Delmar Formation


Torrey sandstone overlying
Delmar Formation

The Delmar Formation is exposed only at the base of the sea cliffs from just south of the main reserve entrance to just south of Flat Rock. Flat Rock is a sea stack remaining after wave erosion of the Delmar Formation. The Delmar is mostly a greenish yellow mudstone and siltstone (like sandstone but with smaller grains). At its bottom, layers of harder rock composed almost entirely of fossil oyster shells protrude. The shells stand out on top of the oyster beds as the rock erodes because they are harder than the siltstone.


Eocene oyster bed

Layers of sandstone up to ten feet thick are also in the Delmar. All of the formation was deposited in a lagoon full of life like our modern lagoon so fossils are abundant. Features showing deposition underwater include regular cross-bedding. Each wave deposits a layer of sand on the lee side of a sand bar, inclined at the angle of the end of the bar. Then, a new surge of water cuts off the top of the bar. Note that the cross-bedding in our picture is in opposite directions in the top two layers. The water flow had to be in opposite directions at the times the layers were deposited.

If enough animals burrowed in the sand and mud, the cross-bedding was destroyed and the rock is “bioturbated.”


Crossbedding in
Delmar Formation

Fossil worm burrows
in Delmar formation

Next formation: Torrey Sandstone