546 INTRODUCTION TO PLANT GEOGRAPHY [CHAP, 



* thaw-sink ' topography. Erratic boulders of various shapes, sizes, 

 and provenance constitute a minor depositional feature in many 

 formerly glaciated areas. 



The residual features left by glaciers are also various, though 

 commonly exposed, rocky, and very poorly vegetated. Such are 

 both aretes (the ' knife-edge ' ridges left between glacial troughs) 

 and matterhorn peaks, which latter are left when several cirques have 

 so cut into the different flanks of a mountain as to isolate it and re- 

 duce it to a spectacular horn or needle. The vegetation in such 

 instances is almost invariably poor, as befits the lofty exposed situa- 

 tion and rockiness of the substratum ; it is often poorer than on 

 constructional mountain surfaces, owing to the instability of the 

 residual ones. In high alpine situations it may consist of little more 

 than Lichens on the rock-faces, crevice plants where they can find 

 a roothold, and tussock or other herbs where soil accumulates {cf. 

 Fig. 137, B). In hanging valleys, the other important member of 

 this category, the vegetation may be much less poor, owing to the 

 sheltered situation ; it is, however, usually less luxuriant than in the 

 associated main valley, at least when compared with the floor of the 

 latter. The roches moutonnees or ' sheep rocks ', those asymmetric 

 rocky hills smoothed by ice-action which characterize many formerly 

 glaciated areas, are at once erosional and residual. As with large 

 erratic boulders, their rock surfaces are often to this day devoid of 

 other than lichen and similarly dwarfed cryptogamic growth. How- 

 ever, in favourably sheltered situations, soil and higher vegetation 

 may largely occupy all but their steeper sides, crevices being especially 

 favoured, while a thick cap of humus, often supporting trees, may 

 cover their domed tops {cf. Fig. 92). 



Erosional features made by ground-water include caverns and 

 tunnels, the vegetation of which becomes drastically reduced with 

 decreasing light away from the orifice and, of course, limited to 

 saprophytes, etc., in the dark. Even near the orifice where the light 

 seems fairly strong and green plants prevail, these are chiefly vege- 

 tative forms of shade-plants such as various Ferns, the types occurring 

 farthest in being commonly Bryophytes, Algae, and long-drawn-out 

 seedlings utilizing stored food-reserves. Sinkholes or swallow-holes 

 are funnel-shaped depressions in the surface of the ground in regions 

 of soluble rock, formed either by the collapse of a cavern roof or, 

 directly, by the solvent action of descending surface-water which 

 enlarges cracks or joints. The bottoms of such sinkholes may be- 

 come choked with sediment, so that water can no longer drain out 



