Geographic History

The rocks forming the shoreline of Larchmont’s Manor Park are really part of the Appalachian Mountains, a chain of mountains that developed millions of years before the dinosaurs of Jurassic movie fame. The Appalachian Mountains and the shoreline of Manor Park were formed over 600 million years ago by glaciers during the Paleozoic and Precambrian geological periods.

The melting glaciers left behind the rocks that you see today. All along the Park’s coastline, you will see deposits of sandstone, limestone and shale, but most evident are the deposits of granite. These have been changed and tortured by the earth’s violence millions of years ago by metamorphic actions to form layers of shale, gneiss and schist. Originally the various rocks and minerals were in horizontal layers easily viewed by standing at the edge of the wall at a number of locations along the shoreline (1). These horizontal layers were folded, turned, and pushed together many times. The pressure, heat, and violent crinkling and bending action melted the mineral layers causing thicker folds and the irregularity of the layers suggest that the rocks flowed like candy taffy as a result of the intense pressure and heat. (2)

The Appalachian Mountains show what happens when thick formations of rock are squeezed as part of the earth’s surface change. The rock bends, crumbling into vertical fold that form hills and valleys. There were many geological phases, including both colder and warmer phases. Glaciers first melted at their base, from the heat of the core of the earth. Molten rock lava from deep underground, caused by change by breaking some rock into chips and compressing and smoothing other rocks. Steam from this heat dissolved mineral matter that was folded into cracks and pores. (3)

Metamorphic rocks are rocks that have been changed in appearance and mineral composition. The changes caused by the hot molten lava, pressure and heat from the earth’s movement, and the chemical action of gases and steam cause the minerals to crystallize and form wavy bands. Minerals were transformed into flat plates or arranged in layers. Granite was turned into banded rock known as gneiss. Several types of rocks were turned into the layered shiny material known as schist.

Gneiss is a metamorphic rock made from the same materials as the granite, but in the Park’s rocks the minerals are arranged in layers (4) – quartz (white), feldspar (white or pink), and mica (shiny, seemingly almost transparent). Some metamorphic rocks are uniform in composition and contain minerals that appear in parallel plates and these are called schist.  Some of our schist rocks have hornblende (5) black and garnet (6) red dots in addition to the mica. The schist rocks have flaky particles or fine bands of layers. Schist sometimes feels slippery and may sparkly in the sunlight. Sometimes the schist will split into thin plates. The red dye that we sometimes see on top of the rocks is the mineral iron.

The galactic ice which sometimes towered hundreds of feet high melted at less than an inch a year, and acted like sandpaper. This “sandpaper” (7) caused scratches (those lines on top of the rocks) and holes (8) – look at the rocks behind the north gazebo as they taper down to the water.  But also like sandpaper, rock worked against rock and made some areas smooth. Today, the Sound’s wave action constantly carries small stones that continue to wear away and smooth the rock surfaces.

Even today changes are caused in the rocks by the wind, weather and water, although geologists do not classify the effects of weather as metamorphic changes. Alternating light and dark gray layers show on freshly exposed rocks, while rocks that have been chemically and physically exposed to atmospheric conditions over a period of time turn dark gray or black. Today the loss of our ozone layer continues to change.

Finally, our most well known rock, the dinosaurs egg. It is called a glacial erratic (9). It was left behind at this site by the melting glaciers for children to climb on. Enjoy

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