Like a caramel bar on a hot day, some rocks are ductile: They flow, or change their shape, in response to stress. If stress is applied over millions of years to hot, layered rocks within the Earth, the layers may flow into folds. Flowing rocks can also be stretched into elongated forms.

Once, both of these trilobite fossils looked like the one on the left. But deformation related to faulting flattened the black fractured rock and its fossil at an angle to the original layers. You can see the result: a long, asymmetric trilobite.