On the road in Allisona

— post by Susan London-Sherer, Historical Archaeology/Historic Preservation Graduate Intern

Although the timeline related to the road that we exposed in our excavation unit remains uncertain, the research that I have been doing related to it has been fascinating.  We now know that a road from James Allison’s property was ordered by the court, and that local men were appointed to build this road from 1807 to 1809.  We cannot be certain yet that the road in our unit is indeed part of this particular court-ordered road.  So, it is necessary to investigate further, to consider other avenues…take a detour, so to speak, into alternate possibilities.

If you will remember back to my last blog post, I wrote about the uniform-sized rock that my colleagues diligently shoveled and screened for days.  In researching early road construction, and in discussion with Co-Director Jesse Tune, we came up with an interesting concept that just might apply to the road in our excavation unit, and place a slightly later date on our road.

Map of the First Surveyor's District of Tennessee, circa 1807-1808 Digital Image © 2008, Tennessee State Library & Archives.

Map of the First Surveyor’s District of Tennessee, circa 1807-1808
Digital Image © 2008, Tennessee State Library & Archives.


We came across the name John Loudon McAdam, a Scottish engineer, who developed a new system for road construction in 1820.  The size and weight of each stone was the most important element of McAdam’s technique.  Men sat with small hammers and carefully broke up rocks into different sizes, which were then laid in layers with the largest on the bottom and the smallest on the top.  The largest stones, which comprised the bottom 7.9 inches of road surface, were to be no bigger than three inches in diameter.  The top two inches of the road consisted of stones no larger than .79 inches in diameter.  The road building crew was under strict supervision and the size and weight of their rocks could be checked in a variety of ways.

Sometimes road crew supervisors carried scales with them and made sure the rocks did not exceed a maximum weight of six ounces.  If no scale was available, sometimes supervisors carried metal rings that each rock had to pass through to certify its size.  The road crew also had a third option of guaranteeing the size of their rocks.  If they were able to fit the rock inside their mouths, then it was approved to go on the roadway.  McAdam’s technique also required that the men lay the road one careful shovelful at a time.

Workers break up rocks for a McAdam’s road.   This is where the term “macadam” road surface comes from. Photo courtesy of wikipidia.com

Workers break up rocks for a McAdam’s road.
This is where the term “macadam” road surface comes from.
Photo courtesy of wikipidia.com



If indeed, our road dates to a later time period, specifically after the first macadam roads were constructed in the United States in 1823, a whole new set of questions begs to be answered.  Where exactly were the early residents of Allisona going?  What kinds of places did they want, or need to visit when they traveled along this road?  What kinds of businesses developed in the region due to the construction of the road and the access it provided to the settlers?

As you can see, with each day that passes at the Magnolia Valley site, and with each new discovery that we unearth, our research questions take on new shapes and dimensions.  The more ways that we can confirm human activity on the landscape, the more significant our research becomes.  This journey began when a revealing snapshot of something that “looks like a road” was discovered in the data from our initial geophysical survey.  The road was later confirmed through the controlled excavation of a two-meter by two-meter excavation unit laid out on a grid with the same coordinates as the geophysical survey.  The historical document research revealed a contemporary road-building technique that appears to back up the description of the uniform-sized rocks that covered the road in our unit.  In this case, the detour that I took on the road to discovery led me on a quest to a precious hidden gem of knowledge.



One thought on “On the road in Allisona

  1. It has never been visually clear to me how your road traverses your Magnolia Valley site. Does it pass fairly near the front of a historic house that is still present on the site in a location where one might expect a road to traverse or is it off on the “far boonies periphery of the site?

    Can you already definitively eliminate the possibility that this is just an old farm service road that was used by previous owners to get from one place to another on the farm? Many farms have had such roads (forgotten ones), and many still do. I know of one farm in Sumner County where the farmer converted an abandoned railroad bed that traversed his property into a farm service road.

    With regard to McAdam road surfaces, it might be wise to look not just at that but also at the entire civil engineering history of rural road construction in that area of Middle Tennessee. I work with civil engineers a lot, and they have a tendency to avoid fixing things that are not broken. In other words, if it is easy, cheap, and does the job well, stick with it through time.

    I am bringing this up because I strongly suspect that the McAdam approach to building gravel roads, and even asphalt paved roads, is still in use. Like you said, it involves first laying down crushed limestone rock that is about 3 inches in diameter and then overlaying this with finer grade crushed limestone with a much smaller diameter. I have actually seen this done on gravel roads within my own lifetime (past 61 years). This same approach rolls over to current-day asphalt pavement roads in the form of a base asphalt pavement layer of crushed limestone pieces about 3 inches in diameter. This is often overlain by a layer of asphalt pavement containing 0.79 inch (or thereabouts) crushed limestone. Sometimes this is the last layer, depending on economics and the planned use of the paved road. In other instances, a third layer of pavement with much smaller crushed limestone pieces is applied to make the road especially smooth for the comfort of drivers. I guess my overall point is that a McAdams approach to building a rural road may not necessarily be temporally diagnostic to the 1820s.

    Do you have some technical means of differentiating between hand-crushed limestone rock from the 1820s and limestone rock that has been crushed mechanically in a current-day, large-scale commercial crushing operation?

    I am asking because of something odd that I witnessed recently here in East Tennessee—something I had never seen before. Most of the time, the crushed limestone in the Knoxville-Oak Ridge area contains no Knox Black Chert inclusions. (I have no idea why, how they sort it out, or even if they do any sorting at all.) I have recently run into several limestone gravel roads in the Oliver Springs area that have numerous pieces of crushed Knox black chert included with the pieces of limestone. I am told that this rather unique crushed limestone comes from a quarry operation in nearby Campbell County. Here is the interesting thing though. With your 0.79 size gravel, the Knox black chert pieces tend to be randomly angular, just like the same size limestone pieces. However, when you get into larger sizes, the limestone comes out randomly angular, but the black chert that goes through the same mechanical crusher with it tends to come out in diagnostic rectanguloid and cuboid shapes.

    That being the case, I would bet my bottom dollar that crushed limestone made by men with hand-held hammers in 1827 is physically different in some definably diagnostic way from crushed limestone that has gone through a commercial crushing operation in the very late 1800s or the 20th and 21st centuries. If it has not been done already in the realm of historical archaeology, this might be an interesting thesis topic for someone.

    (I apologize for writing a book. That was not my intent. I just find the work that you folks are doing “danged interesting.” I hope other people do too. Keep up the good work!!!!)

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