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Short geology thread. So while walking (no, the bike didn't have a puncture, we were on holiday and didn't have bikes with us) along the shore near Weymouth we encountered these. Lots of them. And they seemed most odd... 
A rock, you say? What's strange about that. Size, uniformity and location. See, now I've worked out what they are, where they are needs some explanation. It turns out they're septarian nodules, from the Kimmeridge Clay deposit. But they're entirely in the wrong place...
...now the Kimmeridge deposit kind of goes right across the country, from Dorset up to North Yorkshire, and you get outcrops of it in various places. It continues our into the North Sea and yes, it's indicative of where to drill for oil and gas... And it doesn't look like this
...but that's where these nodules are coming from. Kimmeridge material is late Jurassic, early Cretaceous. So it's 145 million years old. But underneath that earthy material is this...
...yes, that's soft, grey clay with oyster shells in it. I -think- that can only be a deposit from a warm interglacial - oyster beds like this thrive below the low tide line and this is a good 2-3 meters above that. So, 120-130,000 years old?
The same nodules, where they come directly from the Kimmeridge deposit, aren't sorted by size. Here's one from up the coast near Osmington. Bigger than any I found on this beach...
What are the nodules? Think muddy limestone. You get a piece of organic matter, anything that when decaying can bind calcium ions. That nucleates growth on a warm, tropical ocean and the deposit grows. Then they get buried, time passes...
...with pressure and time they become more mud/limestone, shrink, and crack. In the cracks there is a supercritical solution of calcium ions, which over millions of years deposits as calcit crystals. If the fracture, it's down those cracks. Here's a fragment...
...so we know how old they are from the rocks they're from, and we know they're many millions of years older than the deposit they're in. And we also know some process has sorted them according to size. Because the beach is covered with similar sized ones...
...but they aren't size geaded out with other materials in the (tertiary?) deposit they're in now. So, the initial sorting was by water, probably glacial melt waters washing down from churned up Kimmeridge clay. But then another process dropped buried them into this deposit...
...and I'll confess I'm totally stumped on the last part! But geology is an ongoing process. What next for these nodules? Now they're being redeposited back at the same level, size sorted Jurassic nodules on top of ice age mud. Who knows how that will look in the distant future?
