A system and method for excavating a wellbore through a subterranean formation that includes a drill string having an inner passage for flowing a suspension of fluid and impactors. The drill string includes a flow control device to block flow in the drill string passage or in the annulus formed between the drill string and the wellbore inner wall from lowing into the bit when flow is reduced or stopped. The flow can take place due to fluid density differences resulting from impactors being in suspension in the fluid. The flow control device includes a selectively openable and closable valve. Valve embodiments include valves having flapper elements, a plurality of cables suspended in an annular configuration, and whisker elements.