The Entiat Mountains, a spur of the Cascade Range in central Washington, are composed dominantly of a metamorphic complex of pre-Ordovician age. Although originally made up of a diversified series of rocks partly of sedimentary and partly of igneous origin, metamorphism has been so complete that the original heterogeneity of the group, save that inherent in the chemical composition, has been almost completely obliterated, and the entire metamorphic mass now participates in a common foliation and forms a single definite structural unit. Subsequent to the development of the foliation, mechanical movements superimposed cataclastic structures upon the metamorphic rocks and locally developed flat thrusts of some magnitude within them. Along the thrust surfaces mylonites are developed. Three distinct periods of batholithic intrusion of pre-Tertiary age are recorded within the area, and these, together with the Miocene batholiths to the west, establish the presence of four periods of plutonic invasion in the Northern Cascades. A striking fact brought out by the study is that the late Tertiary folding which trends northwest-southeast and east-west transverse to the main axis of the Cascade Range was preceded by several episodes of pre-Tertiary deformation that occurred along the same axes.
No takes yet. Share an insight, caveat, or question.
A. C. Waters (1932) studied this question.