Wednesday 8 April 2015

Late Pleistocene lithic artefacts from the Bear Creek Site on the Puget Sound.

The earliest Human settlers are thought to have entered the Americas across the Bering Straits during the last glaciation when sea levels were much lower than today, spreading across the Americas during the Late Pleistocene and early Holocene. Several tool making cultures are known from across the Americas during this time interval. However the Pacific Northwest of North America, where these cultures are thought likely to share a common root, is notoriously lacking in archaeological sites from this era, and those that are known are mostly problematic to date.

In a paper published in the journal PaleoAmerica in January 2015, Robert Kopperl, Amanda Taylor and Christian Miss of SWCA Environmental Consultants, Kenneth Ames of the Anthropology Department at Portland State University and Charles Hodges of Pacific Geoarchaeological Services describe the newly discovered Bear Creek archaeological site (site 45KI839) at Redmond on the western Washington Puget Lowland in Washington State, which has yielded a variety of stone tools from the Late Pleistocene-Holocene transition.

Location of the Bear Creek site (45KI839) near the north shore of Lake Sammamish, Puget Lowland, Washington. Kopperl et al. (2015).

The site was first identified in 2008 when scattered lithic artefacts were found above a layer of tephra and peat. Excavations in 2009  and 2013 revealed a distinctive horizon, identified as Stratum Vc, which yielded a high number of lithic artefacts, including bifaces, scrapers, gravers, projectile points, and unifaces, as well as hammerstones and edge-modified flakes and cobbles, made from chert and fine-grained volcanic rock.

Projectile point bases recovered in situ in Stratum Vc from 2009 test excavation (#2009–50 and –181) and 2013 data recovery (#2013–1156). Kopperl et al. (2015).

Stratum Vc lies on top of a layer of glacial sediments, Stratum VI, and is overlain by a layer of peaty wetland deposits, Stratum Vb and then a layer of Diatomaceous Earth, Stratum II. This is consistent with a warming climate at the end of the Pleistocene, with the glacial deposits formed in a glacial setting, then replaced by wetlands and a lake (Diatomaceous Earth is formed by freshwater Diatoms, a form of single-celled Algae, settling out of the water column far enough from shore to that other sediments are not being laid down). The entire assemblage is cut through by an erosional sandy channel deposit, Stratum III, which also contains some lithic artefacts, apparently reworked in nature.

Schematic profile of stratigraphy across the Bear Creek site (45KI839). Primary in situ artifact-bearing LPH deposit is Stratum Vc, shown in red. Kopperl et al. (2015).

The artefacts show a mixture of traits associated with the Palaeoindian Period (roughly from 18 000 to 8000 BC) and Palaeoarchaic Period (roughly from 8000 to 6000 BC), suggesting that they may date from close to the Pleistocene-Holocene boundary (about 10 000 BC). Importantly it was possible to obtain Radiocarbon Dates from several pieces of carbonized and uncarbonized wood and peat samples within the sequence, including a piece of carbonized wood from within Stratum Vc, dated to 12 770-12 596 years before the present, suggesting that the site may date from the Younger Dryas Period (a short burst of cooling and return to glacial conditions during the thaw at the end of the last glaciation), though a single carbon date is not generally considered absolutely reliable. The clay layer at the base of Stratum Vc has yielded two pieces of uncarbonized Willow wood, which yielded dates of 12 597-12 116 and 12 690-12 420 years before the present – this is actually slightly younger than the date from Stratum Vc, suggesting that it may be inaccurate, but still within the Younger Dryas. Stratum Vb has produced a range of dates, ranging from 10 586-10 298 to 8543-8376, i.e. the very latest Pleistocene and Early Holocene.

See also…

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