550 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



desirable soil. However, if unprotected by vegetation it is easily 

 eroded. 



Residual features left by wind include mushroom rocks and some 

 mesas and buttes : their tops are usually poorly vegetated owing to 

 exposure and dryness, their sides being often barren owing to 

 abrasion and erosion. 



Erosional features due to waves and currents include wave-cut 

 benches, clijfed shore-lines, and sea-caves. Of these the benches 

 extend down to the lower limit of wave erosion and tend to be well 

 vegetated by fair-sized Brown and other Algae as indicated in Chapter 

 XVI. The cliffs commonly introduce a host of crevice and other 

 microhabitats and consequently support very various plant com- 

 munities in different spots and instances, though their exposed and 

 unstable nature usually prevents the attainment of anything approach- 

 ing the local climax. Deep and narrow inlets and sea-caves are 

 also very variously vegetated according to local conditions, though 

 owing to their rocky nature and the frequently poor light in them, 

 there is considerable limitation of the flora and of sturdy growth. 

 Sometimes, however, where salt water does not reach and a last- 

 ing supply of fresh water percolates, so that the atmosphere 

 is continuously damp, a fine growth of Ferns and Bryophytes 

 is developed in the mouths of caverns and as far in as light 

 allows. 



Depositional features of waves and currents include beaches, tidal 

 deltas, and various kinds of bars. The beaches are liable to be 

 barren, especially where the surface is exposed and dynamic, though 

 in sheltered situations alongside salt as well as fresh water the 

 surface may be colonized and, where silty, often well vegetated. 

 This is especially true in tidal creeks and about the off-shore 

 periphery of deltas, which in the tropics typically support luxuriant 

 mangrove vegetation of shrubs or trees as described in Chapter 

 XIV. In extra-tropical regions a swarded salt-marsh often develops 

 in such situations. Sand and, especially, mud bars or spits may 

 also be so adorned when formed in sheltered situations such as 

 deep bays ; more often, however, they are devoid of higher plants, 

 though in cases of intermediate stability the surface may be bound 

 by a close investment of Schizophyta or lowly Algae. 



Residual features formed by waves and currents include stacks 

 and arches which, being cliffed and exposed, tend to be poorly 

 vegetated although they introduce an array of microhabitats com- 

 parable with those found about cliffs. Though distinctive and often 



