The main belt extends from the Cerrillos Hills in the north through the Ortiz Mountains to the San Pedro Mountains () and South Mountain (). Cerro Pelon () is also geologically a part of the belt. Each of the clusters of mountains is a laccolith where magma intruded between sedimentary rock beds and cooled to form a dome-shaped body of rock. Magma preferentially intruded along bedding planes in the country rock, particularly in more ductile beds such as shales of the Chinle Formation or Mancos Shale. In some places, the magma broke through to the surface as volcanoes. The overlying volcanic rock and sedimentary beds subsequently were eroded away to leave the more resistant laccoliths.