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Quartzite Quest Fulfilled:

Exploring the Source of "Spanish Diggings" Quartzite


While doing fieldwork as a budding geologist in Colorado, I found a few small arrowheads made out of quartzite.  I was amazed that any stone could be shaped by hand into sharp–edged tools and projectile points.  Quartzite is typically pretty rough and tough, so I was all the more fascinated that Native Americans could craft arrowheads with this material.

Through the years I gradually learned the art of flintknapping, and I became proficient at makin g arrowheads and knives out of a variety of stone types, including obsidian, jasper, chert, flint and agate.  Then, more than a decade ago, a friend in South Carolina sent me the first of several packages containing "Spanish Diggings" quartzite.  He wanted me to see what I could do with it  Needless to say, I was a bit apprehensive at first.  But to my surprise, this variety of quartzite flaked quite well and produced tools with very sharp edges.  The photos below show two projectile points that I made with stone that my friend Perry had sent me.


I really liked the stone, so I did a little bit of research.  First of all, there are two basic varieties of quartzite.   Meta–quartzite is sandstone that was buried several miles below the ground surface and subjected to enough heat and pressure through metamorphism to fuse and re–crystallize the sand grains into a solid mass.  Almost all of the quartzite cobbles that can be found in modern and ancient streambeds are of the meta–quartzite variety.  This stuff is really tough and difficult to knap.

The other variety of quartzite, which is applicable to Spanish Diggings material, is ortho–quartzite.  Unlike its metamorphic cousin, ortho–quartzite is a sand deposit that was buried deep enough to be compressed into sandstone, but not so deep as to have the sand grains fused together by heat and pressure.  In ortho–quartzite, the spaces between the sand grains (pore spaces) have been filled with silica cement.  The result is a very hard stone with almost zero porosity.  The silica cement holds the sand grains together such that flakes can travel through the grains themselves, as well as through the silica cement.  Interestingly, on a microscopic level, the flake edges of ortho–quartzite have micro–serrations where the edges pass back and forth between quartz sand grains and silica cement.

So, what is the story behind the name "Spanish Diggings"?  Discovered by a cowboy in 1879, the site location includes numerous deep pits surrounded by broken stone.  The original assumption was that Spanish Conquistadores had excavated the pits searching for gold.  So much for assumptions... The pits actually were dug by Native Americans while seeking knappable stone of high quality.  The site has been a source of quartzite for tool making for at least 10,000 years.  The distinctive quartzite obtained from Spanish Diggings was widely traded in antiquity, and artifacts made of the quartzite have been found throughout the Rocky Mountain area, and as far east as Ohio and Indiana.

It turns out that my friend Perry had visited the source location for Spanish Diggings quartzite, and he was kind enough to describe the route to get to this remote site in eastern Wyoming.  Three years ago my wife and I attempted to travel to the source location in a rental vehicle, while on a road trip to Wisconsin.  Although we were able to follow Perry's directions to the final leg of the "road" to Spanish Diggings, we were stymied by access conditions that the rental car dared not challenge (see the following two photos).  At that point, I put this location on my bucket list for future exploration.


In September 2016 I repeated my quest to visit the Spanish Diggings site location, along with my Jeep, my small teardrop trailer, fellow flintknapper Chris Dunlap, and Chris' son Ethan.  Upon reaching the turn-around spot of three years before, we were surprised to encounter the property owner at the cattle guard on his way out! I had assumed that the site was on public land, but alas, it is not.  After a brief conversation, the property owner somewhat grudgingly allowed us to pass, but he advised against it and warned that we "were on our own" if we got into trouble.

The road beyond the cattle guard offered plenty of challenges, with steep climbs, ruts, washouts and side–hill sections.  Nevertheless, our 4–wheel–drive vehicles had relatively little trouble negotiating the two miles beyond the cattle guard to the Spanish Diggings site, even while towing my teardrop trailer.  We stopped in several areas along the route to explore the best ways to bypass trouble spots.  At every stop we found ancient flakes and spalls of Spanish Diggings quartzite on the ground.  We were definitely getting excited! The following photos were taken on our way out and show some of the terrain that we crossed to reach the quarry site.  We arrived at the Spanish Diggings site in mid–afternoon with great anticipation.


