Bridge River Housepit Geophysics

Magnetometry map of the Bridge River Site housepits. Source: Prentiss et al. 2009

Wouldn’t it be nice to be able to see what was underground without having to go through the time-consuming, expensive and destructive process of digging it up?  There are some nice geophysics techniques in archaeology for doing just that, though none are yet a substitute for excavation.  I noticed the other day (and you were quizzed on it) that there is a very comprehensive recent report online (45 meg PDF) by Anna Marie Prentiss and colleagues, on work at the middle Fraser pithouse village of Bridge River (EeRl-4).  This village lies in the territory of Bridge River Band (Xwisten) and the St’át’imc Nation.  While there is a huge amount of archaeological interest across the 350 page report as a whole, it was the use of geophysics on an interior pithouse village which got my attention.

Diagram of a Pit House. Source: Teit 1928.

First, some readers may need a bit of background.  For the last 4,000 years or so, the semi-subterranean pithouse was a common wintertime dwelling on the interior Plateau of the Northwest.  Such pithouses were generally circular, were excavated several feet into the ground, and covered by a heavy-timbered, earthen roof.  Upon abandonment, the wooden elements largely disappeared and the roof collapsed, leaving behind an array of circular, crater-like features behind, each of which might be from 8 to 15 or even more metres in diameter

Aerial view of the Bridge River site. Each cup-shaped depression is a former house. Source: Prentiss 2009.

With their prominent surface visibility, and working from the principle that each house was a coherent spatial unit of analysis which reflected a true social unit, then pithouse archaeology was an early, and leading element of a major archaeological movement of the past few decades known as “household archaeology”.  While the first, pioneering pithouse excavation projects are closely associated with David Sanger and Arnoud Stryd, it was the energetic foray of SFU’s Brian Hayden into the field at Keatley Creek in the middle 1980s which really ushered in the true social archaeology of Plateau Pithouses.

Working in Housepit 20 at Bridge River site. Source: Prentiss et al 2009.

Hayden blazed an original and compelling theoretical path and spawned a veritable flower arrangement of work on that large village site. More recently, there has been a very active debate between Hayden and Anna Prentiss over the timing and meaning of the major cultural historical events at Keatley Creek.  Meanwhile, as noted the other day, the empirical basis for some of Hayden’s conclusions, such as the confident speciation of salmon remains, has also been undercut by new methods.  So Hayden’s original conclusions have frayed somewhat as new researchers have followed his path.  It must be said, though, that when following this debate from my little foxhole on the coast, it is notable how these newer researchers have noted the collegial co-operation of Brian Hayden as they seek to challenge and in some cases refute his conclusions.  Letting other archaeologists have access to your own data, specimens and “your own sites”, knowing that they might reach different and perhaps more persuasive conclusions, is the true mark of a good colleague and that has been refreshing to see.  (I mean, maybe this is just people making nice in their acknowledgments but it doesn’t read that way). (Also, Hayden wrote one of the best and accessibles books about BC Archaeology to date).

OK – so, pithouses are extremely challenging to excavate, moreso than you might think.  The earthen roof is a site-formation nightmare, poising older, dug-out sediments above the more recent occupation levels.  What goes up, must come down.  Further, unless you like living under a creaking, 10-ton roof, you will need to renovate your structure, perhaps every 20 years or so.  In so doing, you may take down the support posts, rip apart the roof, re-dig the floor, and re-assemble and re-cover the roof.  Basically, you, the 2,000 year ago St’át’imc ancestor, thoughtlessly rips the heart out of future archaeologists.  Awkward.

Plateau Pithouse formation processes. Source: Hayden 2005

This process is nicely illustrated by a diagram by Hayden you can see above – click for larger but not great image.  If you follow it through you get the idea about the shifting target of the house floors and the possibility for changing size of house between occupations – both of which were central issues in the debates over the last 5 years or so between Hayden and Prentiss in the pages of American Antiquity and elsewhere.  Hayden’s long-term research was based on the complete excavation of entire Housepits, which surely allowed him to decode many site formation processes and therefore make sensible analytical units of materials which belonged together, in the sense of having been used by the same group of people at about the same time.  However, this limited him to work in a select number of houses, and complete excavation is time consuming and also makes subsequent re-investigations difficult.

Example hand-held magnetometer in use. Source:

So, to finally get to the point – with such large, complex sites, any extra edge you can get prior to digging is a huge advantage, especially if it means you can dig less.  It is therefore welcome to see Prentiss using magnetometry, electrical conductivity, and ground penetrating radar at the Bridge River site.  Just taking the first of these, magnetometry depends on the principle that materials in the ground may have varying magnetic properties, which produce extremely small, but measurable, anomalies in the local magnetic field of the earth.  These can be measured with hand-held instruments carried over the surface.  A positive anomaly (red in the map) indicates a stronger magnetic presence, perhaps the result of fire-alteration of fine sediments, hearths, or, in some other contexts, the presence of ferrous metals or fired clay bricks.  A negative anomaly may be the absence of those positive influences vs. baseline levels, or an excess of such very-low magnetic substances as carbonates.

