During the last ice age, the shoreline of the Gulf of Mexico along the Florida’s Big Bend was far offshore of its current location. At that time, several local rivers, including the Aucilla, Ocholocknee and St. Marks, converged several miles offshore of the current shoreline. At that confluence, there was a significant topographic rise. As sea level rose with the melting of the Pleistocene ice sheets, that hill top became an island and then became completely submerged about 6000 -7000 years before present. The former island currently is a significant shoal in Apalachee Bay with modern NOAA nautical charts showing a least depth of 3 feet.
Before and during, and submergence, this landform was likely the scene of human activity as early as 14,500 years ago and possibly contains evidence of that activity. Further, although the area is not currently impacted by commercial marine traffic the area possibly contains evidence of past naval activity. The navigable channel to the port of St. Marks is immediately adjacent, and during the Spanish Mission era, it was without doubt the location of groundings of early shipping to and from the Spanish colony. As evidence of this, during the Spanish Mission era, the Spanish governor sent a hydrographer to the area to determine a safe route to the Port of St. Marks. In addition, prior to the extension of railroads to Southern Georgia, it was along the route of the shipping of cotton and other agricultural products from the Red Hills area of North Florida to other parts of the U.S. and Europe. It also saw considerable naval traffic during the Civil War during the Union blockade of Confederate ports. As evidence of its threat to early marine traffic, the current NOAA chart for the area indicates reports of sunken vessels in the area around the shoal. In addition, the shoal is also highly likely to retain unique biological, geological, and hydrographic features as an isolated submerged island in the Gulf. Those characteristics make it a prime target for scientific study.
Despite its unique history and historic activity in the area, the shoal has never been mapped in any detail. In 1876, probably as a result of U.S. naval vessels having navigational problems during the Civil War, a hydrographic survey of the Shoal area was conducted by the U.S. Coast and Geodetic Survey. That survey showed a least depth of three feet, but it was based on a few widely spaced soundings run across the shoal. Considering the time period of the survey, the soundings would have probably been made from a rowed skiff using lead-lines with the horizontal positions of the soundings based on sextant angles to prominent features on the shoreline several miles away.
Amazingly, there is no record of subsequent hydrographic surveys of the shoal. Based on NOAA records, the soundings and contours reflected on the current hydrographic chart of the area are based solely on the 1876 survey with its limited sampling. The probable reason for the glaring lack of current hydrographic information for the area around the shoal is the hostile nature of the surrounding waters for modern hydrographic survey vessels. Nevertheless, the precise location and topographic configuration of the shoal are virtually unknown even though it is an area of land relatively close to populated areas.
To remendy this lack of information for a critical part of the prehistoric Aucilla River, the Aucilla Research Institute (ARI) has embarked on a project to map the Ochlocknee Shoal with grant from the Florida Division of Historic Resources and the Ingalls Foundation. To overcome the difficulty associated with the use of conventional hydrographic surveying in this area, the ARI project is using airborne bathymetric LiDAR. Unlike conventional LiDAR which uses red laser pulses reflected off topographic surfaces to map an area, bathymetric LiDAR uses both a red laser too reflect off above water features and a green laser to penetrate the water. By transmitting and receiving millions of laser pulses as an aircraft flies along a path over water, a high-resolution image of the sea bottom, any features in the water column, including fish, as well as any above water objects, may be obtained.
The maximum depths that the system will map varies with the clarity of the water and the reflective characteristics of the bottom surface. Therefore, it is not known how much of the area will be mapped. Yet, based on results from a similar ARI project mapping several miles of the prehistoric channel nearer the present coastline in 2018, the shoal should be mapped in great detail.
For the current Ochlocknee Shoal project, the ARI has contracted with the firm of Quantum Spatial, Inc. to fly the project. The project is scheduled to be flown by that firm during November of this year. Following that, the contracted firm will analyze and “classify” the millions of data points obtained from the flight, identifying which points represent the sea bottom versus those that were reflected from the water surface or items in the water column.
Once ARI receives the classified data, a detailed analysis will be performed using a geographic information system (GIS) to prepare a topo map of the bottom along with unusual features. The grant funding will also allow for a limited amount of field work including “ground truthing” of the LiDAR bathymetry using a fathometer and GPS provided by the Ingalls Foundation grant. In addition, it will allow examination of the area by ARI geology, oceanography and archaeology divers to examine any unusual features noted in the GIS examination. Thus, the project promises to provide some exciting results as well as developing a large amount of data for future research.
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