Besides these species, found on Skidaway island, two species, Mammut americanum and Mylodon harlani, have been found at Heyner’s (or Hainer’s) Bridge. This is about 7 miles south of Savannah, where the road crosses Vernon Creek (Lyell, “Travels in North America,” vol I, pp. 163–164). Here the stream is called White Bluff Creek. In order that the reader may get a clear understanding of the conditions at this important locality, a map found in Hodgson’s Memoir is reproduced (map [40]).
The whole region south of Savannah, between the mouths of Savannah and Ogeechee Rivers, is low and much divided into islands by streams connected with the rivers mentioned. A considerable part of these islands consists of marshes, which are usually overflowed by the tides. Most of the fossil bones were found along the southern bank of Skidaway River, in two places, apparently about 0.5 mile apart and near the western end of the island. On the map Hodgson has named the locality Fossilossa. Here Skidaway River made a bend which caused the bank to be eroded away, thus exposing the bones. According to Couper (Hodgson’s Memoir, p. 40), the bones were embedded in the marsh formation at about the level of very low-water. Lyell (Second Visit, etc., vol. I, p. 314) stated that the bones occurred in a dark peaty soil, or marsh mud, above which was a stratum of sand 3 or 4 feet thick; while below the peaty soil and below sea-level was sand containing many marine fossil shells, all belonging to species yet living on the neighboring coast.
The authors quoted state that at various places along the Georgia coast are found stumps of trees, cypress, cedar, and pine, in the deposits of the salt marshes and at a depth of from 2 to 4 feet below high-water. This is taken as evidence of subsidence in that region.
It is a matter of importance to know how those animal remains reached their place of burial. It has been suggested that whole carcasses had been floated down the streams and sunken where the bones are found. This is possible, but not probable. The peaty nature of the deposit inclosing the bones appears to be opposed to this view; nor could disarticulated bones have been washed down far from above, for they show no signs of attrition. The most probable explanation is that these animals lived and died about where their bones were discovered. At some past time the surface stood at a higher level than at present, although low enough to be more or less marshy. It probably supported a dense forest growth, and hither the species listed above resorted, with many others not yet discovered.
The animals inhabiting the region represent the same fauna found at so many places in Florida and Texas. The writer believes that they existed during the early part of the Pleistocene, approximately during the Aftonian interglacial; and that some of the species, as Megatherium, Mylodon, Equus, and Tapirus haysii became extinct before the advent of the Wisconsin glacial stage, probably a long time before this.
FLORIDA.
For the most recent descriptions of the geology of Florida one must consult the Annual Reports of the Florida Geological Survey, issued by the State geologist, Dr. E. H. Sellards, and Water supply Paper 319 of the U. S. Geological Survey, prepared by George C. Matson and Samuel Sanford and published in 1913. In the latter work are two large maps, one representing the topography of the State and the distribution of the various geologic formations; the other presents a generalized view of the distribution of Pleistocene terraces, as recognized by Matson and Sanford. The Second Annual Report of the Florida Geological Survey contains a map similar to the first mentioned.
From these maps it will be seen that the surface of Florida is largely occupied by Pleistocene deposits. According to Matson and Sanford, these deposits present themselves as disposed mostly in three principal terraces; and these are believed to indicate that the State was at one time largely submerged beneath the sea and that its present condition was attained after three principal upward movements. As shown on plate V of the geologists just named, the northern half of the peninsula at the time of greatest depression was represented by a number of islands, two of considerable size. One of these was situated at the northern end of the peninsula, the other near its center. The materials laid down around these islands and bordering the dry land along the northern border of the western half of the State form what is called the Newberry terrace. Its surface stands now at a height varying from 70 to somewhat more than 100 feet above sea-level. A second elevation exposed the deposits which, at least in part, constitute the next terrace, the Tsala Apopka. Its surface is a plain having an elevation of 40 to 60 feet above sea-level. At this stage the islands of the peninsula had coalesced, and the dry land extended southward nearly to the present Lake Okeechobee. A broad belt along the Atlantic and Gulf coasts, now dry land, was still occupied by salt water. A third elevation of the land left exposed the lowest terrace, the Pensacola, that bordering the Atlantic and Gulf coasts and including the southern end of the peninsula somewhat farther north than Lake Okeechobee. The elevation of this terrace varies from that of sea-level up to about 40 feet.
The materials composing the terraces in Florida are principally sand with local deposits of clays. In the southern part of the State important beds of limestone are found in the Pensacola terrace. These beds are shown on Matson and Stanford’s geological map. At St. Augustine and along the coast southward are beds of sea-shells cemented into coquina. Where cementation has not occurred there are beds of loose shells and of marl and sand.