Magnetometry has been in use in archaeology for over 50 years, and yet, in common with other geophysical methods, it has barely been seen in British Columbia.  I think Arcas used it at Tswwassen in about 1990, and RG Matson used it in the middle 1990s at Shingle Spit, but geophysics is a long way from being the routine part of archaeological impact assessment that it could be, probably should be in British Columbia, and indeed is elsewhere in the world.

Magnetic and conductivity surveys of Housepit 25 compared. Source: Prentiss 2009.

As you can see in the above, different geophysical methods give different results.  To the left, you can see a strongly-defined housepit rim as well as internal structure reflecting, I think, post-holes.  From the report:

In both instances, HP25 displays the typical pithouse magnetic signature. Rim deposits are clearly delineated by anomalous positive-valued gradients surrounding interior floor areas that are largely characterized by negative-valued gradients. Typically, one or more positive magnetic features are indicated within the central area of the house floor, surrounded by mainly negative gradients that are particularly strong over southern-southwestern areas of the house floor and extending into the interior flank of adjacent rim deposits. In the case of HP25 (Figure 5), magnetic gradient readings acquired over the central floor area reveal a particularly interesting pattern of anomalously positive gradients and there is evident correlation between these features and apparently related conductivity features.

These specific, cautious results are probably the result of not many comparable studies being done and I haven’t read the entire report to get a full appreciation of how these data and method intersect through the years of Prentiss’ research.   I note that Prentiss et al. attribute to targeted geophysics  their very high success rate in getting appropriate samples for their research goals, despite not excavating entire houses.  It is this last point which matters most.  As the authors note:

While Hayden’s data are superb the lengthy time required to excavate entire houses was costly and it limited the number of houses that could be explored. Further, it effectively prevented future field investigations associated with these housepit floors. These problems can be avoided by using geophysical methods to identify activity zones associated with major hearth and cache pit features and subsequent excavation sampling.

In other words, geophysics allows one to do more, with less: less money, less labour, and above all, less destructive impact on the archaeological record itself – and yet recover the same amount or more of the information needed to answer questions.

Relatively minor impact at Housepit 24, as excavation units are guided and informed by geophysics. Source: Prentiss 2009.

And, put into a Culture Resource Management context, by creating a road map of the subsurface structure of a site, these methods may allow pre-emptive navigation around crucial parts of important sites.  While at Bridge River the housepits are obvious on the surface, at many sites subsurface features to quite some depth which have no surface visibility could nonetheless be mapped.  You can’t easily find sites with these methods, but you sure can map some of their contents and coarse structure. Furthermore, as you use it more and more, interpretation gets better and better through experience and by comparison of the geophysical map to the material recovered in the ground.  In particular, geophysics on the NW Coast and interior may be especially well-suited to finding human burial features and thereby avoiding them, as well as finding hearth features, ditches, large posts, and house perimeters.  All of these are pointers to high cultural and archaeological significance.

It is for this reason that I think that we need to have a very serious debate about geophysics becoming routine and expected parts of CRM work in this province, certainly any time there is proposed substantial impact onto known sites.  The onus should be to explain why these methods were not used, and not the expectation of praise on those few occasions that they are. Were geophysical methods applied yet at Glenrose Cannery, for example?  Were they considered, and rejected for some reason?  In many countries and jurisdictions, these methods are, indeed, routine and I wonder why they are not in British Columbia.

Geophysics or not, this is always my unit. Sigh. Source: Prentiss 2009.

Reference: Brian Hayden, 2005: The Pithouses of Keatley Creek (2nd edition) SFU Archaeology Press.

14 responses to “Bridge River Housepit Geophysics

  1. The investigations by the Prentiss crew at Bridge River, and methods of reporting, are providing new insights on the formation and occupation of pit house villages, among other points of investigation. Having visited this site several times during the 2009 field season, and reviewing portions of the reporting, I feel this work is cutting edge and certainly useful for CRM as mentioned above.


  2. Interesting blog and you make some important points. One of which is the paucity of geophysical surveys in archaeological assessments. I’ve been struggling for years to understand why there would be any resistance (pardon the pun) to the techniques given the amount of additional information it can add to the plan, depth and orientation of a site. Also, given the time that geophysics can save through potentially unneccessary and certainly destructive excavation and the speed in which the data can be collected seems as though its use would be a given (as it is in much of Europe).
    Also, it just so happens that I conducted a magnetometry survey of a historic cemetery for a First Nation in northern BC in early summer.
    Thanks for the blog.


