Block mountains, or fault-block mountains, form along faults where rock blocks are forced upwards. They have flat and straight foothill margins and steep slopes like cliffs (i.e., fault scarps). There is no transition zone between the steep slope and the adjacent flat ground, and the slope is always abrupt. When such fault scarp is cut across by a transverse valley, triangular slope facets form, which are called triangular fault facets. Block mountains can form in various types of geological structures. Some have a fault on one side, while others are influenced by faults on both sides. Some exist alone, and some are in groups. They can be parallel, echelon (side by side), crossing each other or have a concentric pattern. Common block mountains are the horsts and tilted fault-block mountains that are widespread in northern and northeastern China, such as Taishan, Huashan and Hengshan mountains.