Prospect Hill in western Sydney is the largest assemblage of igneous rock in the Plain. The oval-shaped ridge was made many millions of years ago when volcanic material from the Earth's core actuated upwards and then sideways. Slow erosion of the overlying layers of sedimentary rock by the flow of rainwater have eventually laid bare the edges of the volcanic and metamorphic rocks of the intrusion. The western suburbs lie on the relatively flat, lowly elevated parts of the Cumberland Plain. Though there are a few hilly or relatively elevated regions on the plain. Western Sydney Parklands and the surrounding suburbs (such as Cecil Hills and Horsley Park), for instance, lies on a prominent ridge that is between high.