Among non-mammalian vertebrates, the cloaca is an anatomical structure that functions as

Among the non-mammalian vertebrates, the cloaca is an anatomical structure that function as a shared pathway for the digestive, excretory, and reproductive systems. Cloaca in vertebrates, common chamber and outlet into which the intestinal, urinary, and genital tracts open. It is present in amphibians, reptiles, birds, elasmobranch fishes, and monotremes.  Cloaca is not present in placental mammals or in most bony fish.

The answer would be sand and gravel