Lettuce Lake EarthCache EarthCache

A great place to look at geology is in parks, because even in urban areas parklands have not been developed extensively, and you can see relatively undisturbed rocks, soils and landforms, as well as the way the native plants and animals interact with the land and its geology.

Lettuce Lake Park is a Hillsborough County Park. Here you can see fine examples of cypress forests and wetlands typical of inland Florida, a wide variety of wading birds and birds of prey, and the occasional alligator. And if you look a bit closer, you can see how geology plays into the lay of the land and the living things in the park. The site is open from 8:00 A.M. to 6:00 P.M. in the winter and closes at 8:00 P.M. in the summer dawn to dusk.

This Earthcache and the tour around Lettuce Lake Park should take you no more than an hour to complete. There is now a fee to enter Lettuce Lake Park. I believe the costs is $2.00 per car. Once inside the park follow the signs for the Boardwalk.

Florida’s dynamic karst topography is found in few places around the world. Over millions of years, a unique recipe of limestone, confining beds, organic matter, and moving water has sculpted Florida's seemingly flat terrain into a geologic piece of karstic artwork that includes complex networks of interconnected caverns and caves, drainage basins, disappearing rivers, flowing springs, collapsing sinkholes, circular lakes, and subsurface aquifers. Erosion is the basis of karst development. Florida's limestone bedrock is continuously dissolved by moving water on the surface and underground. The thick layers of limestone and dolomite rocks that underlie Florida are easily dissolved by weak acid that naturally occurs in rainfall. The water dissolves the rock to form openings through which water readily flows. As the rock dissolves, it can cause the surface area above it to collapse, creating a sinkhole. Karst areas are characterized by an abundance of sinkholes, springs and caverns.

Isolated depressional wetlands are also common features in karst landscapes. Dissolution of underlying limestone causes a slumping of the land surface, which creates distinct basins which may or may not be connected to surface water or ground water. Wetlands that form in depressions in karst topography are commonly referred to as sinkhole wetlands.

Boardwalk: Lettuce Lake Park features a raised pathway or Boardwalk that allows you to walk out into the swamps and look around without wetting your feet. Note the vegetation as you walk in from the parking lot: oak and pine trees, with thick palmetto undergrowth, changes to cypress trees with ferns and grasses beneath. This falls into either the Pine Flatwoods or Oak Hammock uplands botanical association. Both the oak and pine & palmetto areas indicate land that stays continuously dry. The cypress-fern flora indicates land which spends at least part of the time submerged in the swamp.

Step away from the path which is comprised of sand and crushed shells, before reaching the boardwalk, and examine the soil at the edge of the cypress-fern zone.

Question 1: What is the physical makeup of this soil? How big are the grains, and what are the grains (i.e., are they minerals, shell fragments, etc.)? How much organic matter is there?

Continue down the path and up onto the boardwalk. Note that you quickly leave dry land and start to cross submerged areas. In wintertime the ground is dry almost out to the edge of the trees. The long woody knobs sticking up out of the water along the boardwalk are called Cypress Knees. The rationale for the knees is not well understood. However, cypress trees, like other plants, need a mixture of water and air around their roots; when the roots are submerged, the knees may serve to absorb oxygen, like snorkels. When you cut them off, as was often done for the tourist trade some thirty years ago, the tree dies. The tops of the knees are typically just above the highest high water that the trees have experienced; the older the tree, the taller the knees, simply because they've had more time to see bigger floods.

Question 2: Based on the tallest cypress knees you can see along the boardwalk, near the observation tower, how much higher has the water here gotten than it is now?

Continue along the boardwalk until you reach the observation tower. Lettuce Lake is basically just a wide spot in the Hillsborough River. It is named for the swamp lettuce that grows in great clumps on its surface. Rivers like the Hillsborough River, which flow all year, are called Perennial or Effluent streams. Such streams are a surface expression of the water table-- the level of the groundwater in the rock and soil near the surface.

Question 3: Given the height of the average cypress knees, not the real big ones, and the fact that all the trees you see are above water at some point during the winter, how much do groundwater levels typically vary around Lettuce Lake Park?

Question 4: How did this "wide spot" in the river come to be? What are the geologic processes, typical of Florida, that might be able to create a large depression through which a river could flow?

Return to the parking lot and follow signs to the Visitor Center. There you will find a map of Lettuce Lake Park outlining its prominent features. Note three or four light green spots on the map: these are Cypress Domes, stands of cypress trees unconnected to the main swamp.

From the Visitor Center, either walk or drive to the other side of the park, following signs for the Parcourse. Follow the parcourse path until you reach the path into the cypress dome. Cypress domes are so called because of the profile effect of the tall cypress trees rising above the surrounding oaks like a green dome. Walk into the cypress dome and follow the boardwalk to the end. Note that you are walking downhill from the parcourse path. Notice the change in ground vegetation, from palmetto to ferns. Cypress trees require considerably more water than pines or oaks; thus their occurrence along Lettuce Lake. Their existence in the dome implies the water table is closer to the surface here, a reasonable conclusion given that you walked downhill into the dome.

Question 5: Why did you walk downhill into the cypress dome, and why can cypress trees grow here, in isolation from the others along the river? Or, to state the question more directly, what natural phenomenon results in small, circular areas of lowered ground levels such that the low ground might be very near, or even below, the water table?

To claim credit for this Earthcache you must email your answers to the five questions you answered on your tour of Lettuce Lake to the email address on my profile page, As of Jan 1, 2011 pictures can no longer be required as part of an EarthCache. However, pictures would be appreciated. DO NOT post your answers in your log, even if encrypted.

Optional Pictures: Post a photo of you and your GPSr with Lettuce Lake in the background and a photo of you and your GPSr in the cypress dome with the cypress trees in the background.

Sources: South West Florida Water Management District (SWFWMD), Hillsborough County Regional Parks, Professor Len Vacher, University of South Florida, Department of Geology