ABSTRACT The Upper Ordovician Juniata Formation, Central Appalachian Basin, USA, is a thick succession of cyclically bedded arenites, wackes, and mudrocks. Sedimentary facies of the formation in West Virginia, Virginia, and Maryland indicate cyclic peritidal deposition along the northern shoreline of the basin. The subsurface Juniata Formation has been drilled throughout the basin, and long, continuous well logs are available for analysis of the cyclic deposition. A 2400-ft-long (731.52-m-long) gamma-ray (GR) log from the Preston 119 well, northern West Virginia, provides a proxy of terrigenous siliciclastic fluxes originating from the Taconic highlands, from the early Ashgillian to the Ordovician–Silurian transition. Strong cycling in the GR log shows evidence for Milankovitch-forced sea-level oscillations, hypothesized to have been produced by dynamic Late Ordovician glaciation in polar (southern) Gondwana. Juniata cycle frequencies are different from those of Quaternary Milankovitch cycles, with significantly higher obliquity and precession index frequencies, consistent with a 21.5 h Ordovician day and an Earth-Moon distance that was 95% of present day. These results support John Dennison’s long-held view that Milankovitch forcing of sedimentation took place in the early Paleozoic Appalachian Basin by action of remotely generated glacio-eustatic oscillations powered by glaciation on southern Gondwana, and that this sedimentary record has tracked “Earth’s movement through space.”

Study of slope movements triggered by the storm of November 3–5, 1985, in the central Appalachian Mountains, U.S.A., has helped to define the meteorologic conditions leading to slope movements and the relative importance of land cover, bedrock, surficial geology, and geomorphology in slope movement location. This long-duration rainfall at moderate intensities triggered more than 1,000 slope movements in a 1,040-km 2 study area. Most were shallow slips and slip-flows in thin colluvium and residuum on shale slopes. Locations of these failures were sensitive to land cover and slope aspect but were relatively insensitive to topographic setting. A few shallow slope movements were triggered by the same rainfall on interbedded limestone, shale, and sandstone. Several large debris slide-avalanches were triggered in sandstone regolith high on ridges in areas of the highest measured rainfall. Most of these sites were on slopes that dip 30 to 35° and lie parallel to bedding planes, presumably the sites of least stability.

Fossil soils in alluvial red beds from the Juniata Formation, near Potters Mills, in central Pennsylvania, provide evidence of soil-forming processes during Late Ordovician time. Paleogeographic and facies considerations indicate that the fossil soils formed on flood plains west of the Taconic uplift. Most studies of paleosols of this age or older have considered soils developed on metamorphic or igneous basement rock. Alluvial fossil soils provide evidence of conditions during shorter intervals of weathering without problems of overprinting by successive and different weathering regimes. They can be recognized by the presence of trace fossils and the development of soil horizons and structures. Problems associated with such fossil soils include establishing the nature of the parent material and distinguishing clay formation in the soil from originally deposited fining-upward cycles. These difficulties can be overcome by comparing paleosols in different stages of development, as indicated by degree of ferruginization, density of trace fossils, amount of clay, and abundance and size of caliche nodules. In modern soils, caliche forms in alkaline conditions under which TiO 2 is stable. Gains and losses of oxides (measured in grams per cubic centimeters) relative to TiO 2 in a strongly developed paleosol were compared with those of a weakly developed paleosol, taken to approximate the compositional range of the parent material. Concentration ratios indicate significant soil development of the strongly developed paleosol beyond the compositional range of the weakly developed paleosol. There was depletion of SiO 2 and enrichment of Fe 2 O 3 , Al 2 O 3 , and K 2 O relative to TiO 2 . Anomalous enrichment of K 2 O has been documented in other fossil soils. Both x-ray diffraction studies and a strong correlation between K 2 O and Al 2 O 3 are evidence that most of the potassium is contained in sericitized illite.