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Relative Dating: Stratigraphy, Seriation, and Biostratigraphy

Relative Dating

  • Relative dating is a method used to determine if something is older or younger than something else.

  • The primary goal is to place these 'somethings' (artifacts, strata, features) into a chronological sequence.

The Law of Superposition

  • The Law of Superposition is a fundamental principle that guides all inferences drawn from stratigraphy.

  • Stratigraphy refers to the succession of soil and sediment layers that archaeologists excavate.

  • Principle: In an undisturbed sequence of strata, the oldest layer is at the bottom and the youngest layer is at the top.

  • Implication for artifacts: Things found within the same stratigraphic layer (stratum) were likely buried at approximately the same time.

  • Chronological Rule: Nothing contained within a given stratum can be younger than the stratum itself.

  • Application: Vertical excavations in archaeology are specifically designed to expose a site's stratigraphy, allowing for the proper chronological ordering of cultural layers and the artifacts within them.

Significance of Stratigraphic Analysis

  • Stratigraphic analysis aims to reconstruct the chronological order in which features and strata were deposited.

  • Importance of Absolute Dates: If an absolute date (e.g., from a coin, radiocarbon sample) can be obtained from any single item within a specific layer, this date more or less corresponds to everything found within that same stratum.

  • Exercise Context: An exercise might involve placing labeled features and strata (represented by letters such as B, E, D, A, C, G, I, F, H, K, J, L, M) in their correct stratigraphic order.

  • Challenges: It is important to recognize that some features or strata might be contemporaneous (deposited at the same time), while others might lack clear stratigraphic connection in a profile, making direct association difficult.

Seriation

  • Seriation is a relative dating method based on the idea that artifact styles, types, or popularity change over time in a predictable and often gradual manner.

  • It arranges objects in a series according to their stylistic similarity, assuming that objects with similar styles are closer in time.

Examples of Seriation:

  1. Technological Evolution (e.g., Music Formats):

    • Early examples (like the Public Image Ltd. album from 1986/1987) demonstrate the transition from formats like cassettes to compact discs (CDs), reflecting changes in recording and playback technology over time.

  2. Product Models (e.g., iPhone Models):

    • The sequence of iPhone models from 2007 to 2017 (e.g., 2G, 3G, 3GS, 4S, 5, 5C, 5S, SE, 6, 6S+, 7, 8+) illustrates how product designs evolve, allowing for a chronological ordering based on the presence or absence of specific models.

  3. Projectile Point Traditions of the American Southeast:

    • This detailed archaeological sequence showcases the evolution of projectile point styles across thousands of years:

      • Paleo-Indian Period (12000-8000 B.C.): Clovis, Hardaway Blade, Hardaway, Hardaway Side Notched.

      • Early Archaic Period (8000-5000 B.C.): Palmer, Le Croy, Badin, St. Albans, Halifax, Corner Notched, Stemmed, Kanawha, Big Sandy, Guilford.

      • Middle Archaic Period (5000-3000 B.C.): Stanly, Morrow Mountain I, Morrow Mountain II.

      • Late Archaic Period (3000-1000 B.C.): Savannah River.

      • Early Woodland Period (1000-500 B.C.): Yadkin, Uwharrie, Gypsy.

      • Middle Woodland Period (500 B.C.-A.D. 800): Tar A-21.

      • Late Woodland Period (800-1600 A.D.): Pee Dee, Caraway.

      • Historic Period (1600-onwards): Hillsboro, Randolph.

    • This seriation provides a robust chronological framework for cultures in the region based on diagnostic artifact types.

  4. Stylistic Sequence from a Cemetery in Stoneham, Massachusetts:

    • Gravestone motifs changed predictably over time:

      • 1720s-1770s: Death's Head (oldest).

      • 1770s-1800s: Cherub Head.

      • 1790s-1820s: Urn and Willow (youngest).

    • This demonstrates how stylistic preferences can indicate relative age.

  5. Southwestern Pottery Seriation (e.g., Pajarito White Ware, Rio Grande Glaze Ware):

    • Pottery types like Kwahe'e B/W (1000 A.D.–1100 A.D.), Chupadero B/W, Santa Fe B/W, Pindi B/W, Glaze I Agua Fria G/R, Glaze II, Glaze V Pecos, and Tewa Polychrome each have distinct chronological ranges.

    • The presence of specific pottery types in a stratigraphic layer can help date that layer relatively within the cultural sequence from 1000 A.D. to 1800 A.D.

Biostratigraphic Dating: Pollen Zones

  • Biostratigraphic dating utilizes biological remains, such as pollen, to establish relative chronological sequences based on changes in flora over time.

  • Pollen Zones: These are divisions of time characterized by specific assemblages of pollen, reflecting broader climatic and vegetational shifts.

  • European Pollen Zones (J. Iversen, 1954) provide a classic example:

    • Zone IX (Sub-Atlantic): 500 BC to present. Dominant type: Spread of grasses and pine/beech woodland.

    • Zone VIII (Sub-Boreal): 3000-500 BC. Dominant type: Mixed oak forest.

    • Zone VII (Atlantic): 5500-3000 BC. Dominant type: Mixed oak forest.

    • Zones V and VI (Boreal): c. 7700-5500 BC. Dominant type: Pine/birch forest and increasing mixed forest.

    • Zone IV (Pre-Boreal): c. 8300-7700 BC. Dominant type: Birch forest.

    • Zone III (Younger Dryas): c. 8800-8300 BC. Dominant type: Tundra.

    • Zone II (Allerød Oscillation): c. 9800-8800 BC. Dominant type: Tundra, Park Tundra, and birch forest.

    • Zone I (1c Older Dryas): c. 10,000-9800 BC. Dominant type: Tundra.

    • Zone I (1b Bølling Oscillation): c. 10,500-10,000 BC. Dominant type: Park Tundra.

    • Zone I (1a Oldest Dryas): c. 13,000-10,500 BC. Dominant type: Tundra.

  • Pollen Diagrams: These are visual representations showing the concentration of different pollen types (e.g., Pinus, Nymphaea, Ferns, Amaranthaceae) at various depths (in cm) within a sediment core.

    • Changes in pollen concentrations define different pollen zones, helping to reconstruct past environments and establish relative chronologies.

Integrated Chronological Approaches

  • Modern chronological studies often integrate multiple lines of evidence to build robust timelines.

  • A comprehensive approach might combine:

    • Stratigraphy: Understanding the order of deposition of layers and the macro-remains, pollen, and charcoal particles found within them.

    • Pollen/Charcoal Analysis: Identifying pollen zones and charcoal concentrations to infer past vegetation, climate, and human activity (e.g., clearances, forest cover changes, wetland environments).

    • Geochemistry: Analyzing chemical properties of sediments (e.g., Pb, As, Zn levels) to identify environmental changes or human impact.

    • Absolute Dating (Chronology): Using methods like ^{14} ext{C} samples (e.g., MAMS-30884, MAMS-30883, MAMS-32962) to provide calibrated absolute dates (e.g., in cal CE) for specific stratigraphic layers, anchoring the relative sequence to a calendar timeline.

  • Synthesis: By combining these methods, researchers can construct a holistic understanding of the chronological sequence, environmental history, and human-environment interactions at a site. This often involves statistical modeling, such as Bayesian models, to refine probability intervals for dating.