A pyroclastic flow can run quite thick. One example is the city of Los Alamos, NM. If you take the main road from Pojoaque to the city center, you will be travelling along the side of the mesa. It does look a bit porous. It's old volcanic ash. This is part of an pyroclastic flow from when the Valle Grande volcano (just west of Bandolier National Monument) blew its stack. That flow is roughly at least 500 feet thick, and probably much more.