Geo Jocks: Eagle Rocks and Sediments

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Like much of the geology around Los Angeles and Southern California, the geologic systems underneath Occidental College and Eagle Rock have been widely studied. As discussed in a previous column, the majority of the Los Angeles basin consists primarily of consolidated sedimentary units. In the Eagle Rock and Highland Park areas, these sedimentary rocks consist of the Topanga, Puente and Fernando formations, in addition to recently deposited surface sediment referred to as alluvium.

On campus, there are several areas to investigate the sedimentary units, familiar to students who have taken the Earth: Our Environment course in the geology department. Of note are the exposed strata along the trails leading to Fiji, outcrops near Bird Road and the cleanly sheared dirt walls surrounding the Greek Bowl.

Two units are easily observable: An older, moderately well-consolidated layer and a younger poorly consolidated layer are distinguishable as the first in the sedimentary sequence. The older unit is a poorly graded conglomerate with clasts of minimal roundness. Despite being moderately well-consolidated, the formation is still broken in hand easily. Sediments are coarse- to fine-grained with clasts ranging from two to 30 cm at the various locations, suggesting a dynamic depositional environment. Igneous cobbles present within the formation indicate a material source near the San Gabriel Mountains.

 

The topmost alluvial layer is very poorly consolidated, hardly cemented and easily eroded by surface processes. It appears reddish-brown in color and has poorly-sorted sub-angular to sub-rounded clasts, though the range of sizes observed is not as great as that in the older alluvium. This younger alluvial layer is infrequent and varied in size, but the smaller and slightly better sorted clasts and cobbles suggest a less dynamic system than the old alluvium and perhaps a more local source material.

Like much of the LA basin, the area has been exposed to many a repeating cycle of sea level rise and fall that allowed for many of the sedimentary rocks to be deposited. Clearly, however, based on the region’s topography, changing conditions have allowed for the formation of low hills and shallow valleys. The active tectonic regime of Southern California and the nearby Raymond Fault running down York Boulevard forced up the hills in the Eagle Rock area including our very own Fiji. At the core of our hills is dense basement material that shakes less vigorously during earthquakes compared to flat areas with less dense sedimentary rocks at depth. From the sedimentary rocks at the surface to the igneous material at depth, the rocks under Occidental reflect the varied geologic history of the greater LA area.