Troy Blodgett

Constraints on the Origin of Paleolake Expansions in the Central Andes

Abstract During the late Pleistocene, at least one episode of lake expansions occurred in the internally draining high plateau region of Bolivia. Some researchers have advocated that a wetter climate associated with a change in atmospheric circulation caused the development of the large paleolakes, while others have hypothesized that deglaciation contributed to the water source for the expanding lakes. From estimates of the potential meltwater stored in the glaciers during their maximum extents, the authors conclude that insufficient meltwater was available to fill the large paleolakes. However, the meltwater hypothesis remains viable south of the main plateau region where, in five small drainage basins, the volume of available glacial meltwater was 3–16 times greater than the volume of water in the paleolakes. Pollen, dunes, and other eolian features indicate that the region surrounding the Altiplano was much drier during at least one interval of the late Pleistocene. Although the timing of the dry perio...

Landslide Erosion Rate in the Eastern Cordillera of Northern Bolivia

Abstract The northeastern edge of the Bolivian Eastern Cordillera is an example of a tectonically active plateau margin where orographically enhanced precipitation facilitates very high rates of erosion. The topography of the steepest part of the margin exhibits the classic signature of high erosion rates consisting of high-relief V-shaped valleys where landsliding is the dominant process of hillslope erosion and bedrock rivers are incising into the landscape. The authors mapped landslide scars on multitemporal aerial photographs to estimate hillslope erosion rates. Field surveys of landslide scars are used to calibrate a landslide volume versus area relationship. The mapped area of landsliding, in combination with an estimate of the time for landslide scars to revegetate, leads to an erosion rate estimate. The estimated revegetation time, 10–35 yr, is based on analysis of multitemporal aerial photographs and tree rings. About 4%–6% of two watersheds in the region considered were affected by landslides ov...

Gia's Symmetry Grading Boundaries For Round Brilliant Cut Diamonds

GEMS & GEMOLOGY WINTER 2011 ations during planning and cutting. Likewise, makers of non-contact optical scanners have been interested in guidelines for how measurable symmetry parameters affect the GIA symmetry grade. The grade boundaries presented here offer a substantive estimate of the symmetry grade for any round brilliant cut diamond. In GIA’s laboratory, polished diamonds are measured with a non-contact optical scanner early in the grading process. Later, polish and symmetry are evaluated visually at 10× magnification, using a standard procedure. As described in Gillen et al. (2005), specific parameterand facet-related features are considered in grading symmetry. This article presents numerical grade limits for 10 important symmetry parameters that can be measured accurately enough to support visual symmetry grading. Although measured values have been available to graders as a guide for several years, beginning in 2012 GIA will use measured values and apply these boundary limits strictly when grading symmetry for round brilliant cut diamonds. Facet-related symmetry features, and the manner in which multiple features combine, may also affect the symmetry grade, and these aspects will continue to be evaluated visually, as they are presently beyond reproducible instrument measurement. Compared to visual assessment, instrumental measurements provide a more consistent way of establishing a symmetry grade, especially when a diamond has very subtle symmetry deviations. Figure 1 shows a diamond with several symmetry flaws: a wavy and uneven girdle (resulting in an uneven crown height), a table not parallel to the girdle, and uneven bezel facets. In the past, the only means of Since 2006, GIA has used certain proportion measurements obtained with non-contact optical scanners to grade the cut of round brilliant diamonds. Improvements in the operation and accuracy of these instruments now enable us to also measure some symmetry parameters during the grading process. Although both Excellent and Very Good symmetry grades meet GIA’s criteria for an Excellent cut grade (Moses et al., 2004), there is a premium for what the trade calls a “triple Excellent”: an Excellent grade for cut, polish, and symmetry. Therefore, many diamond manufacturers would like to be able to predict GIA symmetry grades from measurement data, so they can apply these consider-