24 University of California Publications in Geology [Vol.7 



showing the real attitude of the strata, and the presence of 

 exposures of pseudostrata, have produced considerable confusion. 2 



Occurrence and appearance of exposures. — The usual occur- 

 rence of exposures of pseudostrata is on the sides of stream cuts 

 or small canons, well up towards the top near where the canon 

 slope intersects the normal hill slope. We may usually dis- 

 tinguish, below the soil layer, a layer of friable sandstone gener- 

 ally extending from one to five feet below the surface, sometimes 

 to ten or more feet. Then follows a more distinctly indurated 

 layer with parallel or approximately parallel boundaries and 

 varying in thickness from two to four or five feet in different 

 localities. Below this follows a more friable layer, partly or 

 wholly covered with hill wash or talus, which covers the slope 

 from there down to the stream channel. Sometimes these ex- 

 posures are produced by small landslides or local washouts on 

 some slope. They may vary in length from twenty or thirty feet 

 to a hundred yards or more. 



As a rule the attitude of the exposed layers roughly coincides 

 with the surface slope — most commonly the general hill slope, 

 occasionally the canon slope. In making a section, therefore, 

 across a ridge in areas where no other kinds of exposures are 

 found, it appears as if the ridge were anticlinal in structure. 

 Such an appearance is illustrated on the hills south of Foxen 

 Canon, and is particularly interesting because the real structure 

 under one of the pseudoanticlinal areas is synclinal. 



The appearance of a pseudostratum in exposure is shown in 

 plate 3. Without careful examination with this particular 

 problem in mind, it is not always easy to distinguish such ex- • 

 posures from those of the original stratification of sedimentation. 



Determination of original structure. — The real attitude of the 



formations with reference to their original planes of deposition 



is frequently very difficult to determine. Bare fossil layers, 



thin streaks of gravel in the sand, or shaly lavers, occasionally 

 



2 For the general distribution of formations and for general structures, 

 reference may be made to the geologic map in Bulletin 322 U. S. Geol. 

 Survev, on the Santa Maria Oil District. On this map structure lines are 

 made to stop before reaching the area where pseudostratification is best 

 developed, and the interpretation of the general structure is evidently 

 affected by the occurrence of these exposures and the general lack oi 

 distinct stratification exposures. 



