ly] LANDSCAPES AND VEGETATION 547 



readily and often a pond develops ; or more luxuriant vegetation 

 may grow in the sheltered depression than in surrounding areas. 

 This is especially noticeable on pastured calcareous ' downs ' where 

 water percolates away and the general monotony of the short, grassy 

 vegetation may be relieved by sinkholes containing dense shrubby 

 growth or even trees. 



Of depositional surface features left by ground-water, spring 

 deposits are the most frequent and important, usually appearing as 

 mounds or terraces — commonly of calcium carbonate though some- 

 times of siliceous material. The vegetation varies greatly according 

 to local circumstances but is usually distinctive^as is true also of 

 the ' spring flushes '. These are most noticeable on slopes below 

 springs where the irrigating water constantly supplies fresh mineral 

 salts and the ground consequently bears grassy or forbaceous {i.e. 

 of herbs other than those of grass type) vegetation that is often 

 bright-green in colour in the midst of brownish acid-tolerant vegeta- 

 tion. Such flushes may be evident even if the spring or surface 

 run-off is not ; for they still indicate a local abundance of fresh 

 percolating water. 



Residual features left by ground-water are natural bridges and 

 chimneys, almost invariably of very limited extent. Although they 

 introduce whole series of microclimatic and often local habitat 

 effects, each of which may have its characteristic plant inhabitants, 

 the resulting communities tend to be extremely limited in area and, 

 moreover, similar to the types to be found in comparable habitats 

 about cliffs and steep banks elsewhere. 



Erosional features made by winds include blow-outs, which are 

 broad and shallow depressions scooped out of soft rock or sand in 

 more or less flat regions, wind-caves, due to differential erosion of 

 soft materials on hill-sides, or rarely blow-holes extending right 

 through a hill. These last two types occur chiefly in arid regions 

 and, because also of the dynamic and often eroded nature of the 

 surface, tend to be devoid of macroscopic vegetation. Blow-outs 

 may also be practically barren for much the same reasons — for 

 example in deserts, where they may be some miles in length and 

 hundreds of feet in depth. In other instances the lower levels may 

 become occupied by lakes, complete with attendant vegetation ; or 

 the soft rock may be colonized by plants, particularly in fissures, 

 and any sand surfaces in time become bound by grassy or other 

 vegetation which may gradually cover the area. 



Features due to deposition by wind include sand-dunes and loess. 



