Potsdam, New York, is about 1800 miles from the Grand Canyon, but because of the extent of the Cambrian transgression by the sea, the lowest, oldest Cambrian rocks in both places are quite similar. Both the Potsdam in New York and the Tapeats in the Grand Canyon are sandstones, lithified from sediments laid down in a near-shore marine environment. The Potsdam sand was probably eroded off the high-standing Adirondacks, which may have almost been an island in the Cambrian sea. And both the Potsdam and the Tapeats lie above a profound unconformity, above Precambrian rocks that are hundreds of millions of years older.
The Potsdam in New York is mostly a clean quartz sandstone like you might expect from a beach, with some hematite (iron oxide) cement that makes it pinkish. It makes a good building stone and Canada’s House of Parliament, in Ottawa, is made from it.
One difference between the Potsdam and the Tapeats in the Grand Canyon is that the Potsdam is younger – probably late Cambrian in age, even though it’s the oldest Cambrian layer present, rather than early to middle Cambrian for the Tapeats. That reflects the millions of years that it took for the sea to encroach, to transgress, across much of North America. There’s a lot of that sand though. The Potsdam is as thick as 1,500 feet around Lake Champlain.
|Potsdam near Chippewa Bay, New York, above the unconformity (Precambrian below). Photo by Michael C. Rygel via Wikimedia Commons, under Creative Commons Share Alike Unported license.|
We used to give similar Cambrian sandstones in Ontario, Michigan, Indiana, Virginia, and as far west as Iowa and Wisconsin and even Wyoming the same name, Potsdam – but while the origin is practically the same, and the sandstones may be stratigraphically equivalent, it’s probably not correct to think of the sand as a continuous sheet of sand, at least not at the same time. The sea in which the sand was laid down varied in space as well as time. So these sandstones have different names today.
Today, February 19, in 1792, was the birth date of Roderick Impey Murchison, in Tarradale, Scotland. Together with Adam Sedgwick, Murchison became one of the great early British geologists who helped define many of the Paleozoic time intervals. In a few days, we’ll talk about the feud between Sedgwick and Murchison over the position of the top of the Cambrian in Britain.
Also on this day, February 19, 1600, the volcano Huaynaputina erupted in southern Peru. It was the largest volcanic eruption in South America in historic times. The years 1600-1602 were the coldest in at least 600 years in Russia, and many people starved. The wine harvest in France and Germany was negatively impacted, and climatic effects were noted in Japan and China as well. Ten villages were buried under ash in Peru, where at least 1500 died.
—Richard I. Gibson
Photo by Michael C. Rygel via Wikimedia Commons, under Creative Commons Share Alike Unported license.