The corpses of the murdered were burned in the crematoria. Sonderkommando prisoners used wooden mallets to crush not fully burnt bones into powder. Everything was then loaded onto trucks and carried to the banks of the Vistula river, beyond the woods in Birkenau, where it was shoveled straight into the waters of the river. Human ashes were also dumped into the Soła river near the Auschwitz I camp, and into holes and depressions in the terrain. They were used as a base for building roads or reinforcing dikes, and as an additive to the compost heaps used on the camp farms. Significant deposits of human ashes are extant in the vicinity of the crematoria, the burning pits, and in the clearings and at the edge of the woods in Birkenau.

(Mini dictionary of terms from the history of Auschwitz)