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|May 05, 2020 Post 116|
This is another excerpt from a work in progress on the Natural History of Lake Ontario to be published early next year by History Press. For more on this project and other lake tales plus a great lake video for rainy day watching visit my website at susanpgateley.com
The lake’s south shore beaches with their colorful variety of rocks from are a pebble picker’s delight. The glacier as it moved roughly southwest picked up rocks from different locations as it traveled. When it melted away the rocks remained behind dumped together in multicultural mix. Ancient metamorphic gniess from the Thousand Islands area, granite and dark sparkly horneblende schist from the Adirondacks, and Ordovician limestone from the Canadian side of the lake, all eventually eroded out to lie side by side on the beach. They and other sedimentary and metamorphic rocks create a delightful mosaic of colors textures and shapes.
Around Sackets Harbor and Chaumont as well as on the Canadian shore around Kingston and Prince Edward County gray limestone pebbles predominate. Most of the beach pebbles on New York’s south shore are of either sedimentary sandstone or various colors of limestone or are metamorphic gneiss or schists. Sometimes the keen eyed beach comber may also spot a few rocks of man made origins. You’ll occasionally encounter water rounded bits of concrete or brick that recall the fate of a cottage or some other man made structure that once stood on an eroding shore.
I’ve found odd water rounded “pumice”- like clinkers and bits of slag as well as pieces of gray dolomite rock fragments from the boulders that have been imported by people to protect shorelines. Once or twice I’ve found water rounded gray beach stones from these imported boulders that have hollows or ‘vugs’ containing small crystals. So you never know what oddities might show up among the native sandstone or other pebbles on the neighborhood beach.
Freshly eroded rocks recently released from the shoreline usually have angular shapes and edges. After a few years of beach life they become the water rounded pebbles that skitter so readily underfoot. However, if you look closely at the stones still embedded in the bluff’s clay some of them may also show signs of some water rounding that must have occurred long before Lake Ontario existed. Now and then a really big rock ends up on the shore or in the lake. They can be of various types- some are sandstone or limestone others are metamorphic. These are glacial erratics, some of which were carried long distances by the ice. ( photo shows two distinctly rounded erratics that have recently eroded out from the land)
Recently I encountered two large boulders recently eroded out from the shoreline. Both appeared to be granite like gneiss, ancient metamorphic rocks that resemble those of the Canadian Shield to the north. Both were clearly somewhat water rounded. Perhaps they were dragged around by the glacier long enough to be rounded off. Or perhaps after they formed a billion year ago miles below the earth’s surface, in the deep time of geology, and then somehow journeyed to the surface they were shaped by water action.
Maybe at some point in their lives they became subject to the forces of an ancient river’s current or were once pounded by the surf of an unknown ocean. Perhaps the waves of the Ordovician sea where the fossil crinoids and horn corals lived, rounded them off. As the guidebook to NY fossils says the vastness of geologic time is difficult to comprehend.
The origins and types of glacial erratics differ from the so called native ‘bedrock’ . Once before the age of geological science, people thought these odd boulders were deposited by the great flood that Noah navigated. By the later 1800’s scholars figured out that they were transported by ice sometimes for hundreds of miles before being left behind after the big melt down. While such rocks can be as big as a house, most of the ones along the lake shore are more on the order of a small compact car or less.
There is a particularly big one just off shore from Dutch Street that lies in about seven feet of water. It’s visible during fall water levels, and is far enough out to be a year around navigational hazard to small boats. The pair on the beach in my photo are much more typical. They were eroded or plucked from the earth by the moving ice sheet after perhaps its massive weight cracked the rock below it.