A laccolith forms when magma has a high viscosity (thick) and cannot spread easily as a thin sill. Instead, the pressure of the magma pushes the overlying rock layers upward, creating a mushroom-shaped or dome-shaped structure. Over geological time, the overlying sedimentary layers may erode, exposing the hardened igneous core as a rounded hill or mountain. Laccoliths are part of a family of intrusive bodies that include Batholiths (large), Sills (flat) and Dikes (vertical), all of which provide evidence of “plutonic” activity beneath the surface.