Fossils , concretions, need help w disc shaped fossil.

WoodsDigger · Apr 28, 2026

I have been picking these up at my local boat launch over the years. Nothing like I have ever seen . Source is the east end of Lake Ontario.

Red-Coat · Apr 28, 2026

Neat. Those appear to be clay or mudstone concretions, given various names according to locality but most commonly generically known as ‘clay babies’ or 'mud babies'. Typically they formed in the bottom sediments of post-glacial lakes (such as in the Great Lakes region) by mineral precipitation and carbonate cementation of the sediments around a nucleus. The nucleus that triggers their formation may have been organic in nature, so a fossil origin (of a kind) is possible, but with an age in the thousands of years rather than millions.

Lake Ontario was formed from glacial meltwater after the retreat of the Laurentide Ice Sheet roughly 12,000–16,500 years ago.

Some examples (but a wide variety of discoid shapes are possible, often showing concentric rings and a central dimple) and sometimes two or more discoids conjoined. They aren't always as 'perfectly' formed as these:

WoodsDigger · Apr 29, 2026

Red-Coat said:
Neat. Those appear to be clay or mudstone concretions, given various names according to locality but most commonly generically known as ‘clay babies’ or 'mud babies'. Typically they formed in the bottom sediments of post-glacial lakes (such as in the Great Lakes region) by mineral precipitation and carbonate cementation of the sediments around a nucleus. The nucleus that triggers their formation may have been organic in nature, so a fossil origin (of a kind) is possible, but with an age in the thousands of years rather than millions.

Lake Ontario was formed from glacial meltwater after the retreat of the Laurentide Ice Sheet roughly 12,000–16,500 years ago.

Some examples (but a wide variety of discoid shapes are possible, often showing concentric rings and a central dimple) and sometimes two or more discoids conjoined. They aren't always as 'perfectly' formed as these:

View attachment 2251995

Thanks RC thats a solve as far as I am concerned. Nice work. I had a biology teacher tell me they were snails. One looks like it has a Mayfly emerging from it .

WoodsDigger · Apr 29, 2026

Red-Coat said:
Neat. Those appear to be clay or mudstone concretions, given various names according to locality but most commonly generically known as ‘clay babies’ or 'mud babies'. Typically they formed in the bottom sediments of post-glacial lakes (such as in the Great Lakes region) by mineral precipitation and carbonate cementation of the sediments around a nucleus. The nucleus that triggers their formation may have been organic in nature, so a fossil origin (of a kind) is possible, but with an age in the thousands of years rather than millions.

Lake Ontario was formed from glacial meltwater after the retreat of the Laurentide Ice Sheet roughly 12,000–16,500 years ago.

Some examples (but a wide variety of discoid shapes are possible, often showing concentric rings and a central dimple) and sometimes two or more discoids conjoined. They aren't always as 'perfectly' formed as these:

View attachment 2251995

I tried to mark this solved. I am not finding the option.