    • Hi Jason – I’m familiar with your work up north, and glad that you chimed in.

      I don’t believe there is resistance to the use of geophysical methods of inquiry – I think that there isn’t the perceived potential or a need/necessity for them at the moment outside of large-site archaeology, academic curiousity or burial (re-)location.

      The biggest obstacle preventing more geophysical surveys from being conducted in non-coastal BC is the very nature of most archaeological work and the kinds of sites encountered in those surveys. I am speaking strictly from the perspective of the CRM world though, where the majority of sites we deal with are unlikely to have features (or so it is thought) – or if they do, they tend to be shielded from impacts and consequently the proponents have no interest in collecting extra data. There are big benefits to conducting geophysical surveys, including GPR, but it is finding the right circumstances, someone willing to pay for it, and more importantly, educating those who must pay for it. A number of trial project need to be completed to build an argument for broader implementation beyond the primary types of projects that presently do benefit from these surveys.


  3. In fact, QXM, you’ll find AH Stryd used both magnetometer and resistivity mapping of the housepits of the Bell and other sites in 1970 in his forays into social archaeology of housepit villages. Here’s a quote I found on-line:
    “Dr. David Huntley of the Department of Physics, Simon Fraser University, joined the field crew for several weeks for a magnatometer and resistivity survey of several pithouse sites.”
    Stryd, AH
    1972 Housepit Archaeology at Lillooet, British Columbia : The 1970 Field Season. BC STUDIES, no. 14, Summer 1972: 17-18.

    I remember being given a very similar lecture to your discussion in my 1974 field school by Arnoud at the Bell Site, although we only used a magnetometer that year.

    There is nothing new under the sun it seems!


  4. Morley, as always, you are a human font of institutional history. 5,000 blog coolness points and one (1) beer for you.

    remi, you say:
    There are big benefits to conducting geophysical surveys, including GPR, but it is finding the right circumstances, someone willing to pay for it, and more importantly, educating those who must pay for it. A number of trial project need to be completed to build an argument for broader implementation beyond the primary types of projects that presently do benefit from these surveys.

    I am going to take issue with this somewhat. Geophysics are not some radical new idea which needs to have an argument built for them. They are not a concept looking for a proof.

    I would accept that a study might need to be done, a “meta analysis” if you like, which brings together and summarizes existing literature on the technology and its potential applicability to BC site types. This could be done simply, and quickly, and in my view it would be a pro-active step the Archaeology Branch could initiate. From that study, start requiring it where appropriate. Your comment reads as if there is something unique about BC archaeology such that the laws of physics don’t apply here 😉

    Your comment on “someone willing to pay for it” is, well, from the ivory tower, misguided. The question here is (a) they don’t have to be “willing”, they have to be “required” to pay for it. That is, required to pay for archaeological impact assessment that meets modern, international standards. And (b) yes education, but included in that, let’s not forget that geophysics can save money, and save the archaeological record, by directing costly excavation and redirecting destructive bulldozers.

    But saving money or not, the principle here is to do the best archaeology and to act in the interests of the archaeological record, and that is something the BCAPA should be promoting as hard as possible and arguably they have a professional ethical requirement to do so, as does the Archaeology Branch.

    Bad analogy perhaps, but MRI machines cost money, but you don’t see doctors resisting them. (archaeologists are the doctors in this bad analogy!)

    I know there are some clear issues – I’ve seen GPR at work in a rainforest and it is a painful process to keep the antennae snug to the ground. Lots of BC sites don’t have features, or so we think. But I don’t believe for a second that there isn’t wide, relevant application of geophysics in BC archaeology and I do believe that all professional archaeologists should consider it their duty to advocate for it. [insert pompous emoticon]

    Jason – welcome and thanks for your comment.

    Hi Karen!


    • I may stand corrected but it is common practice these days (or should be) for Permits to contain a provision for the use of geophysical MOI.

      I am merely being pragmatic – this isn’t Europe. BC is still a very big place, and as long as a developer can avoid something that may need a geophysical assessment, they will. Consequently, the demand or need for geophysical assessments will remain low compared to other jurisdictions (the exceptions being projects that require a very specific footprint). Contrasting BC archaeology to European archaeology – where avoidance isn’t necessarily as feasible or optional, and human remains are treated quite differently – is the proverbial apples ‘n oranges scenario.

      To continue the analogy, as Doctors we seem to practice preventative (avoidance) therapy rather consistently, thereby resulting in a lessened need for MRI’s. We are our own worst enemy in that respect – advocating preservation and protection does not engender investigative funding.


