The reservoir is located within the Sespe Formation consisting mostly of soft shale and thin sandstone beds estimated to be about 2,600 feet thick. It is exposed in a broad east-west band across the upper portion of the foothills.
It is variable in character both vertically and laterally. the red and green shales alternate with reddish-tan sandstone beds with the sandstone being more coarse and more prominent in the lower part of the formation.
The formation weathers into a soft sticky clay that is inclined to creep and form surficial slides. It erodes rapidly, and gullies are common on steep slopes. these characteristics are a problem in maintaining road-cuts, ditches, support for pipelines, or any structure placed on steep slopes.
Danger of developing large or deep slides into the reservoir is slight. the slumpage into reservoir results only in minor loss of reservoir capacity and possible turbidity of the water.