1. Compression: This is the most common force causing folds. It occurs when rocks are squeezed together, typically by tectonic plate movement. This pressure forces the rock layers to bend and fold.

2. Tension: This force occurs when rocks are pulled apart, causing them to stretch and thin. While not as common as compression, tension can also create folds, especially in combination with other forces.

3. Shear: This force occurs when rocks are pushed past each other horizontally. Shear forces can cause folds, but they are often more associated with faulting.

It's important to note that these forces rarely work in isolation. Folding is often a complex process involving multiple forces acting on the rock layers over time.