In the study of earth history one of the important goals is to understand how much time was involved in producing the principal fossil-bearing sediments of the Phanerozoic.  The research reported here examines one measure of time that can be compared with the time indicated by radiometric dating.  On the earth today animals and plants are continually burrowing into the substrate, and this disturbance is called bioturbation.  Bioturbation can, in time, homogenize the sediment, destroying any record of the boundaries between layers of sediment.  In the modern world the rate of this process can be measured, and bioturbation generally homogenizes the sediment in hours, days, or weeks.  Under normal environmental conditions it does not take years.  To quantify this process in the rock record we measured vertical sections through 37 sedimentary formations in western United States, from Cambrian to Eocene, recording the amount of observed bioturbation on these rocks.  In all measured sections, 97% of the thickness showed no bioturbation or occasional isolated burrows.  The remaining 3% of the vertical surfaces contained some bioturbation, with a very small amount (<1%) being thoroughly bioturbated.  Such a low level of bioturbation is inconsistent with sediment accumulation over the time indicated by radiometric dating.