THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 7 



Such deposits would be indicated by the presence of large amounts of 

 water-soluble salts in connection with the peculiar structure of the beds. 

 The sparse vegetation and the equally sparse animal life, both of a peculiar 

 kind, would leave the deposits singularly barren of fossils and devoid of the 

 excess of carbonaceous matter which would produce the black, blue, or 

 green colors so common in normal swamp deposits. Salt swamps occur 

 to-day in some arid regions, as in the Salt Plains of Oklahoma or in Australia, 

 and are apt to be but phases in the life of playa lakes. The periodic desicca- 

 tion is likely to produce bright red colors by the oxidation of the iron in the 

 debris swept into the swamp by the winds and waters of violent storms.^ 



BRACKISH-WATER DEPOSITS. 



Brackish-water deposits may accumulate in tidal estuaries and marshes, 

 in regions subject to periodic flooding by the sea, or near the mouths of 

 streams. Less common are brackish-water deposits in lakes approaching 

 salinity, but the fauna of these is so distinct from that of bodies of water 

 rendered saline by the admixture of marine waters that the distinction 

 would not be difficult in the deposits of past geological periods. 



Estuarine deposits. — These are characterized by the mixed fresh-water 

 and marine fauna and by the rapid alternation and irregular position of the 

 beds due to the changing composition of the water and the shifting currents 

 as the tide meets the river. Could the whole length of the estuary be laid 

 bare, the gradual change from fluviatile deposits and faunae to marine 

 deposits and faunae would be apparent; river gravels and mud banks 

 would give place to the mixed and irregular deposits of the region of tidal 

 influence, and these to the regular deposits of the open sea. Where the 

 invading tides checked the river current the deposits would be in the nature 

 of delta deposits, but a delta would not form, because the ebb of the tide, 

 with its resultant quick outflow of the marine water and the dammed-back 

 river waters, would scour out the deposited sediments. However, there 

 would be much material left on the sides of the main channel where the 

 retarded stream and the inflowing tide would spread over the adjacent 

 lowland. The deposits here would be delta-like and floodplain-like in char- 

 acter, but would differ from the subaerial portion of a delta or a flood-plain 

 in the inclusion of marine or brackish-water fossils. Times of especially 

 high tide or of great flood in the streams would carry the deposits and life 

 of one region far into the domain of the other and might leave very puzzling 

 accumulations for the paleogeographer to interpret. 



The upper reaches of such a large estuary as Puget Sound in the early 

 Tertiary would show all the conditions of a fresh-water swamp. The retarded 

 rivers far above the reach of salt water would spread abroad and by the filling 



1 Gould, C. N., Geology and Water Resources of Oklahoma, Water Supply Paper No. 148, 

 U. S. Geological Survey, 1905. 



