Scientists now believe that the buried cypress grove unearthed last winter during excavations for the Ravens football stadium in downtown Baltimore flourished 34,000 years ago, at a time when mastodon foraged in Maryland's woods.
Maryland's climate was cooling rapidly then, in advance of the last Ice Age, and the deepening cold may have killed off the warm-weather cypress, geologists said.
The date was established by radiocarbon tests, and reinforced by a careful study of pollen found in clay samples recovered by the Maryland Geological Survey.
The pollen findings reported last week revealed a change in the mix of trees in the forest. That is consistent with the cooling geologists believe was under way in North America 34,000 years ago. The continent was in transition between the height of the last warm interglacial period, about 125,000 years ago, and the coldest depths of the last Ice Age, 18,000 to 20,000 years ago.
"It was a very dynamic period," punctuated by short periods of warming and cooling, said Dr. Thomas Cronin, a research geologist with the U.S. Geological Survey (USGS). "The climate in the North America region and Europe was turning on a dime every 2,000 to 3,000 years."
Workers uncovered the 50-yard-long cypress deposit during site preparations for the stadium. It contained huge logs, roots and stumps sealed and preserved in gray clay about 25 feet below street level.
The deposit now lies beneath the rising grandstands, under tons of concrete.
The discovery recalled similar cypress deposits found during 1922 excavations for the Mayflower Hotel in Washington, and stumps dredged from Baltimore Harbor in 1905. In those days, scientists estimated the wood at both sites to be 100,000 years old.
This time, geologists brought new scientific tools to the question -- radiocarbon dating and studies of microscopic pollen preserved in the clay.
Radio carbon dating works because the proportion of carbon 14 -- a natural, but unstable isotope of carbon -- in living tissue is fixed at the time of death. The isotope then slowly decays at a known rate into a stable form of carbon. After about 40,000 years, all the carbon 14 is gone.
By measuring the ratio of carbon 14 remaining in plant or animal material, scientists can determine approximately when it died -- provided it falls within the last 40,000 years.
Carbon dating of the stadium cypress, performed last spring by Geochron Laboratories, of Cambridge, Mass., produced an estimated age of 34,130 years -- with an uncertainty range from 30,340 to 41,530 years.
But as useful as it is, carbon 14 dating can be tricky. Contamination by even a tiny bit of modern carbon -- such as mold growing on a specimen -- can skew a date by thousands of years.
As a check, the USGS was asked to conduct pollen studies on the clays in which the wood was found.
Pollen is the male element produced by parts on flowers called anthers. Pollen is to flowers what sperm is to animals. The yellow dust that accumulates on cars in the spring in Maryland is largely oak pollen. Where conditions preserve it, it can still be identified after millions of years.
Pollen experts carefully wash a sample of dirt or clay and "float" the pollen it contains. Collected under a microscope, the grains are identified and counted. That reveals the mix of flowering plants that lived at the time the soil was laid down.
Dr. Debra Willard, a pollen expert at the USGS in Reston, Va., reported her findings on the stadium clays last week.
Dr. Randy Kerhin, of the Maryland Geological Survey, said pollen extracted from the deeper, old- er portion of the gray clay reflects a forest that was 27 percent cypress and 3 percent hemlock. But higher in the clay layer, in younger deposits, the cypress had diminished to 20 percent, while the hemlock had increased to 17 percent.
"It's very interesting that you would find a mix of cypress and hemlock, which don't go together," said Dr. James P. Reger, chief of environmental geology and mineral resources at the Maryland Geological Survey.
Cypress is a warm-weather tree now found from Southern Maryland to Florida. But the hemlock densities correspond with the cooler climate today in upstate New York.
"It's a neat package," Reger said. "It's not just a cypress swamp. We're actually looking at a change in conditions at that location. I think what we're seeing here is a cypress swamp that ceased to be with the change of climate."
Geologists found a rich, black layer of organic muck, deposited after the gray clay in which the cypress was found. Pollens from that later period indicate the cypress had disappeared. In its place was a cooler forest dominated by pine, oak and hickory, with plenty of asters, daisies and shrubs typical of an open forest floor.
The pollen results made Reger "fairly comfortable" with the radiocarbon date. But he remained a bit puzzled by the presence of a cypress grove in Baltimore at that time.
Reger said modern cypress groves in Maryland and Delaware are all within 20 feet of sea level.
But 34,000 years ago, the sea level was falling toward the lows reached 18,000 years ago, during the last Ice Age, when much of the planet's water was locked up in ice and snow.
The stadium cypress would have been growing 160 to 200 feet above the sea level of that time. For Reger, "that's still sort of problematic."
Plenty of wildlife would have been in the cypress groves and hemlock stands along the Patapsco River valley 34,000 years ago. Among the biggest would have been mastodon -- a primitive form of elephant -- and ground sloths the size of grizzly bears, said Dr. Thomas Holtz, a vertebrate paleontologist at the University of Maryland.
Elsewhere on the continent, if not actually in Maryland, he said, there were camels, horses, saber-toothed cats, a North American lion, and vultures with 15-foot wingspans. All would disappear.
"The changes that have happened since then are almost all extinctions, not the appearance of new forms," Holtz said.
Humans would not find their way from Asia to North America for at least another 20,000 years -- near the end of the last Ice Age.
By the time Europeans arrived in Maryland, the surviving fauna were smaller and more familiar. They included puma, bobcats, black bear, wolves, elk, bison and other species that would eventually be chased out, killed off or reduced in numbers.
Pub Date: 10/04/97