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The present-day Gulf of California apparently began to take shape during the Miocene (11 to 25 Ma). According to Gastil and others (1972), the Baja California Peninsula started forming in mid-Cretaceous time. Subduction caused andesitic volcanism from Arizona to south of Sinaloa. At the end of the Cretaceous, the peninsula began to move as a block toward the northwest, and at the start of the Tertiary, it separated from the continent, opening the proto-Gulf of California. The present morphology of the peninsula began to develop between 4 and 6 Ma. There is clear evidence (Miocene sediments in the Island of Tiburón and others) of the existence of the incipient Gulf before the Miocene, but not in its present form.

Wisser (1954) believes that the geologic history of the Baja California Peninsula is similar to that of the southwestern Great Basin and, in its western part, to the Sierra Nevada in the United States. Whatever the origin of the peninsula, I agree with Wisser about the close resemblance between northwestern Sonora and the Great Basin of the western United States, and the relation between Baja California and the California Sierra Nevada (Salas, 1980).

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