The boundary was marked in North America by a widespread layer of black sedimentary rocks covering the bones of many large fauna (including mammoths); such remains were not found above the layer.
The widespread layer clouds associated with cyclonic depressions (see below Cyclones and anticyclones), near fronts and other inclement-weather systems, are frequently composed of several layers that may extend up to 9 km (5.6 miles) or more, separated by clear zones that become filled in as rain or snow develops.
Several post-tsunami sediment sampling surveys have been performed in the present study area or nearby, but the expected widespread layer of tsunami fine-grained deposits was never detected (Szczuciński et al., 2006; Di Geronimo et al., 2009; Feldens et al., 2009; Sugawara et al., 2009; present study).
Their report read, in part: In 1843, explorers reported a widespread layer of newly fallen rock fragments "like a snowfall" and that the forest was "on fire for miles around".
Major- and trace-element compositions of volcanic glass shards of the more regionally widespread tephra layers were investigated by Nagahashi et al. (2004, 2007, 2015).
Refractive indices of volcanic glass and associated minerals have been documented as an effective means for correlating widespread tephra layers (Machida and Arai 1983).
As discussed above, we only used six widespread tephra layers as age constraints, where their relative stratigraphic positions with respect to the oxygen isotope stratigraphy are known.
Therefore, two more widespread tephra layers, Aso-4 and Ata-Th, can also be used to correlate features between Sites U1426 and U1427.
However, the long-term tephrostratigraphy of JS sediments, its correlation with known widespread tephra layers, and the tephrochronology for the past several million years have not been established to date.
