432 Ti-ansactions. — Geology 



and along with heaps of angular rubbish were slowly carried away. Some- 

 times the stones and boulders fell into crevasses, or between the ice and the 

 rock of the mountain slopes, and so got ground and polished on one or more 

 sides, but they always travelled farther and farther off from their parent 

 mountains. The tops of the Lowland hills, peering above the ice, caught some 

 of the wanderers as they drifted past, but many were borne out to the terminal 

 front of the ice and dropped into the sea, where they mingled with the 

 scratched stones that were being pushed out from underneath the glaciers. As 

 the ice continued to melt, erratic and angular debris was stranded at ever- 

 decreasing heights upon the mountain slopes and hill sides, and at last the ice 

 drew back from the sea and the glaciers then dropped their rubbish upon the 

 land. Great streams of water escaping from the melting ice swept the 

 morainic matter down the valleys, and angular stones and rubbish, as they 

 were pushed along, became rounded by attrition and arranged by the rivers in 

 great flats of gravel and sand. Thus, ever as the glaciers withdrew, the 

 angular debris that gathered in front of them was ploughed down and dis- 

 tributed over the bottoms of the valleys by the swollen rivers, the perched 

 blocks at great elevation on the sides of the valleys, and upon the slopes of the 

 Lowland hills, still remaining to indicate the heights formerly reached by the 

 glaciers. There being no great river valleys draining from the mountain 

 regions in the low grounds of Lewis and Caithness, the absence of glacial 

 deposits from such districts is easily accounted for. To what extent the ice 

 was eventually reduced we have no means of ascertaining, neither do we know 

 much of the climatal condition which at this period obtained in Scotland. All 

 that we can safely assert is, that the ice disappeared entirely from all the low 

 lands, and drew back into the deeper mountain valleys. Of the plants and 

 animals which at this time may have clothed and peopled the land, we know 

 next to nothing. I have here and there, in the gravel and sand beds, detected 

 some vegetable matter, but in too decomposed a state to enable me to say what 

 it was. In one section, however, near Carhum, on the Tweed, I obtained from 

 a bed of sand in the series numerous remains of water-rats and frogs. It 

 would be hard to believe that these were the sole denizens of the land ; as yet, 

 however, they are all we have got to show. After such conditions had lasted 

 for a longer or shorter period, the land gradually sank into the sea. As it 

 slowly went down the waves and currents ploughed up and re-distributed much 

 of the old glacial accumulations and river deposits. Broad terraces of gravel 

 and sand were cut into, and their materials winnowed and re-arranged. Here 

 and there, also, ridges of gravel and cones of sand were heaped up in places 

 where no sand and gravel existed before, the sea using up for the purpose the 

 till and morainic rubbish." 



If such an ice sheet as Dr. Ilaast has conjured up had ever occupied the 



