John T. Whetten, 1972. "Matrix-Rich Pleistocene Sediments from Western Washington: Incipient Graywacke-Type Sedimentary Rocks?", Studies in Earth and Space Sciences, R. Shagam, R. B. Hargraves, W. J. Morgan, F. B. Van Houten, C. A. Burk, H. D. Holland, L. C. Hollister
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Pleistocene gravels with abundant interstitial matrix of silt and clay are common in western Washington. At one locality, the matrix appears to result from chemical alteration of detrital clasts. In an adjacent sand bed, hypersthene is altering to clay. A similar process may have formed matrix in graywackes. Some matrix in the gravels may have been introduced by other mechanisms.