4. Shale and siltstone, light-olive-gray to medium-dark-gray, with all intermediate varieties of color and texture; silt and clay intimately interlaminated in lensing layers and in part burrow mottled, blocky to fissile, hard and brittle where dry and fresh, in part limonite- and jarosite-stained. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 3. Siltstone, light-olive-gray, soft, argillaceous, in two beds separated by a 0.3-foot-thick layer of dark-gray, blocky, very argillaceous silt- stone or silty shale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.7 2. Shale and siltstone, medium-dark-gray to dark-gray to olive-gray; lithology completely gradational from moderately silty shale to siltstone; shale intimately interlaminated with or mottled by light-olive-gray laminae and lenses of quartzose silt, with silt predominating in some parts of unit; irregularly bedded layer of light-olive-gray siltstone, 0.2-foot- 厚, lies 5 feet above base; unit stained in part by limonite and jarosite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.9 1. Shale, medium-dark-gray to dark-gray, fissile, weathers flaky, slightly silty to very silty, with numerous thin laminae and lenses of light-olive- gray siltstone; unit stained in part by limonite and jarosite . . . . . . . . . 2.8 Thickness of exposed part of Blue Hill Shale Member . . . . . . . . . . . . . . . 22.5 Total thickness of measured section . . . . . . . . . . . . . . . . . . . . . . . 39.3 69