Separating deep litter

Hey all,
Looking for advice. Using deep litter over 6 inches of river rock to assist with drainage. Been constantly raining for many weeks. Now that its time to clean everything out I'm trying to come up with an efficient way to separate the gravel from the litter, which will go into the compost pile. The girls really got the gravel and litter mixed together very thoroughly. I've thought of using a big sifter, or using a compost tumbler, but both seem pretty labor and time intensive. I would greatly welcome any suggestions.