Folds are bends or buckles in layered or foliated rocks (Figure 10.1) that generally form in compressional tectonic environments related to the movement of crustal plates. They can affect a whole rock sequence across a wide area or be restricted to narrow zones, for example adjacent to faults or close to the intrusion of plutonic igneous bodies and salt domes. However, some folds are not tectonic in origin, but form within a wet sediments shortly after their deposition by slumping on a sloping seabed or lake floor (Figure CS9.2.4c), or can be found in recent glacial sediments related to the movement of ice. Folds can occur on various scales, and in general, the minor structures seen in hand specimen, core or even rock thin sections commonly mirror the larger-scale structures. Thus, the presence of small folds in boreholes will indicate the presence of more significant folds with similar origins, geometries and orientations.