Obsidian Traces: Mapping Majuba

Patterned Ground Video Series #3

Obsidian, a form of volcanic glass, is a key constituent of traditional toolkits — and the detritus of tool manufacture — found in archaeological sites throughout the Great Basin of western North America. It was preferred, in many ways, because of its absolutely sharp edges and the relative ease with which tools and edges can be created. Skilled toolmakers can create complex, almost artistic, hunting technology, while even the most unskilled user can fracture a piece of obsidian to create a utilitarian cutting edge, if need be.

In the volcanic rocks of the Basin and Range geologic province, obsidian is relatively common and can be found in prominent glassy outcrops or as cobbles and gravels spread across the landscape. Its utility to traditional foragers needing a reliable toolkit and the availability of obsidian as raw material result in its common presence in archaeological sites — sometimes it is the only material present to indicate the passing of people through the Great Basin past.

Recce route at Poker Brown Wash in the region of Majuba and Seven Troughs obsidian sources.

What kind of stories can obsidian tell archaeologists? Formed during volcanic eruptions, each obsidian has a unique geochemical signature. The obsidian from Volcano #1 can be easily distinguished from the obsidian of Volcano #2. If we find obsidian from Volcano #1 in our site, we know that people visited Volcano #1 to get toolstone or that they traded with people who had visited Volcano #1. We can think similar things if we find artifacts or waste material made of obsidian from Volcano #2. If Volcano #1 is far away from our study-site, we can surmise that either people or obsidian trade items moved a long way. Maybe obsidian from Volcano #1 is common in older parts of the site, while obsidian from Volcano #2 is only from more recent deposits; it becomes clear then that the movement of people, raw material, and artifacts changed over time — maybe it was different people or maybe access to Volcano #1 was cut off by new inhabitants around that obsidian, so the villagers began to rely on Volcano #2. Any way you look at it (or argue about it), there is a lot of potential information in obsidian artifacts. But to fully realize that potential, we also need to know about the volcanoes!

Where were the original eruptions? Is the obsidian localized or is it spread over vast areas? Many obsidians erupted millions of years ago; in the meantime, mountain-building earthquakes and eroding rains and rivers have erased the original volcanoes, but yet the obsidian can still be found. If we are going to ask questions of the obsidian at archaeological sites, we need to know the natural distribution of obsidian raw material created by past volcanic events.

I have been searching the Patterned Ground of ancient obsidian nodules to help my archaeological colleagues create better maps relating archaeological sites and their artifacts to the raw material of obsidian toolstone. We know many of the ‘primary’ locations (i.e., a few spots on a map), but the forces that rearrange the volcanic remnants of the Great Basin landscape leave many unknowns — many sources are not spots, they are vast blobs, often tens, even 100s of miles across. Sometimes a single geochemical source — obsidian from the same eruption — is separated by mountain ranges with little or no obsidian in between. In Obsidian Traces, I continue to search for clues in the patterns of the obsidian landscape of the Great Basin.

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