I remember finding stones like these on lakeshores in northern Minnesota. Never knew they had a name until I encountered a discussion of a particularly odd one -
- at the WhatsThisRock subreddit. Here's the wiki:
"Omars are a distinctive type of glacial erratic that consists of dark siliceous greywacke and exhibits prominent rounded, often deep, hemispherical voids and pits. The hemispherical voids and pits result from the selective dissolution of carbonate concretions within the greywacke... Omars are typically rounded and range in size from pebbles to boulders. Their rounded shape, whether found in glacial tills or glacial-fluvial (outwash) gravels, indicate that they were eroded from pre-existing littoral or fluvial deposits.The name given these glacial erratics refer to their source, which is the Proterozoic Omarolluk Formation in the Belcher Islands in southeast Hudson Bay. The Laurentide Ice Sheet eroded omars from the Belcher Islands, an archipelago limited to only about a quarter of 1% of Hudson Bay. Glaciers moved omars from the southeastern part of Hudson Bay to central Canada and into the U.S. where they were deposited on moraines. Because scientists know precisely where they came from they are very valuable in documenting the movement of glaciers."
I remember seeing stones like these in great quantities along the chalk shores of northeastern France. I was told that they were the result of sand and small pebbles settling into a depression in the larger stone which would spin around in a micro eddy as water washed over the stone, resulting in a drilling effect. The stones were flint, or silex as they were called there. I probably have 1 or 2 knocking around the house somewhere.
ReplyDeleteThat's certainly the process behind the creation of potholes in river bottoms -
Deletehttps://naturalwonders.substack.com/p/how-do-large-holes-form-in-river
Traveling through the Grand Canyon by boat there are potholes all sizes made by swirling stones. But these Omars are amazing considering the huge area they're found in vs the tiny area they were created. Vikings of the stone world.
DeletexoxoxoBruce
This kind of stone has been found near the Sea of Galilee where the indentation was bored through to use as a net weight for catching fish, by the way.
ReplyDeletethey also exist at the German shores, and they are called 'Chicken-God': https://de.wikipedia.org/wiki/H%C3%BChnergott
ReplyDeleteFormations like that in the Pacific Northwest (with cylindrical holes, not spherical) are the burrows of piddock clams.
ReplyDelete