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Toward a Comprehensive Taxonomy of Earthen Architecture
Edward Crocker, Gustavo Araoz


Most practitioners in the field of earthen architecture, whether in new construction or conservation, probably believe that they understand and can define "earthen architecture." Until they try.

The vagaries of the term became clear to many members of the ICOMOS Specialized Committee on Earthen Architectural Heritage during a broad-based email exchange in the Fall of 2006. It was then widely acknowledged that we seem to have a very clear understanding of, and have cogently articulated, the various methods of production that lead to earthen buildings. These range from defensive earthwork trenches to sun-dried bricks, puddled mud and extruded soil blocks. What we don't seem to have articulated well are the structural typologies that result when the materials are assembled.

The email exchange highlighted some differences of opinion such as, for example, is a non-load bearing curtain wall built of earth, earthen architecture? And, as another example, is there a point at which stone laid up in mud mortar ceases being earthen and is typologically cast as something else? The repartee was loaded with bits of esoteric knowledge and, occasionally, a sense of the absurd.

The authors note, with some disappointment, that though other ICOMOS Scientific Committees (Vernacular Architecture, Archeological Heritage, Structures and Wood) were invited to participate in the formulation of the taxonomy, we had no substantive responses.

This discussion is not intended to be conclusive, but rather to advance discussion of the taxonomy of earthen architecture. The entire string of emails that originally inspired the dialogue has been chronologized, but is of such a length that it cannot be including in these proceedings. It is available here.

What is taxonomy, and why do we need one?

A taxonomy is a classification of related objects into an order that allows us to (1) facilitate the retrieval of information, (2) distinguish between varieties of similar things, and (3) help explain the basis of variation among typologically associated items.

Taxonomies vary depending on the use they are created to serve. Thus, identifying the intended use must precede the establishment of any good taxonomy. In the simplest of human taxonomies, there is a given name and a name that links each human to a patrilineal or matrilineal line and/or clan. Such taxonomy is insufficient in the world polity, which requires many additional classifications, all depending on the intent, such as country of citizenship, race, religious affiliation, sex, age, profession/trade, etc. The many classifiers used to group humans in ways that they can be studied statistically for different purposes is proof that intended use must precede the development of any taxonomy.

Taxonomy, in other words, allows us to place objects in rational order while simultaneously footnoting their distinguishing characteristics. Good taxonomy is what enables naturalists to point at critical issues and identify species that are endangered or disappearing.

Various taxonomies have been loosely developed to classify the built environment for a variety of purposes, but none has proven to be universally satisfactory in terms of broad applicability. For instance, there are building taxonomies based on the use for which they were built, such as religious, residential, civic, commercial, industrial, agricultural, et al. Within each of these overarching headings, there are subheadings, such as churches, chapels, shrines, cathedrals, basilicas, monasteries, convents, etc., for religious buildings within in the Christian faith. These taxonomies are commonly used for inventories, surveys and registers.

Buildings have also been classified according to their structural systems and construction methods (lintel-arch, wood framed, steel framed, vaulted; plus stacked masonry, form-based).

Buildings forms and embellishment, together with the establishment of acceptable time spans, are used to develop a taxonomy or classification of historic architectural styles.

The field of urban morphology constantly develops another type of taxonomies based not on intended building use but on the physical form of the buildings (e.g., volume, shape, lot location, relation to other buildings). Such taxonomies are useful only within the narrow context or the urban environment in which they exist, and are applied as a tool in protecting the character of historic urban districts.

In the case of earthen architecture, and of historic structures in general, a good taxonomy with universal or, at least, multiple applications would similarly help identify uniqueness, patterns, best representatives, etc. -- all important not only for establishing significance, but also priorities in treatments and protection.

On the World Heritage level, in order for us to be able to assess Outstanding Universal Value, we need to know the global context, taxonomies and geographical spreads. One of the big challenges in evaluating nominations to the World Heritage List lies in our inability to say with unwavering certainty that a nominated site is the only one of its kind, the best of its kind or perhaps even the most representative of its kind.

