While skiing up to Cardiff Summit, we encountered a hard slab avalanche that had slid at an unknown time. The avalanche broke in a small isolated pocket, but was a deep hard slab avalanche leaving large blocks of avalanche debris. It occurred on the small area of North facing terrain, on the north side of Cardiff Summit / weather station, that is typically traversed to access the ridge that leads to Superior. The slope that slid drops off into some very steep trees, so it was difficult to see where the slide path ultimately went.