FACIES OF THE SHORE ZONE 655 



though percolating waters should dissolve away the shells a mold 

 or impression of the same will remain, which no agent short of 

 metamorphism can obliterate, and that not always. The abundance 

 of organic remains in sandstones of all kinds and colors and of all 

 ages shows that there are no inherent characteristics in sand which 

 prevent the preservation of such remains. In practically all cases 

 when organic remains or their impressions are wanting in sand- 

 stones, we have a right to assume that they were not present at the 

 time of formation of the deposit. Such absence suggests a sub- 

 aerial rather than subaqueous origin for the deposit, and as such it 

 should be considered unless other unmistakable characteristics point 

 to a subaqueous (marine or lacustrine) origin. (For application to 

 basal beds see Pumpelly-75 '.^i"/.) 



5. Muddy fades. 



This generally occurs in intimate association with an organic 

 facies in the shore zone. Where sands accumulate in sheltered mar- 

 ginal lagoons, plants (eel grass) and animals commonly contribute 

 their remains to enrich and color the mud. Salt marshes are the 

 normal successors of the mud flat, the organic element here being 

 in the ascendency. (See ante. Chapter XI.) The purely inorganic 

 structures of such a mud flat are, in addition to stratification, the 

 mud cracks, rain prints and rill marks, and the tracks and trails of 

 animals frequenting the shore. During very high spring tides ex- 

 tensive portions of a very flat shore may be covered with a layer of 

 mud, which on the retreat of the tide may become marked by mud 

 cracks and footprint impressions. Since such areas will be uncov- 

 ered for a fortnight, the clay may become sufficiently hardened to 

 permit the permanent preservation of the mud cracks. It must, 

 however, be borne in mind that when the mud deposit made by 

 one inundation is comparatively thin, as is apt to be the case, this 

 layer will on drying curl up into shaving-like masses and be blown 

 away by the wind. While under exceptional conditions the mud 

 cracks, rill marks and tracks formed in the shore zone may be pre- 

 served, such preservation is far from being characteristic. 



Flocculation and the conditions of mud deposits. The formation 

 of mud deposits at the mouths of great rivers emptying in the sea, 

 as in the case of the Mississippi, is favored by the ])resence of the 

 salt in the sea water. Flocculation, or the drawing together of 

 particles, takes place much more extensively in salt than in fresh 

 water, and as a result such particles will sink more quickly in sea 



