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| April 14, 2020 Post 114 |
| Beaches on the move |
No place in our region is more ephemeral and subject to change than the shore of Lake Ontario. Part of its appeal and fascination is its endless shape shifting as sand, silt, pebbles and boulders are moved around by wind and water. Here, the lake’s energy ceaselessly sorts and re arranges the sediments that make up the lake’s shoreline.
Sorting by size is especially visible in the berms that often form the beach. On our south shore cobble and pebble beaches the ridges or berms that run parallel to shore are usually made up of pebbles of strikingly uniform size. They, like sand, are sorted by wave energy. The berm represents the upper reach of the surf generated by storm winds. Sometimes on a wide beach you may see two or three such berms of varying heights, each representing a recent storm event. If a berm consists of those bigger hard to walk on cobbles you know that waves from a major gale pounded that shore recently.
As you observe the berms you’ll notice areas where the pebbles are all the same size. Usually the larger stones are located near the top of the ridge. This is because when the wave crashes against the land its energy pushes those heavier rocks inland. Then as the water withdraws and flows back into the lake, its lesser energy drags and sorts the smaller lighter pebbles and deposits them down low on the shore near the water’s edge. In a really heavy surf the waves may even wash entirely over a barrier bar on the lake, pushing the bigger stones over the top of the berm and into the marsh.( see photo)
Barrier bar beaches are especially dramatic in their change. They form between marshes and the open lake and can undergo dramatic shape shifts during big storms. Willow trees help stabilize the bars with their roots, however as erosion wears away the shore the barrier bar moves inland. Often though not always the trees move with it by tipping over away from the lake. Being willows they have the ability to re root and they often do so as the photo sh
Our south shore beaches are made up of a fascinating variety of rocks from Canada and northern and central New York. The colorful variety of stones all gathered in one place are very popular among pebble picking beach combers. The ice age glacier as it moved roughly south west picked up a variety of rocks as it traveled. When it melted away they remained behind dumped together in multiculturally mixed hills or drumlins.
Ancient colorful metamorphic gniess from the Thousand Islands area, granite, dark sparkly hornblende schist often with pockets of garnet crystals from the Adirondacks, Ordovician limestone filled with fossils from the Canadian side of the lake, and Potsdam sandstone from the St. Lawrence valley all then eventually eroded out of the shoreline till to lie side by side on the beach. They create a delightful mix of colors textures and shapes.
More on fossils by the lake coming soon-
Stay tuned for another excerpt from a forth coming book for History Press “The Natural History of Lake Ontario due out in 2020