Formed by glaciers, shaped by time Laura Rosensweig Land planner

Sat, 11/04/2000 - 6:00am

Ecological geography describes the environment in which animals and plants live. Block Island's geography begins with its birth as mixed outwash from a glacier, consisting of sand, gravel and clay. These soil materials do not provide barriers to development on an island that is within a half-day's drive of 20 million people. Ten years ago, the year-round population was 836, but a summer's day would find 10,000 persons here, she said.

The island was originally forested, but after settlers cleared land for farms, the island was open land for 300 years. Until the 1950s, it was intensively farmed. In the 1930s, tree planting began in Nathan Mott Park. (In the 1960s and ‘70s, many landowners planted trees — Japanese black pine and others.) Today, much of the land has not been disturbed for years and there are moderate stands of trees and extensive brushland.

Thirty-eight percent of the island (1,000 acres) is legally protected open space.

Rosensweig, who now lives in Switzerland, was an intern with the Block Island Land Trust and a student at the URI Department of Natural Resources Science when she started developing the island's geographic information system (GIS).

John Boothroyd

URI Geologist

Block Island was formed by the deposits from the lower edges of two glaciers, the first one more than 120,000 years ago, the next 22,000 years ago. The first glacier brought rocks and sediment from Narragansett Bay to the north; the second came across Connecticut and southwest Rhode Island from the northwest. Evidence of the grain of this second glacier can be seen in the northwest-southeast orientation of Rodman's Hollow and other valleys on the island.

The two layers of glacial deposits show clearly in the exposed bluff to the left of the steps near the Southeast Lighthouse. The bottom layer, from Narragansett Bay, is fine-grained, dense and hard; the top part is coarser, sandy and gravelly, with some Narragansett Bay material mixed in. A layer of windblown sand lies like icing on top.

As the second glacier retreated, 18,000 to 19,000 years ago, a lake formed between Montauk Point and the south shore of Rhode Island in what is now Block Island Sound. Folds of sediment left by the lake can be seen in the exposed bluffs.

As the land appeared above the retreating water, Block Island was at first two islands, eventually connected by a barrier beach built up by the flow of sand from one island to the next.

The ponds that dot the island were formed by ice blocks which melted, leaving a depression.

Boothroyd opened his talk by acknowledging the contributions that the late Les Serkin, who died in June, made to understanding Block Island's geology.

Anne Veeger,

URI hydrologist

The two different glacial deposits determine whether a homeowner's well water will be rusty or low in iron.

The U.S. Geologic Survey sampled 76 wells around the island and found that those in the eastern part had high concentrations of iron. These wells were drilled into the first glacial deposit that came from Narragansett Bay. The rust comes from iron pyrite in the rocks and sediment from the bay. In this area, shallow wells are recommended because they will stop short of the iron-rich layer.

Material left behind by the glacier also determines the height of the water table. At the bluffs, the water table is 150 feet above sea level; a mile away in Rodman's Hollow, the water table is much lower. Bands of gray deposits in the bluffs act like a basin to prevent rainwater from flowing downward through the soil.

Block Island's aquifer yields 10 million gallons of water a day, all of it naturally stored rainwater. Eighty percent of that water drains into the sea offshore, leaving two million gallons a day to feed springs and the island's relatively few streams. The intimate relationship between this underground water and the ocean surrounding the island can be observed at Sands Pond, where the level of water in town wells responds to the ebb and flow of the ocean's tides.

Colen Peters

Geologist, Duke engineering

During the OPEC oil embargoes of the 1970s, a search began for alternative fuels. On the island, interest developed in a fuel of the past — peat dug from swamps around the island. Peters, then a graduate geology student at URI, and Boothroyd studied peat resources here under a Department of Energy grant.

Island residents turned to peat, a form of vegetable matter, to heat their homes after the island was deforested in the 1720's. Some of the open ponds that exist in the larger swamps on the island are tug-holes — areas where peat was extracted.

These swamps — Ambrose and Fresh, on the east side of the island; Franklin, on the west side; Great, behind the power station — and other wetlands started life after the glacier as ponds formed by ice blocks. The ponds eventually filled with water lilies, then marsh plants such as reed sedge, then moss peat. Peat dug for fuel is 90 per cent water.

Peters' study measured the volume of usable peat remaining in 14 wetlands by taking core samples in a line from north to south and east to west in the wetlands. The study found that the 14 wetlands held enough peat to provide supplemental heat to 100 homes for 64 years.

The core samples also yielded clues to the island's geological history. In Franklin swamp, the sample showed two layers of sediment alternating with layers of peat. This meant that Franklin swamp had been a pond twice, and the plant succession from lilies to reed sedge to moss peat had happened twice. For the future, that means that wetlands can adapt to changing conditions as long as their water source is maintained.