Shiva's supremacy was established after the Fiery Linga incident and he commanded Vishnu & Brahma to worship him using the Agamas. They did so for sixteen thousand years. Skanda Purana 1.3b.16.62 :-
Viṣṇu and Brahmā wore matted hair and applied sacred ash over their bodies.
So where did they get this sacred ash from? Did they only stay in one cremation ground or multiple ones? Which ones were these?