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There are sills of shonkinite intruding between layers of lighter rocks.
Whenever there are active volcanoes in the area, there is bound to be magma moving around below the surface as well. In some cases the magma works its way into cracks, between layers, or simply melts its way through surrounding rock, hardening before it makes it to the surface. If the magma cuts across layers of sedimentary rocks (or other types) the formation is called a dike. If the magma forces its way between layers of sedimentary rock, as it did here, the formation is called a sill. The dark sills in the cliffs around the lake are associated with a larger formation called the Shonkin Sag Laccolith, a formation hundreds of feet thick, formed as magma forced its way between layers of sandstone and pooled in a mushroom-shape below the surface.
Whenever there are active volcanoes in the area, there is bound to be magma moving around below the surface as well. In some cases the magma works its way into cracks, between layers, or simply melts its way through surrounding rock, hardening before it makes it to the surface. If the magma cuts across layers of sedimentary rocks (or other types) the formation is called a dike. If the magma forces its way between layers of sedimentary rock, as it did here, the formation is called a sill. The dark sills in the cliffs around the lake are associated with a larger formation called the Shonkin Sag Laccolith, a formation hundreds of feet thick, formed as magma forced its way between layers of sandstone and pooled in a mushroom-shape below the surface.