Grizabella
If she doesn't get to go outside, she was probably just taking a rare opportunity to roll around on some "rocks". Animals do things like that, maybe as a way of giving themselves a natural massage. They like the feel of it. Our cat, Sushi, wasn't ever able to be outside when we lived in our apartment so now she love, love, loves to roll around in the rocks on our gravel driveway.
Just a silly thing about Sushi, who is a very uncharacteristic cat in many ways---she also likes to run through the garden sprinkler outside. At first I thought I had just caught her with it by accident and she was running away, but then she very obviously took over from there and would run through it on purpose. She's even started lying next to it, batting at it, while waiting for us to turn it on in the evening. She also loves to dump the dog's water bowl over, so I have to be sure to put it in something she can't turn over. She paddles around in it with her feet anyway.
Just a silly thing about Sushi, who is a very uncharacteristic cat in many ways---she also likes to run through the garden sprinkler outside. At first I thought I had just caught her with it by accident and she was running away, but then she very obviously took over from there and would run through it on purpose. She's even started lying next to it, batting at it, while waiting for us to turn it on in the evening. She also loves to dump the dog's water bowl over, so I have to be sure to put it in something she can't turn over. She paddles around in it with her feet anyway.