[5] Overall the unit shows a persistent alternation of shale and sandstone layers, of widely varying thickness, indicating frequent and erratic changes in water depth at the point of deposition.

South of the river, on the north slope and on the crest of the Santa Ynez Mountains, the shales are darker and a thick interior sandstone member can be mapped as a separate unit.

[10][11] Sometime during the late Cretaceous period, the sea withdrew from the area that was to become the California coast in the region of Santa Barbara and Ventura, exposing the sediments that had accumulated during tens of millions of years underwater – the Espada and Jalama Formations – and they began to erode.

[12] At this time, the crustal block on which these units were deposited was nearer to the present-day position of San Diego; it has twisted approximately 90 degrees clockwise during the last 15 million years, as part of a process of regional, extensional deformation associated with motion along the boundary of the Pacific and North American Plates.

[6] Another peculiarity of the unit is that fossil foraminifers from very different depth environments are often mixed together; this has been interpreted as the result of turbidity currents and other submarine mass movements.

However, some of the stratigraphically lowest beds – such as the portion which is in contact with the Sierra Blanca limestone formation – contain larger fossils characteristic of nearshore and reef environments.