100 Leonard Hawkes — Rock Stream in East Iceland. 



liparite commonly weathers out in very large blocks. A striking 

 demonstration of this is to be found at the head of the Jafnadal, 

 Sto^varfjord, S.E. Iceland. The valley ends in a large cirque, the 

 walls of which are chiefly composed of liparite. The plain about 

 a thousand feet below the top of the cirque wall is dotted over with 

 blocks of liparite which have rolled from above over the steep snow 

 slopes. The largest of these blocks, measuring 90 feet by 45 feet by 

 40 feet, has split in two at its final resting-place (see PI. VI, Fig. 2). 

 The complete weathering down of such a block would give rise to 

 a mound like the largest of those in the Lodmundarfjord valley. 

 The mounds clearly result from the weathering of large blocks in 

 situ, and the only agency which can be imagined to have brought 

 them to their positions so far west is that of ice floating in the sea, 

 which once stretched far up the valley as evidenced by the raised 

 beach deposits. Thus the " Hraun " is. of raised beach age and 

 was not formed subsequently (2, p. 159), and it dates from the 

 end of the last Ice Age, when the glacier of the fjord valley had 

 retreated. The raised beaches along the fjords of the Folden district, 

 Salten, IS". Norway, are considered by Rekstad to be formed of 

 morainic debris, and to date from the time when glaciers flowed 

 down side valleys to the shores of the fjords (7, pp. 10-11). 

 A similar explanation suffices for the deposits described in the 

 Bardarstadadal, and thus the "Hraun" is to be regarded as the 

 moraine of a glacier flowing down Hraundal. 



The morainic theory is rejected by Thoroddsen on two main 

 grounds. These are (1) that " the whole mass of debris and blocks 

 consist exclusively of one particular rock, spherulitic liparite, whilst 

 a moraine must contain both ", and (2) " no ice scratches are found 

 on the blocks" (2, p. 160). As mentioned above, besides obsidian 

 basalt does enter into the composition of the "Hraun", though to 

 a remarkably small extent. This is partly understood when we 

 consider the brittleness and fissility of liparite as contrasted with the 

 toughness of basalt, whereby the former succumbs more easily to 

 erosive agencies than the latter. The readiness with which liparite 

 breaks up would itself account for the absence of ice-scratches on the 

 blocks. During a field examination, extending over several weeks, 

 of the acid rocks of East Iceland, the writer never saw an ice-scored 

 surface of liparite. 



I regard the " Hraun " as a surface "block-moraine ", and it would 

 not bear ice-scratches. The material probably did not fall slowly on to 

 the glacier as a talus stream, but descended in great landslides, as has 

 been suggested for some of the rock streams in the San Juan Mountains, 

 Colorado, which are characterized by "the remarkable quantity of 

 relatively coarse material comprising them, and the fact that the 

 greater part of this must have been carried on the surface of the ice " 

 (8, p. 25). The "Hraun" presents many analogies to the rock streams 

 of the San Juan Mountains. These latter are Comparable in size, 

 the hummocky surface is similar, they are often composed of Tertiary 

 acid volcanic rocks, and the topography of the district is that of 

 a " dissected and glaciated plateau of more or less horizontally bedded 

 volcanic rocks " (9, p. 11). 



