Heulandite, with thomsonite, stilbite, scolecite, calcite and chalcedony, occur as infilling minerals.
Indeed, so rapid is the infilling by the torrents which sweep down detritus from the surrounding heights that even the existing lakes are visibly diminishing.
The ground was once marshy so that drainage and infilling was necessary.
Much of the infilling probably occurred early in the Holocene, as the landscape adjusted to increasingly temperate conditions.
The infilling material may be warmer, softer ice from below the surface, or water from a subsurface ocean.