ABSTRACT

Weathered gravels and sands that can be cut easily with a knife form impressive deposits of Eocene, Miocene, and early Pleistocene age in Oregon and Washington. The deposits are believed to be profiles of weathering developed at or near the surface and are derived mainly from quartz-free fragments, chiefly from basalts and andesites, which had been deposited by streams. The minerals of the weathered gravels and sands include: kaolinite-halloysite, beidellite-nontronite, montmorillonite, gibbsite, iddingsite, siderite, ilmenite, limonite, vivianite, celadonite, feldspars, quartz, muscovite and minor amounts of others. No apparent change in volume accompanied the change of the original silicate minerals in the pebbles, cobbles and boulders to clay minerals. Chemical and petrographic obs rvations indicate that readjustments by the clay minerals formed in the rounded fragments and matrix were accomplished with the preservation of outward shapes and textures of the original rock fragments.