is covered with a layer of sandy 

 shingle, averaging about fifteen feet in thickness, we can form an idea of the 

 size of the river and the magnitude of the floods which existed in former times. 

 This terrace is about two hundred feet below the main pampa level ; conse- 

 quently the river valley of which it once formed the floor must have been cut 

 down very considerably through tertiary rocks before this shingle was 

 deposited. Now on this same terrace we find an occasional flat-topped hill, 

 rising to the level of the main pampa, and the hill is covered with pampa 

 shingle, which shows- that the cutting-down action of the valley must have 

 occurred subsequent to the pampa-shingle period, and the terrace shingle 

 must be of far later date than the first Ice Age. The first phase was a cutting 

 one, when the floods were enormous, and all the materials eroded from the 

 valleys were swept to sea ; the second occurred when the floods had con- 

 siderably abated and the cutting action had been replaced by deposition. The 

 map of this part of Patagonia shows that from the top of the highest ridge of 

 the Andes to the Atlantic is not more than one hundred and eighty miles, and, 

 as this narrow strip is intersected every thirty miles or so by a huge river 

 valley with branching cafladones, it seems wonderful that such an area could 

 have collected enough water to cause the floods we have been considerino-. 

 If, however, this country was once covered with a great mantle of snow and 

 ice which had been accumulating for centuries, and when this had reached 

 its climax the climate rapidly began to get warmer, enormous floods would 

 be prevalent every spring ; this flood-epoch would last until the accumulated 

 snow and ice would disappear in the spring and summer. ISTow at first during 

 this period the pampas and highlands would be protected by a deep layer of 

 ice; consequently they would not suffer much erosion, but the whole force of 

 the floods would be concentrated on the floors and sides of the river valleys, 

 with the result that the latter would be eaten down and the debris swept 

 away to sea. Later on, when the climate had become still more approximated 

 to what we have at the present day, and the pampas and highlands had 

 become denuded of their protecting mantle of ice, the floods would begin to 

 wash down huge quantities of shingle and sand into the river valleys. The 

 force of the floods having in the meantime considerably abated, this deposit 

 would accumulate until the floods died down to their present-day insigni- 

 ficance, when, the climate having become mild, vegetation would spring up 



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