🦴Intro to Archaeology Unit 5 – Stratigraphy & Site Formation

What's Stratigraphy?

• Stratigraphy studies the layers of archaeological deposits to understand a site's chronology and history
• Involves analyzing the composition, thickness, and sequence of strata (layers) in the earth
• Helps archaeologists reconstruct past environments, human activities, and site formation processes
• Relies on the principle of superposition younger layers are deposited on top of older layers, providing a relative chronology
• Stratigraphic analysis is crucial for understanding the context and associations of artifacts and features within a site

Key Principles of Stratigraphy

• Law of Superposition in undisturbed sequences, younger layers are deposited on top of older layers
• Principle of Original Horizontality layers are deposited in a nearly horizontal position, and any deviations indicate later disturbances
• Law of Lateral Continuity a layer will extend laterally until it thins out or reaches the edge of the depositional basin
• Principle of Cross-Cutting Relationships a feature (fault, intrusion) that cuts across a layer is younger than the layer it cuts
• Walther's Law facies (layers) that occur in conformable vertical succession also formed in adjacent environments
  • For example, a sequence of shale, sandstone, and limestone indicates a transition from deep to shallow marine environments

Types of Stratification

• Geological stratification natural layers formed by sedimentary processes (deposition, erosion) over time
  • Examples alluvial deposits, volcanic ash layers, glacial till
• Archaeological stratification layers created by human activities, such as occupation, construction, and abandonment
  • Includes floor surfaces, fill layers, middens (waste deposits), and destruction debris
• Biological stratification layers formed by organic processes, such as the accumulation of plant or animal remains
  • Peat deposits, shell middens, and guano (bat droppings) are examples of biological stratification
• Pedological stratification soil horizons developed through weathering and soil-forming processes
  • Soil profiles can provide information about past environments and land use practices

Site Formation Processes

• Natural processes physical, chemical, and biological factors that affect the preservation and alteration of archaeological remains
  • Erosion, deposition, bioturbation (mixing by organisms), and diagenesis (chemical changes) are examples of natural processes
• Cultural processes human activities that contribute to the creation, modification, and destruction of archaeological sites
  • Includes construction, occupation, abandonment, looting, and modern land use practices
• Post-depositional processes factors that affect the site after its initial formation
  • Soil compaction, frost heaving, and groundwater fluctuations can alter the original stratigraphic relationships
• Taphonomy the study of how organisms transition from the biosphere to the lithosphere (fossil record)
  • Helps understand the biases and limitations in the preservation of organic remains

Excavation Techniques

• Stratigraphic excavation removing layers in the reverse order of their deposition to maintain stratigraphic control
  • Involves carefully defining, documenting, and removing each layer separately
• Arbitrary levels excavating in predetermined increments (e.g., 10 cm) when stratigraphic layers are not visible or difficult to distinguish
• Single context recording treating each depositional event (layer, feature) as a discrete unit and recording it individually
• Harris Matrix a diagram that represents the stratigraphic relationships between layers and features in a site
  • Helps visualize the sequence of events and identify contemporaneous units

Recording and Interpreting Layers

• Stratigraphic profiles vertical sections of the excavated area that show the sequence and characteristics of layers
  • Drawn to scale and annotated with layer descriptions, elevations, and sample locations
• Soil and sediment descriptions recording the color, texture, composition, and inclusions of each layer using standardized systems (Munsell color charts, particle size scales)
• Stratigraphic relationships documenting the interfaces and contacts between layers (conformable, unconformable, gradational)
• Correlation linking layers across different parts of the site or between sites based on their characteristics and stratigraphic position
• Interpretation inferring the depositional environment, human activities, and site formation processes based on the stratigraphic evidence
  • For example, a layer of burnt clay and charcoal may indicate a destruction event or hearth feature

Dating Methods in Stratigraphy

• Relative dating establishing the chronological order of layers and events without assigning specific dates
  • Based on stratigraphic principles (superposition, cross-cutting relationships) and artifact typology
• Absolute dating determining the age of layers or artifacts using scientific methods that provide calendar dates or ranges
  • Radiocarbon dating organic materials (charcoal, bone) up to ~50,000 years old
  • Optically Stimulated Luminescence (OSL) dating sediments exposed to sunlight during deposition
  • Archaeomagnetic dating clay materials that recorded the Earth's magnetic field orientation during firing
• Chronometric hygiene evaluating the reliability and consistency of dates based on sample context, pretreatment, and calibration
  • Helps identify and exclude anomalous or contaminated dates from the chronological sequence

Real-World Applications

• Reconstructing site formation history understanding the sequence of events and processes that shaped the archaeological record
  • Helps distinguish between natural and cultural deposits, identify post-depositional disturbances, and assess the integrity of the site
• Paleoenvironmental reconstruction inferring past landscapes, climate, and ecology based on the sedimentary and biological evidence in stratigraphic layers
  • Pollen, phytoliths (plant silica), and faunal remains can provide insights into vegetation, land use, and subsistence practices
• Cultural chronology establishing the relative or absolute dates of archaeological cultures, phases, or events based on stratigraphic relationships and diagnostic artifacts
  • Contributes to regional and inter-regional comparisons and the study of cultural change over time
• Site management and conservation using stratigraphic information to prioritize areas for protection, develop sampling strategies, and mitigate the impact of development or erosion
  • Helps ensure the long-term preservation and accessibility of archaeological resources for future research and public outreach