# Cooper’s Ferry in Idaho: authors challenge ASI index inference of vertical mixing at the site

- Published: 2026-11-05 23:38 UTC
- Section: Science › Research
- Tags: Cooper’s Ferry, Nipéhe, Apparent Stratigraphic Integrity Index, peopling of the Americas, Late Pleistocene, postdepositional mixing, coastal migration model, LU3 loess, Idaho archaeology
- URL: https://realtech.media/news/rt-dd66d45660f9ccea93e1f39a_cooper_s_ferry/

**Authors argue that ASI, a statistical index built on binned artifact counts, does not support claims of large-scale mixing at the Idaho site, dated to ∼16,000 calibrated years BP.**

![Cooper’s Ferry in Idaho: authors challenge ASI index inference of vertical mixing at the site](https://sgzwpqqelzmajkcubgkl.supabase.co/storage/v1/object/public/rt-media/2026/11/6009dc2edf7434b5.jpg)
*John Atherton, Wikimedia Commons, CC BY-SA 2.0*

A paper in Science Advances re-examines the Apparent Stratigraphic Integrity Index (ASI), a statistical measure used to infer postdepositional mixing at Late Pleistocene sites and to support a relatively late occupation of temperate North America. The authors tested it on Cooper’s Ferry/Nipéhe in Idaho, where the occupation is dated to ∼16,000 calibrated years before the present [1].

ASI describes vertical discontinuity in artifact-frequency profiles split into bins. According to the authors, it does not directly measure artifact movement or stratigraphic integrity [1]. At Cooper’s Ferry, the 5-centimeter absolute-elevation bins used for the calculation do not match the actual stratigraphic positions of artifacts across the irregular LU2 surface, on which LU3 loess accumulated [1].

The authors conclude that the resulting ASI value cannot support claims of large-scale vertical mixing. They cite independent chronological, stratigraphic, pedogenic, archaeological and excavation evidence for the integrity of the occupation [1]. In their view, the site provides early inland evidence of the kind expected under coastal-migration models, and models should be evaluated against archaeological evidence rather than used to override it [1].

## Sources

1. [Science Advances](https://doi.org/10.1126/sciadv.aek3138)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-dd66d45660f9ccea93e1f39a_cooper_s_ferry/