SANKATYS GEOLOGICAL SIGNIFICANCE Glacial Flour: A distinctive white layer of these fossils is clearly visible from the beach, and shell fragments, freed by wind erosion, tumble down the bluff face onto the beach below in a steady stream of fine, powdery sand known as glacial flour. Sankaty’s natural history from Chapter 1, A Laurentide Junk Heap, by our own Peter Brace. A little technical, but the uniqueness of the eroding bluff to our islands environment is readily apparent. How is that it that the beach it has been continually creating for millennia can now be engineered into a wasteland, as one dismayed islander has asked? Here is more from Peters book: You can peel back the layers of Nantucket’s sandy onion and learn about her past at the exposed bluff below Sankaty Head Lighthouse, called a “marine scarp” by geologists. Here, three Pleistocene Epoch stages are visible. Two glacial stages sandwich an interglacial stage of sediments in a section of the bluff beneath Sankaty Head Lighthouse. The oldest, the Illinoian Glacial Stage, occurred 125,000 to 350,000 years ago; it displays glacial drift materials and is believed to lie lowest down, closest to the beach. Atop that are layers of sediment that were once part of a seabed of an ocean that existed during the Sangamonian Interglacial Stage, 75,000 to 125,000 years ago. These layers contain fossils of mollusks and Ostracodes, tiny crustaceans commonly called seed shrimp, which are 0.75 to 1.2 inches long and resided on the surface or just beneath the ocean bottom in this area during the Sangamonian Interglacial Stage. A distinctive white layer of these fossils is clearly visible from the beach, and shell fragments, freed by wind erosion, tumble down the bluff face onto the beach below in a steady stream of fine, powdery sand known as glacial flour. Geologist Robert Oldale says he also found there the intact shells of mollusks, including soft-shell clams, quahogs, oysters, mussels, whelks and snails. And atop the Sangamonian layers is the glacial till laid down by the last glacier during the Wisconsinan Glacial Stage, between 10,000 and 21,000 years ago. But these layers of sediment and till are not resting in their original state; the glacier pushed them into standing ripples and wrinkles of layers of glacial drift and till. They resemble a rug being pushed forward onto itself, with each tall wrinkle falling onto the next, scraped from the land surface and gouged out of the ground to be thrust upwards and either on top of or beneath the other exposed layers. The entire bluff runs about two miles from just south of Sesachacha Pond to the village of ’Sconset, measuring several feet tall just south of Sesachacha Pond to around 100 feet at Sankaty. It came into being, and had these layers exposed, as the surging fingers of the Cape Cod Bay and Great South Channel lobes of the glacier advanced southward side by side. When the glacier receded, the material left behind between the two lobes — including, in the vicinity of the lighthouse, a giant erratic, called an interlobal moraine — became the marine scarp that is Sankaty Bluff. Although a peak once existed on this interlobal moraine after the two glacier lobes melted away, the ocean and wind have long since eaten it westward from the east so that today, Sankaty Head stands on the lower elevation of the western slope of this interlobal moraine. NOTE: The material above is an excerpt from a book written by Nantucket writer Peter B. Brace: “Nantucket: A Natural History”. It is printed here by courtesy of the Mill Hill Press, 4 Winter St., Nantucket, MA. 02554, which retains all rights to that book. Mill Hill Press, an affiliate of the Egan Maritime Foundation, publishes books that explore and record the art, history, literature and traditions of Nantucket. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval system, without the written permission of the author’s agent and the publisher, except where permitted by law.
Posted on: Mon, 03 Feb 2014 11:54:49 +0000
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