PHYSIOGRAPHY OF THE SEA-SHORE 37 



river is subject to marked fluctuations, or maintains a more 

 or less steady flow. The number, size, and proximity of 

 large towns usually seriously affects its purity, through 

 pollution with sewage, or the waste of manufactures. 

 Coastal configuration, and local currents and wind direction 

 determine largely the subsequent direction taken by the 

 mass of river water on reaching the coasts, and even when 

 no river is present a shore-line may be considerably affected 

 by seepage and the action of springs. 



The shore can be considered in relation to one or other 

 of three chief physical types : (i) rock, (2) sand, (3) mud, 

 with a number of sub-types within each type and between 

 the different types. The rocky type varies greatly accord- 

 ing to the kind of deposit forming the shore, and the manner 

 in which its erosion proceeds. A vertical cliff face descend- 

 ing sheer into water of considerable depth ; slopes of varying 

 degree ; and reefs, may all appear in the same locality, and 

 show either considerable uniformity or diversity of surface, 

 as the strata or rock masses are uniform in texture or resist- 

 ance, or dissimilar in structure, or show different degrees of 

 contortion, fracture, or faulting, and hence weather irregu- 

 larly, giving a series of exposed and sheltered situations which 

 offer different types of environment for plant and animal 

 life. A steep cliff may have at its base a mass of boulders, 

 shingle, an erosion plane, or sand (rarely mud) and pass 

 laterally into the sandy or muddy types, or into shingle beds. 

 Several distinct types may occur along a line drawn straight 

 from high to low water levels, where a successive geological 

 series may appear. Volcanic rocks frequently present a 

 smooth surface, unsuited for attachment, while a sandstone 

 of fine texture will support a richer fauna than one of coarse 

 grain. Limestone often wears into many hollows and 

 fissures and gives ample protection. Conglomerates and 

 drifts weather into loose detritus and are faced with, and 

 provide material for, shingle banks. Sand, whether derived 

 from the erosion of sandstone, or comminuted shells, etc., 

 varies considerably in texture, weight, and other characters, 

 and according to degree of admixture passes into mud. 



