FAQ: What is a burned rock midden?

A burned rock midden (BRM) is basically a refuse accumulation of fire-fractured cooking rocks ("burned rocks") that vary greatly in size and thickness. Some have just a single layer of rocks and are only 4-6 meters in diameter (13-20 feet). Others may be up to six or seven feet thick and can cover several acres. BRMs formed because prehistoric peoples often returned to the same spots on the landscape and built cooking or roasting pits. Within these pits, which were often little more than shallow scooped-out basins, prehistoric cooks used layered arrangements of heated rocks to bake plant foods (mainly), especially bulbs and roots. Plants such as sotol, agave lechuguilla, wild onion, and wild hyacinth have starchy bulbs that are inedible and sometimes poisonous without prolonged baking; cooked right, the complex starches break down into simple sugars that are readily digestible and sweet. The layered arrangements are called earth ovens because, when they were in use, they were capped by a thick layer of earth that sealed in steamy heat while the plants cooked for as long as several days. A "well-developed" BRM is simply a substantial accumulation that can vary from 18 inches thick to 4-5-feet thick at some sites.