Figs. 151-154—These four diagrams represent the physical history and the corresponding physiographic development of the coastal region of Peru between Camaná and Mollendo. The sedimentary beds in the background of the first diagram are hypothetical and are supposed to correspond to the quartzites of the Majes Valley at Aplao.
The effect has been not only the general aggradation of the valley floor, but also the development of a combined delta and superimposed alluvial fan at the valley mouth. The seaward extension of the delta has been hastened by the gradation of the shore between the bounding headlands, thus giving rise to marine marshes in which every particle of contributed waste is firmly held. The plain of Camaná, therefore, includes parts of each of the following: a delta, a superposed alluvial fan, a salt-water marsh, a fresh-water marsh, a series of beaches, small amounts of piedmont fringe at the foot of Pliocene deposits once trimmed by the river and by waves, and extensive tracts of indefinite fill. (See the Camaná Quadrangle for details.)
With the coastal conditions now before us it will be possible to attempt a correlation between the erosion features and the deposits of the coast and those of the interior. An understanding of the comparisons will be facilitated by the use of diagrams, Figs. 151-154, and by a series of concise summary statements. From the relations of the figure it appears that:
1. The Tertiary deposits bordering the Majes Valley east of the Coast Range were in process of deposition when the sea planed the coastal terrace ([Fig. 151]).
2. A broad mature marine terrace without stacks or sharply alternating spurs and reëntrants (though the rock is a very resistant granite) is correlated with the mature grades of the Coast Range, with which they are integrated and with the mature profiles of the main Cordillera.