A good taxonomy, which does not exist in this field, is very likely to accelerate the inscription of earthen architectural sites on the World Heritage List as well as inform conservation approaches, site management and identify the threat of "extinction" of a typology.

What kind of taxonomy would be useful and how do we derive it?

A review of taxonomical approaches indicates that there are hundreds, perhaps thousands, of taxonomies for everything from snails to software architecture.

Without doubt, the most well known taxonomies are those advanced by Carolus Linnaeus (1707-1778) in his Systema Naturae (1735), in which he outlined the hierarchical classification of the natural world, dividing it into the Animal Kingdom, Plant Kingdom and Mineral Kingdom. The usefulness and popularity of his system led eventually to thirteen editions, each greatly expanded, from eleven pages in the first to three thousand in the thirteenth. Linnaean taxonomy successively refines and specifies according to Kingdom, Phylum, Class, Order, Genus and species. Thus, a domestic dog would be identified as Animalia, Chordata, Mammalia, Carnivora, Canis and familiaris. It is an elegant and successful methodology that works well for animals, less well for plants and falls apart entirely with minerals. It also fails in helping to clarify an approach to the understanding of earthen architecture.

However, Linnaeus' contemporary and chief rival, Baron George Leclerc Buffon (1708-1788) in his Histoire Naturelle, comes to our aid by providing a nonhierarchical system that joined each species to some others by physiology, to a different group by anatomy, and to a still different set by ecology.

Nevertheless, attempting to isolate a system that could be readily adapted to the present question was somewhat frustrating to the authors who, under duress, think that it makes sense to start from scratch despite the noble contributions of antecedents.

There are five essential steps to building a taxonomy:

  1. Determine the scope of the system
  2. Ascertain the authorities whose findings will be accepted by the interested constituency
  3. Extract concepts from the authorities and other sources
  4. Organize the concepts into a useable format
  5. Validate, review and refine the taxonomy through use.

While structuring the format, we must also keep in mind the following questions:

  1. How best can we access/utilize the information?
  2. How best do we apply it to treatment and protection (standards) in earthen architecture?
  3. How best can we make the information useful in the World Heritage forum?

Building a taxonomy of earthen architecture

The review of taxonomies noted above makes one thing very clear: agreement about relationships does not guarantee consensus about importance. Therefore, in an effort to remain scrupulously objective, we have abstracted some of the key words and concepts from the email string and made some assumptions that we feel address the purpose of the exercise as outlined above.

Step One: Determine the scope

The initial impetus for deriving a taxonomy for earthen architecture came from an enquiry (Crosby) to colleagues to help create ". . . a map of the US broken down in regions that seem most appropriate for brief descriptions of the architectural types in those areas." The idea caught hold, but very rapidly expanded first to a global scale (Araoz) and subsequently to include experts in other, related heritage fields.
In terms of technical inclusion, the authors are embracing the full realm of possibilities; anything that anyone thinks should be classified as earthen architecture will be included in discussions.

Step Two: Ascertain the authorities

The authors note that the attempt to include practitioners in other heritage fields has not yet been productive. Nevertheless, we fully expect that those practitioners will come forward as the discussions proceed. Thus, though the door is open, the present authorities are members of the ICOMOS Specialized Committee on Earthen Architectural Heritage.

