As I understand it, these crust fragments wouldn't just be lying flat on the boundary between the lower and upper mantle. They would likely have rotated during

As I understand it, these crust fragments wouldn't just be lying flat on the boundary between the lower and upper mantle. They would likely have rotated during their movement in the flowing upper mantle, and gotten stuck at the boundary in some ~random orientation. As an analogy, consider some layered drink. Say peppermint schnapps over creme de cacao. Then add some chunks of dark chocolate. I'm guessing that they'd settle out at the interface, being denser than the peppermint schnapps, but less dense than the creme de cacao.