5.1 Principles of Stratigraphy

Stratigraphy is like reading a history book written in layers of dirt. By studying these layers, archaeologists can piece together the story of a site, from oldest events at the bottom to newest at the top.

Understanding stratigraphy helps us uncover how people lived and how their cultures changed over time. It's like peeling back layers of an onion, revealing the past one layer at a time.

Principles of Stratigraphy

Concept of stratigraphy

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  The Geologic Time Scale – Physical Geology Laboratory View original

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• Study of layered materials (strata) and their chronological sequence
  • Layers deposited over time, with oldest at bottom and youngest at top (sedimentary rocks, archaeological sites)
• Crucial in archaeology for establishing relative chronology, interpreting sequence of events, and providing context for artifacts within layers
  • Helps understand cultural changes and human activities over time (transition from hunter-gatherer to agricultural societies)

Law of superposition

• In a sequence of undisturbed layers, oldest layer is at bottom, and each successive layer is younger than one below it
  • Applies to sedimentary rocks and archaeological sites
• Allows archaeologists to establish relative dates for artifacts and features
  • Artifact found in lower layer is older than one found in upper layer (pottery shards, stone tools)
• Helps determine order in which events occurred at a site and provides framework for understanding cultural changes over time
  • Transition from one cultural period to another (Paleolithic to Neolithic)

Principles of stratigraphic relationships

• Original horizontality: layers deposited in nearly horizontal position
  • Deviations from horizontal indicate later disturbances or deformation (folding, faulting)
• Lateral continuity: layers extend laterally until they thin out or reach edge of depositional basin
  • Layers can be correlated across a site or region (ash layer from volcanic eruption)
• Cross-cutting relationships: feature that cuts across another is younger than feature it cuts
  • Establishes relative chronology between features (walls, pits, intrusions)
  • Example: a pit dug through an existing wall is younger than the wall

Types of stratigraphic units

• Layers (strata): distinct, relatively uniform units of sediment or soil deposited horizontally
  • Represent a specific period or event in site's formation (flood deposit, occupation layer)
• Lenses: small, lens-shaped deposits within a layer
  • Often represent localized activities or short-term events (hearth, trash pit)
• Features: non-portable elements of an archaeological site
  • Can be constructed (walls, buildings), excavated (pits, ditches), or naturally formed (post holes, tree roots)
  • Provide information about site structure and human activities (storage pits, burial sites)

Interpreting Stratigraphic Sequences

Using stratigraphic sequences to reconstruct past events and environments

• Stratigraphic sequences provide a timeline of events at an archaeological site
  1. Oldest events at bottom, most recent at top
  2. Layers represent different periods of occupation, abandonment, or destruction
• Archaeologists use stratigraphic sequences to:
  • Infer changes in human activities over time (shift from hunting to farming)
  • Reconstruct past environments and landscapes (climate change, deforestation)
  • Identify periods of occupation, abandonment, or destruction (natural disasters, wars)
• Stratigraphic sequences can be correlated with other sites in the region
  • Establishes a broader chronology and understanding of cultural interactions (trade routes, migration patterns)
  • Example: a distinctive pottery style found in multiple sites can indicate cultural exchange or shared traditions