An explanation is suggested for the origin of thin, laterally persistent pebble beds commonly found in the Columbia Formation of Delaware. The pebbles in the thin beds are usually less than 16 mm in diameter, well rounded, spherical, and composed mainly of resistant material: chert, vein quartz, and quartzite. The process thought to be responsible for the origin of these beds is a combination of erosion and transport of sand and pebbles in suspension by highly turbulent streams resulting in selective deposition of pebbles in thin, laterally persistent layers. An attempt is made to estimate quantitatively the amount of sand that has to be eroded to produce thin pebble beds. The results suggest that such pebble beds mark significant erosional unconformities within the fluvial Columbia sequence.