      • BC is indeed a big place, but the sites are comparable in size to Europe, or smaller – and generally speaking, geophysics are used on known sites, not used to find unknown sites. So size of the Province is a red herring.

        Sure, preventative medicine might lessen the need for MRIs, but every excavation is more like an “operation”, and most operations could be guided by better imaging. I imagine it is pretty rare these days for a major operation to happen without some kind of x-ray, CAT scan, MRI, PET, whatever.

        So, even if we want to talk only about those sites where excavation is actually happening, then I would say, every one of those should have an explicit argument made why geophysics are not appropriate or needed, and that that argument should be based on archaeological best practices which themselves are not really, at heart, about what the developer is “willing” to pay.

        Best business practice does not equal best archaeological practice, in other words. I am arguing for the latter, admittedly without having to deal with the exigencies of the former.


  5. qmackie says: “The onus should be to explain why these methods were not used, and not the expectation of praise on those few occasions that they are.”

    So how bad is it really? To get a sense I did a key word search in the permit report library. Keep in mind that permits are not usually required for this kind of work and additional studies will have been done to those found in permit reports. If the permit report referred to a non-permitted study I included that study in this analysis. I doubt there are many more such studies lurking around out there.

    Well, the first use of resistivity, magnetometer or ground penetrating radar (GPR) methods in BC, conducted under a permit was in 1970. This is the use by Stryd at 5 pithouse sites that Morley mentions. Since that time these methods have been used 47 times in 33 permitted projects. That is out of approximately 5000 permits (that number is a guess). Magnetometer has been used 24 times, GPR 16 and resistivity 7, in many instances more than one method has been used on a single permit.

    They have been used in AIA settings (exclusive of burial assessments) 4 times.
    For mitigation projects other than burials 2 times
    For burial assessment or burial mitigation 18 times.
    On historic sites (usually Fort excavations) 7 permits (but 3 Forts)
    For house excavations 8 times, mostly research.
    For underwater projects, mostly inventory, 8 times.

    In addition, and separately from the above, the use of such methods have been included in recommendations for future work 16 times.

    1970’s – 5 times/4 permits
    1980’s – 9 / 8
    1990’s – 22/14
    2000’s – 11/9

    Looks like a lot of explaining is in order.


  6. The yearly summary at the end of previous post is for the use of these methods, not for the recommendations that they should be used.

    25 uses for CRM
    22 uses for Research

    magnetometer – 25 uses
    GPR – 16 uses
    resistivity – 7 uses.


    • Are all of these uses on individual, separate sites ? So 47 sites out of 47k ?


      • No, its 47 permits. I think there are 2 permits at a few sites – such as the SFU Rocky Mountain Fort project, and some of the wreck inventories – the Tonquin for instance might even have had 3 separate studies over the decades. I forgot to add one that I have knowledge of – the Archaeology Branch commissioned a magnetometer study on Long Beach in the 1970’s to try to find a reported wreck of a Spanish Galleon (without success). Another not in the list was an examination of the Pioneer Square cemetery in Victoria (not sure which method but maybe GPR). I also think that there might be quite a few instances of Parks Canada doing such studies – these would not have permits and, sadly, most archaeologists in BC do not reference the Parks Canada reports (probably because they are pretty hard to come by) and thus I would not have picked them up in permit reports. If you think 50 projects in 40 years you won’t be far off I expect.


  7. Thanks for those numbers, APM, which are suggestive of a very low incidence of use. The triad of burials, historic sites, and underwater (all underwater would be magnetometry I’ll bet) makes sense as reflecting a coarse understanding of what these tools are “good at”. But I’d still like to see that understanding become more nuanced and account for other things they are good at, even if these are less common site types, or regionally restricted. Also I’d like to have a more formal comparison to other jurisdictions about the incidence of use.

    I am guessing the roughly “5000” permits includes CMT permits where these methods would be irrelevant?

    I also note that there is actually a decline in recommendation of use from the 1990s to the 2000s!

    Anyway, I don’t want to be a one note pony, but a formal comparison to other jurisdictions on the use of these methods should be done. Maybe there is confirmation bias, that is, whenever I hear of a cool Dutch or Danish site with extensive geophysics I remember it, when I hear of one without I don’t. and thereby build a mental model of “they always use it and we never do!!”. Hence the need for a study.

    As I mentioned before, I think the Archaeology Branch could show leadership on the CRM side by funding a study of these methods and a critical commentary on their utility across the bewildering variety of BC environments and site types.


  8. I would like to include some of this geophysical technique in my research/assessment… I wonder how much it costs?


  9. This site is large enough that it is actually visable on Google Earth, amazing.


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