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Geologic Formation

key, currents, atoll, florida and tortugas

GEOLOGIC FORMATION.

The geological formation and the history of south Florida and the adjacent regions have been investigated thoroughly by T. Wayland Vaughan, of the United States Geological Survey, and in one paper he discusses particularly the Tortugas Atoll' From this paper the writer quotes largely as to the geology of these islands. The Dry Tortugas consist of eight small islands: Loggerhead Key, Bird Key, Garden Key, Long Key, Bush Key, Sand Key, Middle Key, and East Key; these, together with large submerged banks and several shoals which were, until recent hurricanes, charted as islands, form an irreg ular ellipse with its longer axis directed from northeast to southwest. The lagoon inside of this atoll has a depth of 5 to 7 fathoms.

As stated in Vaughan's interesting account, two lines of investiga tion were taken up in determining the geology and origin of the Tor tugas atoll: first, whether submarine solubility phenomena played an important part in their history; second, whether the wind and current 'Vaughan, T. Wayland, The Building of the Marquesas and Tortugas Atolls and a Sketch of the Geologic History of the Florida Reef Tract. Carnegie Inst.. Wash. Pub. No. 182, pp. 55-67.

111 action were responsible. To determine the first, a chemical research was conducted into the dissolving power of sea-water due to the it contained, and the material from the bottom of the lagoon was exam ined. This latter examination showed the bottom to be fine calcareous mud, precipitated by denitrifying bacteria, as discovered by Drew.' The former chemical research was made by Dole' on daily observations of the amount of COt in the sea-water flowing into the lagoon and the amount of carbonates in the water flowing out. The result of this work showed that the dissolving action of in the water was negli gible and from the bottom samples it was ascertained that deposition was going on at a rate far exceeding any solubility.

The above study eliminated the first theory of action by solubility, but the theory of formation by waves and currents remained. As observed by various writers, sand dunes and water-carried detritus are alike molded into two shapes by certain wind and water currents. The latter are formed by an obstacle or a counter-current shearing a constant current, which drifts material to both sides and a crescent is formed with the bow of the arc facing the current. Vaughan further says that there are three kinds of currents working in this region: first, wind-formed currents, accompanied by waves; second, the Florida counter-current; third, tidal currents. The prevailing wind in the

Tortugas is from northeast to southwest, and the islands also lie in the sweep of the Florida counter-current, which moves west, so that these two sorts of currents cooperate; on the other hand, the general tidal current is from north to south—i. e., flowing transversely to the two before mentioned; thus the arc of the Tortugas Atoll is bowed toward the east, against the prevailing wind and the counter-current. The southwest part of the perimeter trails along the direction of these cur rents. The southwest and southeast passages in the atoll are attrib uted to the influence of the counter tidal current. White Shoal and Brilliant Shoal indicate the direction of this current, while Loggerhead, the largest of the group, shows the elongated "linear-ridge" shape due to the deposition of material by currents trailing along from the tails of the crescent, and the indurated beach rock on this island shows it to be wave-built, by its seaward slope.

On Garden Key, as seen by borings made by Vaughan, the larger rock-masses were found to be massive dead coral-heads. All of the Tortugas Keys are composed of calcareous detritus, the remains of various organisms, mollusks, corals, nullipores, echinoderms, and the calcareous algae. The coral fauna of the Tor tugas, which has also been extensively studied by Vaughan, is quite rich and has contributed a great deal to this detritus. The atoll is therefore formed by certain currents arranging this loose calcareous detritus. This material above sea-level is unconsolidated, but below 8 feet indurated beach-rock is found. This constituted an older formation for the growth of reef corals. "It is probable," Vaughan says, "that this rock was sub-aerially indurated and then depressed i. e., the atoll was outlined previous to coral growth in this region." The whole of the south Florida region was deposited in Pliocene times during depression; then in Pleistocene time there was an uplift and again a depression with a succeeding uplift. During the Pleistocene period it is supposed that some of the coral reefs stood as much as 18 feet above sea-level. In recent times there has been a depression, which, however, has left the keys slightly higher than they were before the Pleistocene depression. The evidence of these oscillations is borne out by the knowledge of the growth habit, in relation to sea-level, of the reef-forming corals in the Florida region.