Field Trip: The Florida Keys
In the early spring of 2010, St. Norbert College geology was feeling the chill, and decided to spend a week in the sub-tropical Florida Keys. A hard rock geologist's nightmare, the Florida keys are a fascinating and unique sedimentary environment. Essentially untouched by plate tectonics for hundreds of millions of years, sand, mud, and limestone dominate the the geology of southern Florida. Over the course of the trip we explored an abandoned quarry, kayaked in the Everglades, and snorkled on a coral reef to get a hands-on feel of the geology and ecology of the Keys.
The Everglades is a very unique and fragile ecosystem. The Everglades, it is said, “are the only Everglades in the world.” Nostalgically referred to as a “river of grass,” the Florida Everglades is a hundred-mile long swamp. Every spring, Lake Okeechobee in central Florida overflows on its southern bank. Over a period of years, this water makes its leisurely way through miles and miles of flat swampland. It exits into Florida Bay, a very shallow bay on Florida’s western side. Because the Everglades have such little relief, changes of only a foot in elevation can have great implications in terms of soil moisture and hence in what plants will grow. The mounds of trees seen in this photo represent an elevation of only two feet above the swamp. The Everglades sits on flat limestone bedrock, which is exposed in some areas. The soil of the everglades is notoriously poor.
Of course, given the unique geology of the Everglades, the Everglades also boasts some unique biology. Despite how nutrient-poor Everglades water is, this water can support an amazing array of life. In this artificial slough dozens of 'gators are sunning themselves. The Everglades are also home to crocodiles, spoonbills, pelicans, herons, egrets, and a wide variety of other animals, including a few panthers. Anhinga birds, lacking oil on their wings, climb onto a rock or log and sun their feathers after every dive for fish.
Biology exists in the context of geology. If it weren’t for southern Florida’s flat sedimentary character, the Everglades would not be possible. The Everglades are a study in the ecological feedback between the world of rocks and the world of plants and animals.
Kayaking in Florida Bay. Florida Bay is a shallow bay, at most 12 feet deep. It is home to a variety of birds, including ospreys, as well as sharks, dolphins, and manatees. Warm shallow seas have very different geochemistry than open oceans. Seagrasses with calcite shells produce the calcitic sediment which characterizes these bays. Warm shallow seas like this used to cover America’s midland, and are responsible for thick sequences of carbonate rock so important to Midwestern geology.
Windley Key State Park
At Windley Key State Park, an abandoned limestone quarry, we saw fossilized coral. The Keys are built upon ancient coral reefs. Coral needs very specific conditions to grow, including warm, shallow, clear water. As water levels rose and fell during Pleistocene glaciation, so did the position of Florida’s coral reefs. Fossils of many varieties of coral can be seen in these rocks. As modern biology informs us, each kind of coral tells a specific story about geologic setting. A sedimentary geologist must interpret these fossils to tell the story of the rocks.
John Pennecamp State Park
While in Florida, we snorkled at John Pennecamp State Park. As James Hutton famously said, “the present is the key to the past.” To understand the history of the Florida keys, one must understand the natural history of coral, and to do that why not examine modern coral in the wild? Coral interacts with the environment today in much the same way as it did 18,000 years ago. In understanding fossilized reefs, an understanding of modern reefs is crucial. So, we use this...
...To explain this! In particular, this is a close-up of the intricate folds on the surface of a fossilized brain coral.
Miami's Hollywood Beach
On our last day in Florida, we visited Miami’s Hollywood beach. The geology of Florida is the geology of sedimentation, and nothing is more symbolic of sedimentation than a quartz-sand beach. The mineral quartz is highly resistant to chemical weathering, so extremely mature sands are close to 100% quartz.
Beaches are great places to examine environmental energetics. Larger sand grains and shells were accumulated around the bases of trees. When storms raise water levels high enough to cover the beach, the tree serves to locally slow the waters. As the water slows, the largest material drops out first. We collected two samples of beach sand and brought them back to De Pere to run through a rototap machine. A rototap is a machine which separates sediment into size categories, as a means of classification.
Don't worry, though. It wasn't all work for us down by the beach.
After that we returned to St. Norbert. Florida was an unusual field trip for us. Not many of our field trips focus on sedimentary processes. Nonetheless, Florida provides an excellent case study of sedimentary environments and their interaction with the biological world.