The cliffs consist mainly of beds of Namurian shale and sandstone, with the oldest rocks being found at the bottom of the cliffs. During the time of their formation between 313 and 326 million years ago, a river dumped sand, silt and clay into an ancient marine basin. Over millions of years, the sediments collecting at the mouth of this ancient delta were compacted and lithified into the sedimentary strata preserved in the now-exposed cliffs. The area is considered a geologic laboratory that preserves a record of deltaic deposition in deep water. Individual strata vary in thickness from just a few centimeters to several meters, each representing a specific depositional event in the history of the delta. In aggregate, up to 200 meters of sedimentary rocks are exposed in the Cliffs of Moher. Trace fossils are abundant, comprising two main types: (1) scolicia or worming trails, which are interpreted as feeding trails left by as-yet-unidentified invertebrates, and (2) burrow marks, which are circular features preserved as casts of burrows once occupied by as-yet-unidentified marine creatures. Ripple marks are preserved in some stones.