Lake formation by Glaciers. ' 437 



ALFRED TYLOR, F.G.S., 



ON THE ACTION AND FORMATION OF RIVERS, LAKES, AND 

 STREAMS, WITH REMARKS ON DENUDATION AND THE 

 CAUSES OF THE GREAT CHANGES OF CLIMATE WHICH 

 OCCURRED JUST PRIOR TO THE HISTORICAL PERIOD.* 



{March 22nd, 1875.) 



MY lecture will relate to the consequence of the flow of water and ice, that 

 is, to the motion of rivers and streams and of glaciers, and their effect in 

 producing erosion or denudation, as it is called, of the surface of the earth. 



I hope to prove, by diagrams, the effect of the excessive rainfall and snow- 

 fall in former times, when the earth received its sculpture or superficial configu- 

 ration. The extent of the ancient rivers and glaciers is shown by the dimensions 

 and shape of the present hills, lakes, and valleys. I believe, and I hope to 

 convince you, that the present forms and shapes of the surfaces of the earth are 

 due to events which happened at or about the time of the arrival of man, when 

 atmospheric conditions were extremely different to those at present. These 

 views I have held for twenty years, and they are, of course, entirely contrary 

 to the views of the late Sir C. Lyell, who never admitted a pluvial period. f 



Then I do not think, in the present theory of regelation of ice of Faraday 

 (in 1850) and Tyndall (in 1857), that these writers have taken into account 

 all the circumstances of the case. 



(2) The formation of great and deep lakes is a great difficulty ; it has been 

 asserted by Professor Ramsay and others that some of the Swiss lakes were 

 formed by glaciers, but the present accepted theory of the motion of ice does 

 not, I think, give a possible explanation of the formation of large lakes, thirty 

 or forty miles long and 1,200 feet deep. Such a work involves the carrying 

 out of a vast quantity of material from the lake bed, against the action of 

 gravity. (See Fig. I.) 



I hope to show, by taking account of what I think has been omitted or neg- 

 lected — referring to Fig. 1, which I will now describe — that an upward and 

 forward movement would occur in the lake-glacier, due to the greater weight 

 at the upper end. Then I think one consideration has been omitted, viz. the 

 effect of the friction of ice, during motion, in producing heat, by which a large 

 quantity of water is produced. Fig. I is a glacier one mile thick, occupying 

 and filling a lake 1,200 ft. deep, and excavating it at the rate of half-an-inch 

 over the whole surface in one year in the glacial period. 



I think De Charpentier (in his "Essai sur les Glaciers," 1841) was right in 

 supposing that water, freezing in the glacier itself, and expanding, gave motion 

 to glaciers, although that is disputed. He worked with the great Agassiz, who 

 discovered the glacial period in 1837. Playfair first discovered the geological 

 importance of glaciers in 1802. 



The movement of a lake-glacier, I believe, is assisted by three different forces 

 impelling it forward,|and retarded by two resisting forces, as shown in my diagram. 



The impelling forces are, I . Gravity, measured by the slope of top surface. 



2. Pressure from behind, due to the heavy weight of glacier behind acting 

 on the ice which is treading in water, and causing the front end of the glacier 

 to rise, just as the weight of the Victoria Tower in Westminster caused the bed 

 of the Thames to rise ; or as a weight applied in the formation of a railway 



* [This Lecture was illustrated by some new experiments on ice, and by reference to new ex- 

 periments made by the Lecturer to prove the amount of ice thawed by friction of ice upon ice, 

 which is an important and new element in explaining glacial motion.] 



tA. Tylor, Quart. Journ. Geol. Soc.,vol. xxv., pages 9 and 63, 1869; vol. xxiv., page 105, 186S. 



