Simple, vertical to slightly inclined tubes that have smooth walls with linings and structureless fill. The tubes are sometimes J-shaped, and they never branch or cross. If the burrow walls have scratch marks or a rope-like surface texture, then the ichnogenus name of Scoyenia is more appropriate. Scoyenia traces are made by insects, and they sometimes branch downward (with inclined to horizontal branches), like an upside-down Y, whereas Skolithos traces are straight, near-vertical and do not branch. The trace is usually found in rocks that represent non-marine to shallow-marine facies - the former representing insect burrows, and the latter worm or arthropod burrows. The trace is also found in some rocks generally regarded as deeper-marine facies, such as the example on the right, which is from the Eocene Cozy Dell Formation exposed in Sespe Canyon in the California Transverse Ranges.
The famous "pipe rock" of northwest Scotland is a Cambrian sandstone, in which the "pipes" are closely spaced Skolithos burrows. Another well known "pipe rock" is the Ordovician to Silurian Tumblagooda Sandstone of Kalbarri National Park in Western Australia, a picture of which is shown below. These pipe rocks when viewed from above, looking down on the bed in plan view, have closely-spaced circular marks that may resemble the fossil raindrop imprints we show elsewhere on this site.
Examples of Skolithos traces