A history of the Great Swamp
Editor’s note: The following is the first in a series on the history of the Great Swamp.
Put ingredients in a large bowl and mix. Press flat into bottom of bowl. Pour salt water to cover, then let sit until the sediments settle to bottom of bowl and water evaporates. Press flat again. Pull dough out, fold and knead. Cook at a high temperature. Sound like a holiday recipe? It’s just part of the recipe Mother Nature used to make the Great Swamp!
The Great Swamp, a 6,000-acre wetland at the eastern edge of the Hudson Highlands and southern end of the Taconic Mountains, has a unique history. One of the largest freshwater wetlands in New York state, it drains about 63,000 acres. Its presence and resources have molded the people that moved along its shores through the centuries. But what molded the swamp?
Have you ever found a seashell in the woods? Geologists have discovered evidence of great crustal changes that have taken place over billions of years, shaping and reshaping the surface of the earth. As the crust moved, continents moved about, ancient oceans were created only to vanish again, leaving behind fossil seashells in places now far from any sea.
In what is now the Great Swamp, layers of sand, silt, mud and marine carbonates deposited in the ancient seas were compressed and cemented into sedimentary rocks — shale, sandstone and limestone. Over a billion years ago, tremendous heat and pressure from a continental collision folded up the sedimentary rocks and changed them into tough, dense metamorphic rocks — marble, schist, quartzite and gneiss — making the mountains of the Hudson Highlands and the Adirondacks. The tough, resilient gneiss with swirling bands of white quartzite are visible in the eroded roots of the ancient mountains surrounding the Great Swamp.
Another collision about 450 million years ago created the Taconic Mountains that were pushed right up against the eastern end of the Highlands. These two events created the long north-south valley that contains the Great Swamp.
The softer marble bedrock remained on the valley floor and became the bottom of the Great Swamp. As the water there today flows through and over the marble, its chemical makeup becomes more basic. Areas in the Great Swamp support rare and endangered plants that require very low acidity and low nutrient levels.
The last geologic event to shape the valley was the advance and retreat of the Wisconsin lobe of the Continental ice sheet. The ice, a mile thick at the Canadian border, moved southward at the rate of about a foot per day for 10,000 years. It rounded out the valleys and ground down the hilltops. It plucked up huge boulders and pushed piles of sediment that it would leave behind when it melted. When the ice sheet retreated from southern New York beginning about 12,000 years ago, the land resembled the Arctic tundra. Large chunks of ice and piles of sediment blocked the valleys, collecting the melt water in long shallow lakes. Clay beds in the Great Swamp tell us that for a period of time it was a post-glacial lake whose still waters allowed the fine clay silt to settle out.
As the ice melted, it began to flow toward the sea. A rock ridge near what is now Pawling sent the waters of the Great Swamp draining in two directions — north through a series of riverbeds to Long Island Sound and south through a channel of the Croton River to the Hudson.
Ice-free corridors along the ridgelines and the shrubby vegetation attracted herds of Ice Age mammals, such as wooly mammoths, mastodonts and musk ox. Human hunters followed, and historians have found the spear points designed to hunt the huge beasts.
As the climate continued to warm over thousands of years, arctic plants gave way to mast-producing trees, the deciduous forests of the Northeast. With new food sources came new participants in the food chain, from mammals to reptiles and amphibians. The Native Americans were not far behind.
Jill Eisenstein is a freelance writer, environmental educator, and board member of Friends of the Great Swamp. Judy Kelley-Moberg is a retired earth science teacher. She serves on the Board of the Patterson Historical Society and as Education Committee chairwoman on the Board of Friends of the Great Swamp. Contact them at firstname.lastname@example.org