Activity 3: The Everglades - A Geology All Its Own

Materials

• geologic map of south Florida (in Student Packet),
• geologic column of rock units in south Florida (in Student Packet),
• rock samples (optional),
• fossiliferous limestone (marine),
• freshwater limestone,
• peat,
• sandstone, and
• marl.

Background
Much of south Florida is underlain by limestone. The surface topography reflects the underlying geology. Low areas associated with limestone erosion allow for water to pond, thus supporting the wetlands environments necessary for peat deposition. The sequence of freshwater and marine limestones provide a record of sea-level rise and fall over the geologic history of the Everglades. Also, the Everglades themselves lie in a basin that was most likely an ancient atoll-like structure that formed a lagoon. The higher regions that allowed for the development of the Everglades wetlands are fossil reefs.

The process of limestone deposition in the area began about 5 million years ago. During the last 5 million years, the sequence of marine and freshwater limestones preserved the record of seawater inundation of south Florida. About 5,000 years ago, post-glacial rise in sea level slowed enough to allow the build-up of coastal structures, which impounded freshwater in the lowlands that are not the Everglades. The Everglades have evolved since then as a result of the deposition of peats and marls (mixture of 35-65 percent clay and 65-35 percent calcium carbonate formed under marine or freshwater conditions known as calcitic muds.)

Just as geologists use limestone to determine past conditions in the south Florida region, they also use peat cores to reconstruct the more recent history of the region. Peat is a chemical filter that holds a number of cations and anions. Scientists have used these peat cores to document the build-up of pollutants in the Everglades over the past 100 years.

Procedure

1. Refer each group to the geologic map of south Florida in the Student Packet and provide a copy of the geologic column. If students are unfamiliar with geologic maps, explain that geologic maps show the surface distribution of rock formations in the area. Have students use the geologic column to identify the oldest and youngest formations in the area. Ask them to identify which geologic formations border the Everglades and which formations underlie the Everglades.
2. Ask what kinds of rocks and unconsolidated deposits are represented on the geologic maps. (Show samples of these rocks if you have them.) Do they have any experience with these rock types? Have them speculate on what the difference is between marine and freshwater limestones. How did these rocks, which form underwater, end up on dry land? How did marine limestones, which are deposited in seawater, end up associated with the freshwater Everglades?
3. Use these discussions to introduce the concept of sea-level rise and fall related to the retreat and advance of continental ice sheets.
4. Have students use the geologic map and column to reconstruct the history of the Everglades region by relating marine and freshwater limestones to sea-level rise and fall. Ask them if they can determine the last time the region was under seawater? What has happened to the region since then?
5. Now would be a good time to discuss the use of peat cores in determining the environmental health of the Everglades. See the Discussion of Peat Cores below.
6. Have the students compare the land uses on the watershed map to the geologic units which occur in the area. What conclusions can they draw about distribution of rock types and present day land use?

Discussion of peat cores

USGS scientists are using peat cores to study peat accumulation rates and the change in peat chemistry from 1961 to the present. They have sampled two sites in Water Conservation Area 2. The first area shows a shift from saw-grass to cattail vegetation due to the increased nutrient input it receives from canal water. Peat accumulation near the canal has nearly doubled in this area. The second area, with near-pristine, low- nutrient conditions, shows slower rates of peat accumulation. The differences in these areas are recorded in peat cores as differences in preserved plant material and differences in peat chemistry.