The following rules can be used for rockslides as well as avalanches.

An avalanche is a mass of snow and debris falling rapidly down a mountainside. A typical avalanche is 300 feet wide, 150 feet long, and 30 feet thick. Creatures in the path of an avalanche can avoid it or escape it if they're close to its edge, but outrunning one is almost impossible.

When an avalanche occurs, all nearby creatures must roll initiative. Twice each round, on initiative counts 10 and 0, the avalanche travels 300 feet until it can travel no more. When an avalanche moves, any creature in its space moves along with it and falls prone, and the creature must make a DC 15 Strength saving throw, taking 1d10 bludgeoning damage on a failed save, or half as much damage on a successful one.

When an avalanche stops, the snow settles and buries creatures in it. A creature buried in this way is blinded and restrained and has total cover. The creature gains one level of exhaustion for every 5 minutes it spends buried in the snow. It can try to dig itself free as an action, breaking the surface and ending the blinded and restrained conditions on itself with a successful DC 15 Strength (Athletics) check. A creature that fails this check three times can't attempt to dig itself out again.

A creature that is not restrained or incapacitated can spend 1 minute freeing another creature buried in the snow. Once free, that creature is no longer blinded or restrained by the avalanche.