This is false
Sills are not formed by magma that hardens vertically underneath the surface.
A sill is a horizontal layer of igneous rock that forms between older (preexisting) rock beds. Sills are formed when magma squeezes between older rock layers and hardens to form a slab of rock. Sills may have vertical to horizontal orientations due to rotation caused by tectonic processes, but the most common sills are the nearly horizontal ones.
