We’re not 100% sure, but we believe this is Gnaphosa muscoum, a species of ground spider. Ground spiders hunt at night and rest under stones or loose bark during the day, often in a silken sac. These spiders come in muted colors, usually uniformly black, a few brown, some with markings. Their long abdomen is slightly flattened, tipped with prominent, tube-shaped spinnerets (silk-spinning organs), which we are unable to see here.
Photo by: Rose Marchak on 8/14/21 in Missoula, MT