I don’t remember when and where I found this pebble. It was probably several years ago somewhere in Estonia. It is very small (about 4 cm across) and composed of limestone. I picked it up because it must have a remarkable geological story.
It had to be a part of some limestone formation. Most likely in northern Estonia where such rocks from the Ordovician Period are exposed. It has a rather distinct appearance and seems to be partly composed of clastic fragments but I am not familiar enough with Estonian bedrock to locate its exact source. I believe it comes from the coast because it is a pebble. Something had to break it from the limestone bed and then polish it to a nicely rounded shape. I guess it was done by sea waves.
What happened after that is harder to explain but it is obvious that something had to crush it. The pebble is now composed of four distinct parts. However, these fragments were not separated from each other which needs some sort of explanation. They are only slightly displaced. Most likely the pebble was surrounded by other rocks which held the fragments in place. What was the crushing force is impossible to tell. It was hardly an earthquake because these occur in Estonia very rarely and are weak. Maybe some bigger rock fell onto it? Maybe the event was associated with glacial activity during the last glacial epoch?
Anyway, the four main fragments of the pebble stayed next to each other and were cemented together again as a single rock and were later liberated from the surrounding material. So the pebble was crushed and then the pieces were glued together again and the same pebble, although seriously wounded, formed again.
Small brecciated limestone pebble with an interesting history.
It looks like a clast from a cracked-pebble conglomerate. Doing a search on that term, though, is finding very little; it’s likely out-dated now. It’s essentially a situation where large-scale plastic deformation is accomplished by small-scale brittle deformation. “Faults” shear pebbles, but not whole rock. Fractures are plentiful, but most often limited to single clasts in the conglomerate.