Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: SUNDAY, January 19, 1992 TAG: 9201190226
SECTION: HORIZON PAGE: B-5 EDITION: METRO
SOURCE: GEORGE & ROSALIE LEPOSKY
DATELINE: LENGTH: Long
FLORIDA'S PETRIFIED FOREST IS A NATURAL WONDER
Two great blue herons, slender silhouettes against the rising sun, stalked breakfast in the shallows. Fiddler crabs bustled about the base of a rocky reef. Probing a finger-sized hole in the honeycombed brown stone, we extricated and marveled at a small snail with black and white bands on its shell. A breeze sighed through nearby Australian pines, the sea whispered wordlessly, a gull's raucous laugh rang across the water. The magic of the moment vibrated our souls.We went early that morning to the northern tip of Key Biscayne, a barrier island less than five miles as the crow flies from downtown Miami, to explore South Florida's petrified forest. This unique geologic structure is a vestige of a vast black mangrove swamp which once sprawled across the key's northern and eastern margins. A few gnarled old black mangrove trees cling precariously to life on the beach beside the reef. However, the swamp in its prime - about the time Jesus was born - covered an area at least 1,000 feet wide and three miles long.
The late marine geologist John Edward Hoffmeister identified the rock reef as a network of fossilized black mangrove roots. In 1965, as a professor at the University of Miami, he wrote a scientific treatise which meticulously compares the rocks' latticework structure to the root system of a living black mangrove tree.
Like the spokes of a wheel, lateral roots spread horizontally outward from a black mangrove's trunk at or somewhat below the high tide level. Geotropic roots reach downward from the lateral roots to anchor the tree and absorb nutrients. Extending upward, breathing tubes called pneumatophores protrude above soil and water like a diver's snorkel to keep the tree from drowning when high tide submerges the rest of its root system.
Where wave action had cut open a cross-section of the rock, we could see clearly this pattern of vertical rods ascending and descending from horizontal rods - all preserved in stone.
What happened to the swamp remains a mystery. A cataclysmic hurricane may have obliterated it in a single blow, or perhaps shifting ocean currents slowly eroded it.
In any case, this reef is the only known example of petrified mangroves. The heat, humidity and abundant bacteria in a mangrove swamp typically promote rapid decay, precluding fossilization. But after the trees in the Key Biscayne swamp were destroyed, something atypical happened.
Hoffmeister believed the roots decomposed slowly. They released carbon dioxide which combined with water to form carbonic acid. The acid dissolved calcite in the sand around the roots, producing calcium carbonate which in turn somehow replaced the woody tissues. How? Hoffmeister thought bacteria played a role in the replacement process.
Whatever happened took place on a grand scale. More than two miles from where we go to explore the reef, Hurricane Betsy in 1965 uncovered patches of the petrified forest along the hotel row south of Crandon Park. The rock formation extends inland for an indeterminate distance, buried beneath the beach. Seaward, it's submerged in the shallows where Bear Cut opens to the sea, with points jutting above the water several hundred feet offshore at extreme low tide. Because the ocean has nibbled away at the seaward margin of the reef for centuries - a process which continues today - the full extent of the petrified forest will never be known.
The petrified forest differs greatly from the sand beaches, grass flats and live mangrove swamps along most of South Florida's coastline. Thus, it attracts many kinds of marine animals rarely found elsewhere in Miami waters.
Plant life on the reef consists mainly of leaves and sprigs washed up by the sea. We recognize two common local forms: the tapered, floating pods which house baby red mangrove trees, and the long, smooth green leaves of turtle grass.
We also find clumps of vegetation with scalloped, ruffled leaves and stems bearing hollow green balls. This is sargassum weed, a species of brown algae which drifts in huge masses south of Bermuda in a region of the Atlantic Ocean known as the Sargasso Sea. The nodules on the stems are air sacs which help to keep the plant afloat. Many animals living in the Sargasso Sea survive by mimicking the color and, in some cases, even the shape of the weed. Among the creatures thus camouflaged are shrimp, crabs and the bizarre-looking sargassum fish.
In the decade we've been coming here, the grass flats offshore have flourished and spread across a once-sandy bottom, increasing the quantity and quality of the habitat for fish and the smaller creatures on which they prey. Snorkelers in the reef area can see many brightly-colored tropical fish, including grunts, porkfish, sergeant majors, surgeonfish, and several types of angelfish and wrasses. Anemone colonies with stubby green tentacles cling to rocks just barely awash at low tide. On various visits we've encountered small barracudas, a young nurse shark, and an octopus.
Because it's in a public park, the petrified forest and all plant and animal life around it are protected by law. Even empty seashells should not be removed, says Jim King, a naturalist with the Dade County Park and Recreation Department. "Nature recycles some empty shells as homes for hermit crabs and other animals. Eventually they break up, replenishing the beach sand and returning calcium to the ocean."
King also recommends that snorkelers and swimmers enter the water in the reef area only at low tide, and always with a partner. "The currents are dangerous and unpredictable," he warns.
Be cautious, too, of your impact on the petrified forest itself. It's a very fragile structure. "Some damage has been done already by general foot traffic," notes King. "We don't encourage large groups to go there for fear their weight will break the fossilized roots. Individual visitation is OK as long as people tread lightly and try not to molest the reef."
If you visit the petrified forest, time your excursion for the cool morning hours on a day when low tide occurs around sunrise. Take a magnifying glass along to examine many minute life forms not readily visible to the naked eye.
The get there, take the Rickenbacker Causeway (toll $1) to Crandon Park (parking $2) and leave your car at the north end of the beach lot near the Marjory Stoneman Douglas Biscayne Nature Center, a portable classroom where the Dade County school system offers environmental education programs. Walk about a mile north along the beach past an area frequented by gays and nude sunbathers, until the sand gives way to rock.
There, on the edge of a place like no other on Earth, a magic moment surely awaits you, too.
George and Rosalie Leposky are a husband-wife team of widely published travel writers and live in Miami. He teaches college writing classes; she has taught cooking. Ampersand Communications