1. Geoarchaeology and Site Formation Processes

2. This chapter will enable you to answer these questions: 1. What is geoarchaeology? 2. What is the law of superposition? How can it be violated? 3. What is the difference between systemic and archaeological contexts? 4. Why does this difference matter?

3. Outline  The Law of Superposition  Reading Gatecliff’s Dirt  Is Stratigraphy Really that Easy?  Site Formation Processes: How Good Sites Go Bad  Conclusion

4. Introduction: Geoarchaeology Geoarchaeology applies the concepts and methods of the geosciences to archaeological research. Objectives: Place sites and artifacts in a context through the application of stratigraphic principles and dating techniques. Understand the natural processes of site formation, the human and natural actions that work together to create an archaeological site.

5. Introduction: Geoarchaeology Geomorphology, the geological study of landforms and landscapes, including soils, rivers, hills, sand dunes, glacial deposits, and marshes.

6. The Law of Superposition  In any pile of sedimentary rocks that have not been disturbed by folding or overturning, each bed is older than the layers above and younger than the layers below; also known as Steno’s Law.  Sedimentary rock, rock formed when the weathered products of preexisting rocks have been transported by and deposited in water and are turned once again to stone.

7. The Law of Superposition  This principle seems simple, but it was a critical observation in the 17th century when formulated by Nicolaus Steno.

8. The Law of Superposition: Fossil Footprints at Laetoli  Mary Leakey (1913-1996) found evidence of hominins, members of the evolutionary line that contains humans and our early bipedal ancestors.

9. Fossil Footprints at Laetoli: Geological Background  Fossil footprints were contained in the upper portion of Laetolil beds, Tuff 7

10. Fossil Footprints at Laetoli: Geological Background  The ash buried the footprints rapidly, soon after they were formed.  Lack of evidence of grasses suggests eruptions took place during dry season.  Raindrop impressions occur along with footprints and widespread erosion, suggesting rainy season downpours.

11. Fossil Footprints at Laetoli: How Old Are the Footprints? Leakey surmised that the age of the footprints at Laetoli was more than a million years older than the oldest known tool use. Leakey, Robert Drake and Garness Curtis, used potassium-argon dating on samples from the major stratified layers recognized in the Laetoli area.

12. Fossil Footprints at Laetoli: How Old Are the Footprints? The fossil hominid footprints are between 3.49 and 3.56 million years old. With the dating of the Laetolil footprints, Leakey showed that humans were bipedal long before they made stone tools.

13. Fossil Footprints at Laetoli: What Happened to the Laetoli Footprints?  When Leakey completed her work, she backfilled the site with 2 feet of soil.  After a few years, trees grew on the spot.  In 1995, archaeologist Fiona Marshall unearthed the trees’ roots without disturbing the tracks.  In 100 years, the footprints will be uncovered again and if possible safely removed to a museum.

14. Reading Gatecliff’s Dirt  Gatecliff Shelter has a 40-foot stratigraphic profile covering more than 7,000 years.  The Gatecliff sediments, like those of all archaeological sites, resulted from both natural processes and human behavior.

15. The master stratigraphic profile from Gatecliff Shelter. The standing figure is exactly 6 feet tall, and the grid system shows 1-meter squares. Gatecliff Shelter

16. An exposure of the bottom half of the stratigraphic column at Gatecliff Shelter. At the very bottom is Stratum 55 (about 7000 years old). Rob Rowan and Dennis O’Brien are standing on Stratum 39 (comprised of sand and fine silt and dating about 6200 years old). They are pointing to Stratum 26 (a rubble unit that is about 5500 years old).

17. Reading Gatecliff’s Dirt: Gatecliff’s Stratigraphy  16 living surfaces, containing fire hearth’s, charcoal, broken stone tools, grinding slabs, flakes, food remains, and occasional fragments of basketry and cordage:  evidences what was brought into the cave by human activity.

18. Reading Gatecliff’s Dirt: Gatecliff’s Stratigraphy  Alluvial sediments, sediments transported by flowing water.  Eolian sediments, materials transported and accumulated by wind (for example, dunes.)