The Spanish Diggings quarry site is truly amazing.  A hilltop area covering perhaps three acres is dotted with pits that are typically about 30 feet across and 8 feet deep.  Each pit is surrounded by broken rock (waste rock) that was removed by ancient miners to reach the underlying premium quartzite.  The rocky areas in the foreground of the photos below show some of the pits and nearby landscape.

All of these pits were originally excavated by hand labor, along with the use of levers and wedges.  A huge amount of human energy expended over many millennia is apparent at the site.  The purpose of the excavation work was to remove the softer, non–cemented sandstone to reach the valued quartzite below.  The cemented zones of quartzite appear to be quite irregular in shape and size, occurring as seams and layers within the sandstone.  The photo of waste rock below shows the dense quartzite surrounded by softer, lighter–colored, non–cemented sandstone.

Spanish Diggings appears to have been an ancient, seasonal workshop.  The goal here was primarily to mine the quartzite, to remove non–cemented zones from the quality stone by percussion knapping, to flake the dense quartzite into disc–shaped bifaces for export, and then to transport the manufactured goods to distant areas for trade and/or for use in tool making.  As seen below, the ground surface for many acres surrounding the quarry pits is littered with the waste rock and spalls created in making trade bifaces.

The amount of work expended at Spanish Diggings over many centuries cannot be overstated.  The waste rock must have been a real nuisance to the ancient miners, constantly getting in the way of good seams of quartzite.  The angular rock partially filling the ravine in the photo below is waste rock that was carried several hundred yards for out-of-the-way disposal.  Wow, that's a lot of work!


Spanish Diggings quartzite consists of fine-grained, silica-cemented sandstone.  The sand grains consist of nearly 100% rounded grains of quartz.  Part of the attraction of Spanish Diggings material is the variety of colors displayed by the quartzite.  The most common color appears to be a pleasing shade of lavender.  Other colors include golden–brown, dark brown, white, gray, tan, rose and (rarely) deep red.  Some spalls show different shades of coloration in a single piece, but typically the color is fairly constant within a single piece of stone.  The photos below show two colorful spalls of quartzite that I picked up from the ground surface.  The photo on the next page shows the color varieties of several small bifaces that I knapped.


The ortho–quartzite from Spanish Diggings flakes very well right out of the ground.  No heat treatment is needed.  My experiments have shown that the addition of heat to Spanish Diggings quartzite offers little improvement to the brittleness of the stone, although a color change sometimes occurs (brown stone changes to a skin–deep color of dark red).  With a little evaluation, it seems logical why heat treatment produces relatively little improvement in this stone.  Likely 85 percent of the stone volume consists of sand–sized grains of crystalline quartz, which does not experience increased brittleness with heat treatment.  The remaining 15 percent of the quartzite consists of cryptocrystalline silica (chalcedony) that fills the pore spaces between sand grains.  Although cryptocrystalline silica typically does get more brittle with heat treatment, the relative volume of silica cementation in the quartzite is so small that it contributes relatively little change in brittleness to the rock mass after the application of heat.


I brought home about 50 pounds of Spanish Diggings quartzite.  Essentially all of the stone that I brought home was material that was discarded in antiquity as being too small, or was rejected as waste rock because of the presence of softer sandstone.  Considering that the stone I kept is really sweet (in my opinion), the ancient miners must have carried away many tons of fabulous and colorful quartzite, probably enough to make hundreds of thousands of flaked stone tools.

Over the years, I have visited several ancient stone quarries in North America, including sites in Oregon, Wisconsin, Washington, Oklahoma and Texas.  The Spanish Diggings site in Wyoming ranks near the top of that list.  The uniqueness and attraction of Spanish Diggings likely will endure for centuries to come, considering its remoteness and size.  It is not easy to access, but worth the effort! After all, there are not many places where you can you find lovely lavender-colored quartzite like the biface below.  If you attempt a visit to Spanish Diggings, be aware that the land is privately owned.  Do your homework and seek owner permission before attempting a visit to this magical place.

Jim Miller

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