36 EARTH FEATURES AND THEIR MEANING 



which live near the border of the sea. Other limestones are de- 

 posited directly from carbonate of lime in solution in the water. 

 A deposit of this nature is forming in southern Florida, both as 

 a flocculent calcareous mud and as crystals of lime carbonate 

 upon a limestone surface. Again, there is the reef limestone 

 which is built up of the stony parts of the coral animal, and, 

 lastly, the calcareous ooze of the deep-sea deposits. 



The marine sediments which are derived from the conti- 

 nents, the so-called terrigenous deposits, are found only upon the 

 continental shelf and upon the continental slope just outside it. 

 Of these terrigenous deposits, it is customary to distinguish: 

 (1) littoral or alongshore deposits, which are laid down between 

 high and low tide levels ; (2) shoal water deposits, which are found 

 between low-water mark and the edge of the continental shelf ; and 

 (3) aktian or offshore deposits, which are found upon the conti- 

 nental slope. The littoral and shoal water deposits are mainly 

 gravels and sands, while the offshore deposits are principally 

 muds or lime deposits. 



Special marks of littoral deposits. The marks of ripples are 

 often left in the sand of a beach, and may be preserved in the sand- 

 stone which results from the cementation of such deposits (pi. 11 A). 

 Very similar markings are, however, quite characteristic of the 

 surface of wind-blown sand. For the reason that deposits are 

 subject to many vicissitudes in their subsequent history, so that 

 they sometimes stand at steep angles or are even overturned, 

 it is important to observe the curves of sand ripples so as to dis- 

 tinguish the upper from the lower surface. 



In the finer sands and muds of sheltered tidal flats may be pre- 

 served the impressions from raindrops or of the feet of animals 

 which have wandered over the flat during an ebb tide. When 

 the tide is at flood, new material is laid down upon the surface 

 and the impressions are filled, but though hardened into rock, 

 these surfaces are those upon which the rock is easily parted, 

 and so the impressions are preserved. In the sandstones of the 

 Connecticut valley there has been preserved a quite remarkable 

 record in the footprints of animals belonging to extinct species, 

 which at the time these deposits were laid down must have been 

 abundant upon the neighboring shores. 



Between the tides muds may dry out and crack in intersecting 



