648 PRINCIPLES OF STRATIGRAPHY 



prints, footprints, etc., can be impressed upon the exposed sedi- 

 ments, and preserved under favorable circumstances. 



Since the magnitude of tides varies with the moon's phases, the 

 exact hmit of the shore zone is never fixed. During spring tides the 

 zone encroaches upon the strand, that indefinite zone just above 

 high water, while the ebb succeeding will lay bare portions of the 

 zone generally submerged. On lakes the shore zone practically dis- 

 appears (it would be wrong, however, to say that there is no littoral 

 district on the lakes) unless the seiche is to be considered as equiva- 

 lent to the tides. 



Fades of tlic Shore Zone. The shore zone may represent one 

 or more of a variety of facies, or types of material, none of which 

 can be considered as strictly confined to it. The most typical facies 

 of the shore zone are: i, the rocky cliff facies; 2, the bouldery 

 facies; 3, the gravelly facies; 4, the sand facies; 5, the mud flat 

 facies, and 6, tiie organic facies. Each of these facies has its dis- 

 tinct physical and organic characters. 



I. Rocky cliff facies. 



This is most significant from the point of view of bionomics, as 

 will be more fully discussed in a later chapter. Erosion is active 

 here, and coarse fragments are broken from the cliff and accumu- 

 late as a submarine talus and boulder pavement. Where rock accu- 

 mulation takes place, a rudaceous phase will be found next to the 

 cliff', the material of the rock fragments being that of the cliff from 

 which they are derived. Since in such cases the rock fragments 

 broken from the cliff may fall into water sufficiently deep to prevent 

 much attrition of the fragments, the resultant rudyte may be a 

 breccia, the fragments being in the main angular. Examples of 

 such cliff rudytes are found in the St. Croix formation of the 

 Dalles region of the Wisconsin River, and in the Lake Superior 

 sandstone of Marquette. Michigan. Some of these may of course 

 be old, subaerial talus slopes reworked by the transgressing sea. 

 This appears to be the case with the basal Mid-Devonic limestones 

 (Dundee) of IMichigan and western Ontario. (See Chapter XIII.) 



2. Bouldery facies. 



Where a rock cliff fronts the waves, the fragments broken from 

 the cliff' by the frost and by the sea are generally ground into 



