Lots of rock. As shown in this example, a close-up of site 41BR65 at Camp Bowie, burned rock middens are made up of quantities of stones, broken up from heating and no longer as useful for cooking. Dense accumulations such as the midden pictured here represent mainly cooking debris discarded from central baking pits.
Excavation shot showing Unit 1, Level 4 (40 cm below ground surface). This excavation unit was placed at the outer edge of the midden to establish depth and to determine if internal features were present.