196 Scientific Proceedings, Royal Dublin Society. 



III. 



Indications that the Shingle was deposited during the Quaternary Ice Age. 



With regard to fixing the date of the first great shingle period, the facts 

 that we have to go hy are that the layer in question is found superficial to 

 practically all the rocks of the district. The only rock overlying it is the 

 lava, which is found over fairly large areas in many places ; also it is 

 unfossiliferous, and is unique, inasmuch as no parallel is found to it in any of 

 the older formations. The higgledy-piggledy arrangement also gives one the 

 idea of its having been carried down by a large volume of slowly running 

 water. The large lava outpourings of this locality occurred during a long 

 period, and were thrown out in gi-eat part subsequently to the beginning of the 

 ice age ; so that, considering also the similarity of this shingle layer to many 

 of the fluvio-glaeial deposits found in the northern hemisphere, we are 

 justified in concluding that the great Patagonia parnpa shingle layer belongs 

 to the earliest portion of the Ice Age. The south of the Gallegos river is 

 low -and undulatory, and does not seem to belong to the same horizon as the 

 north, and there are many points of difference between the pampas in both 

 localities. On the north, as we have seen, the pampa rises suddenly, or in a 

 series of well-marked terraces, to about five hundred feet, and then extends 

 away indefinitely in the form of a remarkably flat table-land, whereas on the 

 south the plains begin only a few feet above the sea-level, and rise slowly 

 and gradually. At first there are no apparent terraces on the south side, but 

 a little up the Gallegos Eio Chico a terrace appears which would seem, on 

 being traced, to correspond to the lowest terrace along the Gallegos river, 

 and a little further still a second terrace is in many instances found. On the 

 south bank of the Gallegos river, however, about thirty miles from its mouth, 

 one meets once more the typical high or first pampa level, extending down in 

 a V-shaped manner between the Gallegos river and the Eio Chico, or, more 

 correctly speaking, in a long nari-ow strip a few leagues wide running down 

 along and parallel to the former river, and separated from it by a broad 

 terrace, about two miles wide, and the low flat of the river valley. 



On carefully examining this strip, one can see that it possesses all the 

 characteristics of the pampa, such as extreme levelness of the summit, and, 

 where eaten into, flat-topped hills. Already we have seen that at the time 

 when the shingle began to come down, the level of the main pampas was five 

 hundred feet lower than it is at the present day. At that time all the land 

 to the south of this pampa line would be under water, and consequently 

 would not be subjected to the great planing down action of the wind; this 



