The plural form is either graben or grabens. The Rhine Graben is a major extensional basin. Alba's tectonic features are almost entirely extensional, consisting of normal faults, graben and tension cracks. Geologically, the gorge is within a structural graben. Formation occurs within a graben that is located on an active rift zone. Tethys is heavily cratered and cut by a number of large faults/graben. Normal faults create the crestal graben and propagate downward. On or near a branch of the Ottawa-Bonnechere Graben lies the Brent impact crater. The faults typically dip toward the center of the graben from both sides. In between these two faults the Chiwaukum graben was created. In this case Ithaca Chasma would be the outermost ring graben of Odysseus. Malta forms the crest of a tilted block on the edge of the Malta Graben. The main axis of the developing graben runs northeast-southwest. In the south east are the Derwent Graben and the Coal River Graben. The Oseberg field is one of a series of hydrocarbon accumulations within the Viking Graben.