Geologic History of the United States

Geologic History of the Santa Monica Mountains
Amanda D’Elia
ESS 1F Lab 1C
15 November 2010

    The geologic history of the Santa Monica Mountains breaks up into many different phases of geologic activity. The area begins as a marine environment eventually becoming a terrestrial environment as a result of erosion, deposition, rifting, and subduction. The formations seen on the field trip provide supporting evidence to the tectonic theory of the Santa Monica Mountains. The theory breaks the geologic history into four main phases, but based on the formations eight smaller more precise phases are possible. However, the geologic history of the Santa Monica Mountains, based on the formations from the field, generally fits with the big picture plate tectonic theory synthesis.
 
  The earliest rocks of the Santa Monica Mountains are from the Lat Cretaceous over sixty-five million years ago and were found in the Tuna Canyon formation. These rocks are marine in nature and provide evidence of the first major phase of the ancient terrain.   The formation consisted of large conglomerate rocks with metamorphosed volcanic and granite clasts as well as bedded sandstones and graded shale.   These findings shed interesting light on the type of marine environment of the ancient surroundings. The mass of large conglomerate boulders indicates that the current of the area was fast. The volcanic and granite composition of the conglomerates suggests that the rocks came from onshore volcanoes and were carried from the mountainous peaks via streams or rivers and dumped in the ocean. Other proof of this deposit is fossils found in this formation of both marine and terrestrial life forms.   The bedded sandstone was the main channel of the canyon in which fast water flowed. The graded shale, which must have formed in calmer due to its graded nature, was on the outer margins of the marine canyon from a fan deposit. These characteristics point to an ancient turbidity current as well as...