This is how Amelie likes to sleep in her crib. I always put her down on her back, and for a little while, she'd roll immediately onto her side. Now she moves around quite a bit in the crib, though.
In the morning, she's always wedged up against the corner of the crib on her tummy with her rear end sticking somewhat up in the air.
It's a little hard to shake the feeling that she's trying to escape.