Skolithos (formerly spelled Scolithus or Skolithus[1]) is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow with a distinct lining.

[7] Vertical Skolithos can also occur in alluvial sediments such as braided river deposits, where the periodic fluctuation of water is an important factor in the formation of this structure.

[8]  This periodic water fluctuation corresponds to tidal activity in shallow marine environments, but also occurs over longer time intervals in alluvial deposits.

[8] The ichnogenus Skolithos was first described as a subgenus of the supposed seaweed Fucoides in 1840, by Samuel Stehman Haldeman, a renowned Pennsylvania naturalist in the early 19th century, who labeled the structure as the “oldest fossil in the state”.

[9] 1943 marked the revival of Haldeman’s research as Benjamin Howell reported the occurrence of the trace fossil in the Hardyston Formation in Pennsylvania.

Variations in observed Skolithos structures include burrow curvature, angle to the plane of deposition, and size of the fossil’s aperture.

[14] The deformation history of the mylonite belt which is characterized by large translation of thrust faults, can be deduced from the apparent clockwise rotation of these structures.

[15] While it is common for Skolithos burrows to form normal to the deposition plane, this is not always true, in which case, the ideal, undeformed state can no longer be used as a reference orientation.

[15] Shallow depositional sediments are also susceptible to damage by erosion and tectonic stress forces, which can influence average measurements and geometric orientations.