After Maitland Volcano became extinct, prolonged erosion destroyed the volcanic edifice. The reason why Maitland eroded away while its neighbour, Edziza, did not is twofold. With the cessation of volcanic activity, Maitland Volcano was unable to protect itself from erosion by providing a cover of younger lavas on top of the older shield. Another factor is that Maitland's basement was built entirely of soft, friable sedimentary rocks of the Bowser Lake Group. Such rocks are more vulnerable to erosion than igneous rocks. The Bowser Lake Group sedimentary rocks are shale, sandstone, conglomerate and siltstone, which were deposited in marine and non-marine environments during the Jurassic and Cretaceous periods. Intervening valleys, deeply incised into the Jurassic shale and sandstone, are as much as below the base of the now deeply eroded Maitland shield volcano. Most of the rugged topography of the Klappan Range formed after Maitland Volcano was built.