I am writing my second D&D 5e home-brewed campaign. As with most D&D campaigns, this is set in medieval (like) times. In this adventure, the party is sent to aid a new settlement from a potential orc invasion from the north. The only thing separating the village from the orcs is a large, craggy mountain range with a single pass. The villagers have hired the party to close the pass using a strange "black rock" mined by the neighboring dwarves.
I've researched gunpowder, and it appears that it would have been something available during this time-period (fantasy or not). I know I can always use a wizard or other spell caster to use a lightning spell to ignite the powder, but I would rather have this newly discovered "black rock" used to cause an avalanche and seal off the mountain pass.
Therefore, I'm looking for a very large, effective explosion, using the black rock, that could be detonated from a (relatively) safe distance.
EDIT TO CLARIFY: How did a person set off an explosive (black powder, gun powder, etc.) in the middle ages?
Thanks for any input!