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Texas Archeology Academies

Curriculum for Geoarcheology Academy

Recognizing and Evaluating the Archeological Potential of the Landscape:
An Introduction to Geoarcheology

I. Landscapes: Archeological Context, Preservation, and Visibility

A. What is “Geoarcheology”?

B. Archeological sites as artifacts of geologic processes

  1. From past human behavior to the current archeological record
  2. Middle range theory and site formation processes

C. Landscape settings influence site context, visibility, and preservation

  1. Archeological Context
    This describes the spatial and temporal relationships between archeological materials and their host sediments and geologic deposits.
  2. Site Preservation
    The archeological record does not accurately reflect the complete pattern of sites that once existed on the landscape through time, but instead reflects the biases of geologic processes.
  3. Site Visibility
    Dependent upon landscape position, geologic setting, and erosion/sedimentation history; site age is a relatively minor influencing factor.

II. Elements of the Landscape: Environments, Processes, Landforms, and Other Features

A. Terrestrial Environments
Landscape development and evolution is driven by lithology, tectonics, climate, and a complex of systems that erode, transport, and deposit sediments from the periphery of basins or uplands to basin or valley bottoms.

  1. Uplands Soil formation (surface stability) or erosional processes prevail, with deposition being very localized.  There is high potential for artifacts and sites of various ages to be in common context.  Archeological visibility tends to be greater, but the potential for sites with good contextual integrity is low.  Common landforms and features include:

    a. Sand dunes
    b. Loess and dust sheets
    c. Playas and lunettes
    d. Sinkholes

  2. Slopes
    Slopes are the most common landform element (every surface has one!).  Slope position determines whether stable, erosional, or depositional processes prevail, therefore archeological context, preservation, and visibility are influenced by slope position.  The greatest potential for the preservation of sites with good contextual integrity is on the lower slopes or toeslope environments, where deposit prevails.  Common landforms and features include:

    a. Alluvial fans
    b. Colluvial toeslopes
    c. Rockshelters

  3. Valley Bottoms
    Depositional environments prevail (erosion tends to be localized), so site preservation prevails.  Archeological visibility tends to be low, but the potential for sites with good contextual integrity is high.  Common landforms and features include:

    Braided streams
    a. Longitudinal bars
    b. Transverse bars
    c. Lateral bars

    Meandering streams
    a. Point bars
    b. Natural levees
    c. Crevasse splays
    d. Oxbow lakes

    All streams
    a. Floodplains
    b. Terraces

B. Coastal Environments
Landscape development and evolution is driven by sea level (eustatic) change, tectonic uplift, and subsidence, and the complex interplay between fluvial processes that deliver sediments to the coast and marine processes (wave and tidal action) that rework, transport, and deposit these sediments.

  1. Erosional Coasts
    Wave action is a major erosional agent.  In terms of preservation, sites are continually lost because the shoreline is transgressive or eroding landward.  Archeological context and visibility will vary, though the surface of these coastlines tend to be very ancient with spotty Holocene depositional mantles, therefore site context tends to be poor and visibility high.  Common landforms and features include:

    a. Wave-cut cliff or terrace and platform

  2. Submerged Coasts
    Submerged coasts have irregular configurations with headlands and embayments.  The embayments are estuaries, river valleys that were inundated by rising sea levels. Biologic-rich and productive estuaries were attractive environments for prehistoric peoples who frequently occupied the upland margins that circumscribe estuaries.  These upland surfaces are typically ancient surfaces with patchy Holocene depositional mantles, therefore site context tends to be poor, visibility high, and preservation variable.  Common landforms and features include:

    a. Estuaries

  3. Depositional Coasts
    Prograding shorelines are typically environments where large amounts of sediment are delivered to the coast by rivers and reworked, transported, and deposited by wave, tidal, and wind action.  Because deposition is prevalent, archeological context and preservation tend to be good, whereas visibility is poor. Common landforms and features include:

    a. Deltas
    b. Beaches
    c. Barrier islands and lagoons
    d. Tidal flats and marshes

III. Soils vs. Sediments

One of the first steps in interpreting natural deposits is recognizing the components of a deposit.  In most archeological sites the two fundamental units are sedimentary deposits and soils.  Although the terms are often used interchangeably, they are fundamentally different things as will be pointed out.  In this section clues to recognizing these two different kinds of features common to every archeological site are examined.

    A.  Sedimentary deposits or sediments
    These deposits are composed of particles that are moved from one place to another by water, wind, gravity, or people and are the most common deposits at most archeological sites.  This section will describe in more detail what they are, their major attributes, and the kind of information they may contain.

    B. Soils
    The in-place modification of rocks and sediments by chemical and physical processes at/near the air/ground interface creates the features we call soils.  These features form in a completely different manner from sediments, and are very common components of most landscapes.  This section will examine the component parts of soils, how they form, and why recognizing and distinguishing them is critical in archeological contexts.

Practical Illustration: Various soil and sediment attributes from hand specimens
This 30-minute “show and tell” will provide examples of features discussed in the Soil vs. Sediment section.  This component can also expand into a profile description exercise in the field.

IV. Stratigraphy

The term stratigraphy refers to a method of organizing deposits with respect to other deposits. Deposits generally follow a suite of stratigraphic “laws,” but also important here are the various ways we can organize deposits.

    A. Basic stratigraphic principles

    B. Commonly used forms of stratigraphic organization

    1. Lithostratigraphy
    2. Chronostratigraphy
    3. Biostratigraphy
    4. Allostratigraphy
    5. The Harris Matrix

    C. Scale in stratigraphy

    1. Test Unit
    2. Site
    3. River Valley
    4. Region

Practical Exercise: Field texture
This 30-minute exercise will cover the fundamental aspects of identifying the texture of a deposit in the field as well as providing samples of sediments of different textures.

V. Dating

Dating geological and archeological deposits is one of the most important aspects of obtaining an accurate historical record.  This topic will cover two different dating techniques in detail, focusing on how they work and what you need to know to interpret the results.

    A. Radiocarbon dating

    1. Basic principles (Libby’s Assumptions)
    2. What we now know about Libby’s assumptions (they are all wrong…)
    3. Factors affecting accuracy of radiocarbon ages

    B. Optically Stimulated Luminescence Dating (OSL)

    1. Basic Principles
    2. Kinds of OSL dates
        a. Single aliquot (averaged populations)
        b. Single grain (personal grain interrogations…)

VI. Integration

Ultimately, to understand the true nature of the archeological record we need to integrate landforms with their underlying deposits (often sediments) and age information to understand the long-term history of the landscape.  This, of course, is an ideal; we rarely have enough data to work with.  This brief discussion will attempt to show how geoarcheological information can be used to tailor archeological surveys to variable landscape conditions, as well as obtain better archeological information.

    Example landscapes
    1. Southern High Plains
    2. Alluvial Lowlands



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April 20, 2013