Compressive forces caused by the Laramide orogeny were followed by some minor stretching as a new equilibrium was established. This created weaknesses in the crust that allowed magma to intrude toward the surface to create composite volcanos west of the area some 25 to 20 million years ago. Within the fold, magma intruded through and between formations about 4.6 to 3.7 million years ago to respectively create dikes and sills. Small basaltic lava flows erupted through fissures at the surface and igneous activity continued sporadically afterwards. Subsequent erosion preferentially removed the softer sedimentary rock that initially entombed the dikes, sills, and volcanic plugs, often leaving them standing in relief. Examples can be seen in South Desert and Cathedral Valley at the northern end of the fold.