GEOLOGY

The earth beneath our feet has always promoted a feeling of stability, solidity and permanence. If we could embark on a journey back through the ages, a different picture would emerge.

Between 500 and 200 million years before the present (B.P.), the southern Appalachians were formed, born of a complex interaction between the land mass that became the North American continent and the land mass destined to become Africa.

During the next 50 million years, erosional forces weathered the Appalachians to a softer form and part of the soil was carried to a lower level, forming the Piedmont.

By about 150 million years B.P. the two continents separated and the elevated Piedmont itself was now subject to erosional forces. The coastal plain, composed mostly of alluvial sand, began to form. The margins of the coastal plain were composed of the sands destined to provide the building material for the sea islands and barrier beaches of Georgia, and another set of influences now began to have an impact.

As the Atlantic widened with time and formed linkages with other oceans, it became more and more influenced by solar energy and the winds’ unhindered fetch, to become an actively circulating, high energy system. During the Pleistocene Epoch, beginning some 2 million years B.P., another set of factors also began to influence the coastline. A series of ice ages or glaciations occurred, interspersed with interglacial periods. When a major part of the world’s water was locked up in ice sheets, the sea level fell more than 100 meters below its present level. During the interglacials it sometimes rose significantly above its present level. During this period of oscillating sea levels, coastal plain and continental shelf sediments were subjected to much sorting and reworking by the high energy of the ever-widening Atlantic.

There are periods called still stands when the water level is comparatively stable, this level being dependent upon the amount of water locked up in the ice sheets. These still stands are important to the formation of saltmarshes and barrier islands. During a still stand the following processes occur: in the high energy zones sand is deposited as barrier islands, while in the low energy zones behind the islands clay deposition occurs. Marsh develops and more clay is trapped and held in the marshland. Only a small clay input occurs in present times, and very little sand is now carried out to sea by the rivers. Most of the movements we see in marsh muds and beach sands are resuspension and redeposition of materials, which have been in the coastal zone for a very long time.

There is another, and more dynamic scenario applying to the periods between still stands. During glacially controlled, transgressive (rising sea level) or regressive (falling sea level) periods, and also in periods when the earth’s crust in a particular area is being deflected upwards or downwards, there is no longer a stable sea level. This sea level change brings rapid changes to the coastline. The sediments are moved across the shallow continental shelf in response to the rise and fall in sea level. The marsh and barrier beach systems become very unstable, narrowing or widening and sometimes disappearing altogether as their location moves across the gentle slope of the continental shelf.

There is a rather difficult contradiction relating to the present sea level. On an extended time scale of thousands of years, the present islands and marshes are well developed and reflect a still stand for the last five thousand years. However, on a scale of tens of years we know that the local sea level is rising quite rapidly (as much as 3-6 mm per year or 1-2 ft per hundred years). Each of these observations appears to be true, stressing the great complexity of attempting to relate long-term geological changes in the face of the earth to the short-term, sometimes geologically minor, changes so important to human values.

Throughout the later part of the Holocene transgression (5,000 years B.P. until today) sea level has been high enough for sediments carried in the water to be deposited on the now underlying Pleistocene age Sapelo Island sediments to form new saltmarshes and beaches over the old ones. However, even these newer sediments are primarily old sediments reworked. Clays and fine sands make up the marsh deposits which form in the more sheltered areas. These are subjected to scouring and rapid water flows only in the areas of the sounds or tidal creeks and are held and trapped by the saltmarsh vegetation. The beaches are formed on the seaward weather face of the older island from the fine sands of the shallow continental shelf which are brought shorewards by the action of waves and currents that erode and redistribute the bottom materials of the continental shelf. Once deposited on the face of the island, these sands become influenced by wind, waves and longshore currents.

These then are the forces which have shaped Sapelo Island, the center more raised and dating back to the previous high stand during the Pleistocene (somewhat more than 40,000 years B.P.). The sea retreated and Sapelo was left as a mainland ridge. As inundation once more occurred, Sapelo became an island again and the modern Holocene islands of Blackbeard, Cabretta and Nannygoat (3-4,000 years B.P. to present) came into juxtaposition with the Pleistocene island. Low-lying saltmarsh areas formed in the lee of each of the islands. The beaches still present mute testimony to the forces bearing upon them, as sloughs form temporarily in the shifting sand. Dunes build and erode, new beach forms while old beach washes away, occasionally exposing underlying earlier Holocene muds. The processes of island movement are so active that, whereas Cabretta was separated totally from the main island in the 1930’s and could be circumnavigated by a 6-ft draft vessel, nowadays a short bridge closes the visibly narrowed and shallowed gap. Blackbeard is still totally separate and is thought to be representative of features present on Sapelo and Pleistocene times.

Geologists perceive time differently from other scientists and from laymen, and their view of the world can put a different perspective on our surrounding. The “other” scientists look for occurrences which can vary from instantaneous to seasonal and more rarely, yearly. The geologist commonly looks for changes over thousands and/or millions of years, with the result that his view may not hold the fine detail of the other scientists. However, his results are more free from seasonal and annual variations, and show long-term trends in an environment, rather than those fluctuations which may be the result of short-term and finite variations such as temperature, rainfall and industrial stress.

Sapelo is a system which has provided the wherewithal for a group of talented geologists to test their mettle, and they used the skills and information gathered here to develop theories on barrier island formation and origin and sea level variation. This is discussed in the section devoted to the Institute history and Appendix I.