Millions of years ago melting icecaps covered this area of the Georgia coast with an inland sea. As the climate warmed the sea retreated and vast, fertile, grassy plains emerged. It extended fourteen miles from the coordinates and eventually as the climate continued to change it also continued to push the sea inland. The barrier islands of Georgia have the distinct appearance of being split in a north-south plane with their uplands connected by marsh. The reason for this appearance is that there really are two sets of barrier islands, each formed during distinctly different geological times. The islands making up the western side of the chain were formed about 35,000 to 40,000 years ago, and those to the east date back only 4,000 to 5,000 years. The older islands formed the beaches when the sea level was about 6 feet above the present level -before the formation of the fourth and last great continental ice sheet of the Pleistocene epoch. Enough sea water was frozen into that last big freeze to lower the sea level 300 to 500 feet, placing the shoreline near the present edge of the continental shelf, approximately 80 miles offshore. About 18,000 years ago, the ice sheets began to melt and the sea level rose rapidly until 4,000 to 5,000 years ago, when the rate of ascent diminished to 4 to 6 inches each century. Along with the rise in sea level, newly formed barrier islands (Holocene islands) began migrating westward toward the older islands (Pleistocene islands). The westward migration of these islands occurred and still is occurring as the advancing seas continue to erode the eastern-facing beaches and redeposit the sediments into the marshes and lagoons behind the islands. The newer Holocene islands over time have become variously juxtaposed to their Pleistocene counterparts. The islands directly south of the Savannah River (Tybee, Wassaw, and the north end of Ossabaw) and those south of the Altamaha River (Little St. Simons and Sea Island) are more separated from their Pleistocene counterparts than the other Holocene islands. The inward advancement of these islands has been impeded by the far more copious output of sediments by the Savannah and the Altamaha Rivers relative to the other smaller rivers. Where the smaller rivers produced less sediment, the Holocene islands have migrated closer and, in some cases, have become attached to the older islands. Today sea level continues to rise in this area, but at an increased rate of 12 to 14 inches per century. Through the action of prevailing winds, waves, and tidal currents, dynamic changes in the shape and size of barrier islands occur constantly, especially at the ends of the islands where they come in contact with inlets or narrow bodies of water between the islands. Generally, the southern ends of the islands tend to accrete (build up by deposition), while growth on the northern ends is irregular and often interspersed with erosion. The north ends of the barrier islands show complicated patterns of growth and erosion which vary from island to island. Large reservoirs of sand in the form of shoals often are seen at the mouths of the inlets, resulting from complex interactions between tidal and longshore currents. Through the action of the southerly-directed longshore currents and wave refraction, the shoals tend to drift downward and inward toward the upper parts of the islands south of the inlets. The frequent incorporation of inlet shoals to the north gives the Georgia barrier islands their characteristic drumstick shape, with the upper ends thicker. Occasionally, a shoal near a barrier Island becomes stable enough to support vegetation, and acts as a wave shelter to the beach. Again, suspended sediments tend to fall out in the quieter waters between the shoal and the beach, which can eventually form a marsh connecting the newly-made beach front onto the older shore. Any number of shoals may likewise become attached to an island, creating a corduroy pattern of old dune ridges interspersed with marshes and lowlands. Such areas are often eroded back by storms only to re-form over time.
To log this Earth Cache as a find you will need to email me the answers to the following questions. Your answers will need to be quotes from the sign at the cooridinates posted above.
1. During what era did the sea retreat forming the vast, fertile, grassy plains?
2. When the sea continued to push inland approximately 10,000 years ago, what formed, how many formed?
3. What under water Geologic feature is responsible for causing a 6-9 foot change between low and high tides here, while elsewhere along the Atlantic sea board the changes are typically 2-3 feet?
4. What does the TTT in the title mean?
5. To get extra credit and appreciation from the cache owner please post pictures of your group or scenic pictures of the area. This is not a requirement for logging a smiley.
I hope you enjoy the local history and the Geology lesson. Who says a hobby can't be informative and fun!
Remember, it is all about the miles and the smiles!
