Unearthing secrets at Menemsha Hills
Those of us lucky enough to get a seat on the bus for the latest Trustees of the Reservations (TTOR) winter walk last Sunday were taken back 100 million years to the north shore in Chilmark at the Trustees Menemsha Hills property. We found bits of fossilized wood, saw the remains of a 100,000-year-old lake bed, dipped our hands into 65-million-year-old marine clay, and held slabs of oxidized iron that has been forming for 12,000 years.
Sarah Trudel, education coordinator for TTOR, was filled with excitement when we met at the TTOR building on State Road in Vineyard Haven for a short talk and slide presentation before we hopped on the TTOR bus to head for Menemsha Hills.
Ms. Trudel described how TTOR ecologist Greg Whitmore was exploring in the north shore area six months ago and found the ore and what appeared to be an old lake bed. He contacted the Unites States Geological Survey (USGS) and a team of researchers came down to collect samples, which they took back to their labs for testing. Through an electron microscopy dating process, they were able to establish a timeline for some of the materials that make up the cliffs. Ms. Trudel took us back in geological time to when the Laurentide ice sheet moved down from Canada, scraping up very old material on the way. When the glacier stopped along the north shore of the Vineyard and Nantucket and began to melt, it left debris in its wake. As the cliffs have gradually eroded over the millennia, increasingly older material has been exposed. Ms. Trudel had samples of recently collected iron ore and lignite, the coal-like fossilized wood, for us to inspect, but we were anxious to begin our walk and make our own discoveries.
Ms. Trudel deftly drove the bus along North Road, ignoring the occasional hand waver who mistook our bus for the Vineyard Transit Authority, until we arrived at our Menemsha Hills Reservation destination. We were fortunate the rain had passed through the day before, leaving us with crystal clear blue skies, and temperatures hovering near 40 degrees. Even the wind, blowing out of the northeast, died down shortly after we made our descent onto the rock-strewn beach. But before reaching the beach we took in the view from the overlook that sits on top of the Great Sand Bank where we could see for miles up and down the north shore, and across Vineyard Sound to the Elizabeth Islands.
The reason for our adventure was geology, and Ms. Trudel brought our attention to the material we were standing on - sand. The 100-foot cliff consists of sand and debris left by the retreating Laurentide ice sheet, which reached 1,500 feet in height along the north shore, and millions of years wind blown sand and dirt creating the unusual sandplain formation.
Once on the beach, we began our exploration for chunks of iron, fossilized wood, the various colored clays, and the newly discovered 100,000-year-old lake bed. The going was slow along the rocky beach where we had to watch each step so there was little on the beach that escaped our attention. Ms. Trudel was always one step ahead, eager to point out naturally occurring ore deposits.
The ore formed in areas where there were layers of marine clay, covered by sand. Rains passed through the porous sand and became trapped on top of the clay. This iron has been forming for 12,000 years, and is still forming today. Because of erosion, there are numerous cites where layered ore deposits are visible. Iron was mined along these shores in the early part of the 20th century after the nearby brickworks operation closed down, according to Ms. Trudel. Kaolin, the white, powdery clay plentiful along these cliffs, was also mined and sent off-Island, along with the iron, for processing. Kaolin is now used primarily in ceramics and paint, but it used to be added to some candies, like bon bons, before it was discovered to contain slight traces of radioactive materiel.
Continuing toward the lake bed we found areas where the cliffs were oozing with 130-million-year-old dark red clay, and some even darker areas that Ms. Trudel said came from 100-thousand-year-old swamps. This is where we encountered the lignite, or fossilized wood. If we didn't mind scooping into the ooze, we could pick up little bits of this black, charcoal-like substance that easily crumbled in our hands.
As exciting as our discoveries were, it was tiring walking over all the rocks and boulders, but Ms. Trudel urged us on, saying "It's just around the next bend in the shoreline." Well, after the third time she was right, and there rose up a wall of extremely fine white sand that dates back 100 thousand years to the Pleistocene era. This newer, or younger material had been hidden beneath layers of the much older clay. According to Ms. Trudel the lake was formed by the retreating glacier, yet over eons of time and rising sea levels, it disappeared, leaving only this newly discovered clue to its existence.
Having reached our final destination, we were ready to head back to the bus. We opted for a speedier return along a track that passes by the old brickworks chimney rather than retrace our rocky footsteps along the shore. It was an enjoyable journey back in time.
On March 30, Ms. Trudel will be leading a walk on to Squibnocket when she will discuss how TTOR protects property through conservation restrictions.