19. Reading Gatecliff’s Dirt: Marker Beds  Marker bed, an easily identified geologic layer whose age has been independently confirmed at numerous locations and whose presence can therefore be used to date archaeological and geological sediments.  Stratum 55 contained sand-sized volcanic ash (tephra) from the eruption of Mount Mazama, 6900 years ago.

20. Hypothetical Rockshelter, Filling with Colluvial and Eolian Sediments, and Rooffall

21. Reading Gatecliff’s Dirt: Soils Soil, sediments that have undergone in situ chemical and mechanical alteration. A horizon, the upper part of a soil, where active organic and mechanical decomposition of geological and organic material occurs.

22. Reading Gatecliff’s Dirt: Soils B horizon, a layer found below the A horizon, where clays accumulate that are transported downward by water. C horizon, a layer found below the B horizon that consists of the unaltered or slightly altered parent material; below the C horizon is bedrock.

23. Is Stratigraphy Really That Easy? Human and natural processes disturb the sediments, moving things up and down. Pithouse, a semi-subterranean structure with a heavy log roof covered with sod, can be dug into a previous campsite moving older material upward in the stratigraphic sequence.

24. Is Stratigraphy Really That Easy? Reverse stratigraphy, the result when sediment is unearthed by human or natural actions and moved elsewhere in such a way that the latest material is deposited on the bottom of the new sediment and progressively earlier material is deposited higher and higher in the stratigraphy.

25. Development of a Hypothetical Archaeological Site

27. Formation processes in the systemic context  A systemic context is a living behavioral system wherein artifacts are part of the ongoing system of manufacture, use, reuse, and discard.  Once artifacts enter the ground, they are part of the archaeological context, where they can continue to be affected by human action, but where they also are affected by natural processes.  Formation processes are the ways in which human behaviors and natural actions operate to produce the archaeological record.

28. Formation Processes in the Systemic Context: cultural depositional processes Cultural depositional processes, human behaviors by which artifacts enter the archaeological record, including discard, loss, caching and ritual interment. 1. Discard – Everything eventually breaks or wears out and is discarded. 2. Loss – Example: An arrow that misses its target or a pot left at a camp. 3. Caching – Some items are intentionally left behind. 4. Ritual – Example: grave goods.

29. Formation Processes in the Systemic Context: Reclamation Processes  Reclamation processes, human behaviors that result in artifacts moving from the archaeological context back to the systemic context, as in scavenging beams from an abandoned structure to use them in a new one.

30. Formation Processes in the Systemic Context: Cultural Disturbance Processes  Cultural disturbance processes, human behaviors that modify artifacts in their archaeological context, as in the digging of pits, hearths, canals, and houses.

31. Formation Processes in the Systemic Context: Reuse Processes  Reuse process, human behaviors that recycle and reuse artifacts before they enter the archaeological record.  This can entail the recycling of some objects:  Potsherds are ground up and used as temper in manufacturing new vessels.  Broken arrowheads are re-chipped into drills.  Note: It is reuse if beams are taken from a currently occupied building; reclamation if taken from a building long abandoned.

32. Formation Processes in the Archaeological Context  Once an object enters an archaeological context, a host of natural as well as cultural formation processes takes place.  These natural processes determine whether organic material will be preserved and where objects will be found.  There is no simple correspondence between the distribution of artifacts in a site and human behavior.

33. Formation Processes in the Archaeological Context: Natural Formation Processes  Floralturbation – a natural formation process in which trees and plants affect the distribution of artifacts within an archaeological site.  Faunalturbation – a natural formation process in which animals, from large game to earthworms, affect the distribution of material within an archaeological site.  Cryoturbation – a natural formation process in which freeze/thaw activity in a soil pushes larger artifacts to the surface of a site.

34. Formation Processes in the Archaeological Context: Natural Formation Processes  Argilliturbation, a natural formation process in which wet/dry cycles in clay-rich soils push artifacts upward as the sediment swells and then moves them down as cracks form during dry cycles.  Graviturbation, a natural formation process in which artifacts are moved downslope through gravity, sometimes assisted by precipitation runoff.

35. Effects of Natural Formation Processes on Distribution of Artifacts

38. How Artifacts Become Oriented to the Direction of River Flow

39. Summary Questions 1. What is geoarchaeology? 2. What is the law of superposition? How can it be violated? 3. What is the difference between systemic and archaeological contexts? 4. Why does this difference matter?