MARK IN CALCAREOUS SEDIMENTS 143 



Hog Island and Providence Island, which generally has a depth of 

 from I to 3 fathoms, furnishes conditions favorable to the develop- 

 ment of tidal currents of varying velocities between Nassau and the 

 reefs of living corals 4 miles east. The currents are strong enough 

 near the corals to keep the limestone bottom to which they are 

 attached swept clean of calcareous sand. 



Between the reefs and the east end of Hog Island, considerable 

 stretches of the bottom are covered with white sand on which 

 numerous patches of "sea grass" are growing, which partially or 

 entirely prevents the development of ripple marks. Farther west 

 where the "grass" is absent the white sand, consisting largely of 

 coarse, shell fragments, is marked by ripple marks with short wave- 

 length (2 to 4 inches) of the ordinary current-mark pattern (see 

 photograph by Kindle).^ Over the easterly part of the white sand 

 bottom, where the currents probably have their maximum strength, 

 the bottom is covered with sand waves or metaripples.^ These 

 have a wave-length estimated at from 15 to 20 feet and an ampli- 

 tude of 6 to 8 inches. 



The waters of Biscayne Bay, Florida, furnish a considerable area 

 of white calcareous sand bottom under water sufficiently shallow 

 (12 to 18 feet) to permit careful inspection from a glass-bottomed 

 boat. Much of the bottom between Miami and Cape Florida is 

 covered with marine plants which prevent the occurrence of 

 ripple marks. Near the Cape, however, waves and currents have 

 kept the bottom free from plant colonies. Asymmetric ripple 

 marks with rather long wave-length were observed near Cape 

 Florida. A short distance from the steamer landing on the west 

 side of the Cape parallel ridges of the sand wave type were seen 

 spaced 25 to 50 feet apart. In the deeper water highly irregular 

 bottom features occurred somewhat comparable in complexity of 

 pattern with the fiat-topped sand ridges at Annisquam, Massachu- 

 setts, illustrated by Kindle.^ 



' E. M. Kindle, "Recent and Fossil Ripple Mark," Can. Geol. Surv., Mus. Bull. 

 25 (1917), Plate VII. 



^ W. H. Bucher, "On Ripples and Related Sedimentary Surface Forms and Their 

 Paleogeographic Interpretation, Parts I and II," Amer. Jour. Sci., Vol. XLVII 

 (March- April, 191 9), p. 181. 



3 Op. cit. 



