Nevertheless, weaknesses do occur along bedding planes and joints naturally found in these materials. We call these types of mass wasting rockfalls . Rockfalls from steep hillsides frequently disrupt road traffic in the mountainous regions of British Columbia, Canada. Larger scale downslope movement of rock can also occur along well defined joints or bedding planes. This type of movement is called a rock slide .
Most rock slides occur because the hillslope and rock layers dip in the same direction.
In these cases, the rock slide occurs when a fracture plane develops causes overlying materials to slide downslope. Not all rock slides are the result of the process described above. The sedimentary rock layers in Turtle Mountain dipped away from the valley containing the mining town of Frank. However, joints in the limestone layers did dip toward the town. The mountain was also composed of some structurally weak limestone, shale, siltstone, and coal layers that were deformed by the weight of more massive limestone located above.