Step Three: Extract concepts from the authorities and other sources

Other writers (Houben, Guillaud, others) have very succinctly and cogently described the various processes involved in the construction of earthen architecture. We do not seek to further refine that work, which by now has self-validated over time and use. What we do seek to refine is the way in which we refer to the products derived from those processes. As an example, the literature and local knowledge is clear as to the classification of an adobe as an unfired masonry unit used in the construction of buildings. What is somewhat less clear is how function (load bearing, non-load bearing, decorative) affects the classification of the resulting building.
Thus, our goal is ultimately to provide a comprehensive list of buildings and structures, define them and place them in a taxonomy that serves our stated purposes.
In order to provoke discussion and perhaps help present a format, the following abstracts some the discussions that took place in the email string.
    Agreement as to conforming
  • Load bearing vs. non-load bearing (relating to function). The consensus seems to be that this distinction is not one that should be used to distinguish earthen architecture from all other typologies, but should be used distinguish function within the Realm.
  • "Built" vs. "formed" (relating to the method of construction). Here, we may need to avoid the multiple meanings of the word "formed" (as in shuttering for pise de terre) as opposed to "excavated." The consensus clearly is that defensive earthworks (e.g., trenches) and other features of whatever purpose, formed by the removal of material, belong solidly in the Realm. Do we, however, need to note, perhaps elaborate upon, the fact that such "formed" earthworks are both a process and a defined typology?
    Lack of agreement as to conforming
  • Majority vs. minority component. Here there is disagreement. Some (Watson, others) think that the Kingdom should include only structures with a principal functional component of unfired soil, the definition of which is also subject to discussion and definition (Thomson). Others (Mold, Hare, Hurd) note that such a definition excludes field stone laid up in mud mortar as well as other applications in which unfired earth is an essential, but not majority component.
  • Composite (with non-earthen materials) vs. traditional. One observer (Crocker) is reluctant to acknowledge that asphalt-stabilized adobes laid up in Portland cement, sprayed with polyurethane and plastered with elastomerics can comprise "earthen" buildings even though the majority component is unfired soil. Another (Hurd) notes that reclaimed cinder blocks infilled with mud should be considered, essentially for cultural reasons.
    Presented but not discussed
  • Treatment as a defining factor. Several observers (Crosby, Dowdy, Crocker, others) suggest or imply that the maintenance or conservation of a structure may be one of the factors that help define it as earthen. The premise appears to be that if one component (earth) among many (stone, wood, lime) is that which requires either the most or the more sophisticated treatment, then a distinction may be drawn that leads to classification.
  • Erudite vs. vernacular. One observer (Hurd) distinguishes between three general typologies: historic, ethnic, and aboriginal. The authors draw a distinction here that may be other than that originally intended, and that is an implied distinction between structures that are no longer in use and those that are. Archeological sites, even reconstructed ones, might typically be thought of as requiring a different approach to conservation than, say, a historic cob tavern that is still in use. Clearly there would be some overlap as, for example, in the case of Taos Pueblo, New Mexico or the center city of Yazd, Iran. In both cases, the architecture is erudite in its singularity, but vernacular in origin; the two examples are ancient in both form and materials, but have undergone regular modification to accommodate use over time. On the other hand, non-occupied sites such as Pueblo Bonito, New Mexico; Paqime, Mexico; and Bam, Iran are subject to very strict conservation principles that do not allow for change over time.
  • Modern derivatives of traditional knowledge. There is general agreement that mass-produced compressed earth blocks and non-stabilized adobes produced with the assistance of machinery can be used in the construction of validly classified earthen architecture. However, do burlap bags filled with a mix of clay, sand and lime stacked into walls and saturated, qualify?

Step Four: Organize

The authors throw open the door to all interested parties to assist in compiling the structure of the taxonomy which might include, among many more, the following key words and phrases:
Earth as a defining characteristic.
Local soils.
Amended soils.
Natural lime.
Local tradition.
Load bearing.
Non-load bearing.
Earthen mixed (or composite) technologies.
Monumental or "massive."
Mixed use.
Defining characteristic must be integral.
Defining characteristic may be purely decorative.
Rammed earth.
Compressed earth block.
Earth sculpture.
We have not attempted to break down regional typologies based on use and function. We see this as being part of a global effort by our colleagues to report upon extant surveys, or to create, implement and report upon new surveys. Such surveys might include, among others, some of the categories listed above.
It is our challenge to the community of earthen architecture conservators, historians, engineers, architects, contractors and all interested parties to begin assembling the body of the taxonomy. The authors and the ICOMOS Specialized Committee on the Earthen Architectural Heritage look forward to advancing the discussion.

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