A scarp from the 1983 Borah Peak earthquake in the Rocky Mountain Basin and Range of Idaho seen 8 years after the event, Prof. Anne Sheehan for scale at left. Prior to this event, the hillslope covered with brush was continuous: the soil by Anne's feet was adjacent to the brush now on top of the scarp at right. The vertical face at right is called a freeface and was made in the earthquake; a small apron of colluvium covers the base of the scarp. As the scarp ages, the freeface vanishes and the colluvium grows; the slope becomes progressively gentler. Prehistoric scarps can be dated by careful analysis of this change in shape over time.