What Is That Gunk on My Rock?

Figure One - Side B of a 4.1-inch-long discoidal biface made from Alibates agatized dolomite. 
Pedogenic carbonate dominates this side, indicating this was the "down" position in the soil. 

For those of you who do not know the inspiration for my prehistoric adventure book series, the SHADOWS ON THE TRAIL Pentalogy, this is the story. It all began on an early summer morning in 2010 on a northern Colorado ranch. I discovered a prehistoric stone tool made from a red and gray striped rock created in a well-known prehistoric rock quarry in the Panhandle of Texas (figures one and two). After studying the flaking patterns on the artifact and based on other Paleoindian artifacts I found on that site, I concluded that it most likely came from the mysterious Folsom Complex around 12,600 years ago, give or take a century or two. 

Figure Two. Side A of the 4.1-inch-long discoidal biface. Side A is not showing as much pedogenic carbonate as Side B. Side A was the original side "up" in the ground.  

When I found that prehistoric stone tool, I recognized the distinctive rock type at once, and I knew the rock's source was Texas. So, what was Alibates agatized dolomite doing in northern Colorado? My mind wondered about the ancient people who made the stone tool and how it ended up on a Paleoindian campsite in northern Colorado, some five hundred miles away from the famous prehistoric rock quarry.  Who made the stone tool and what was he or she like? What happened on that long journey from Texas to northern Colorado some 12,600 years ago? Since it was impossible for me to ask the Paleoindians, I wrote my own version of the trip in the SHADOWS on the TRAIL Pentalogy. 

One thing is for sure, that discoidal biface rested in that soil for a long, long time. I was surprised that some lucky dog before me did not find it, or a cow or horse hoof did not smash it to pieces. And what was all that white gunk covering its surface? Inquiring minds wanted to know. 

                             Figure Three - Book One in the Series. CLICK to ORDER.

The white stuff is called pedogenic (secondary) carbonate and it is mostly CaCO₃ or calcium carbonate. How does pedogenic carbonate end up on artifacts, rocks, and geological features? I am glad you asked. Pedogenic carbonation occurs when rainwater mixes with carbon dioxide in the atmosphere. Together they form a diluted carbonic acid or H₂CO₃ in the soil. That weak acidic mixture dissolves other minerals in the soil and yields water-soluble calcium carbonate, bicarbonate, and other salts capable of precipitating onto other rocks and minerals when groundwater conditions permit it. 

Figures four and five are photographs of a Paleoindian ultrathin knife form where pedogenic carbonate completely took over one side of the artifact. The Paleoindian who made that knife form used obsidian as the raw material. In figure four, Side B is almost completely covered with pedogenic carbonate. 

            Figure Four - Side B of a 6.5-inch-long ultrathin knife form showing extensive pedogenic carbonate, indicating that side was "stratigraphically down" in the soil. 

John Bradford Branney Collection.  

Pedogenic carbonate obviously detracts from the artifact's original beauty and has limited my ability to study the flaking patterns on Side B. I can only guess what the flaking looks like underneath the gunk that covers it. The only good thing about pedogenic carbonate is that it eliminates any doubt as to the authenticity of the artifact. As far as I know, artifact replicators and fakers have not figured out how to fake pedogenic carbonate, but I am sure they are trying their best. 

Figure Five - Side A of a 6.5-inch-long ultrathin knife form showing no or little pedogenic carbonate. Side A was "stratigraphic up" in the soil. John Bradford Branney Collection.  

A geopetal indicator is any rock feature that demonstrates the relationship of top to bottom at the time of formation or placement of the rock. An example of a geopetal indicator in the geology world is the orientation of cross-bedding in fluvial deposit. In the archaeology world, pedogenic carbonate lets us determine up or down for an artifact, bone, or feature.  

Pedogenic carbonate accumulates on or between sediment grains in the soil, occluding and cementing the sediment. Pedogenic carbonate forms a geopetal structure that first accumulates on the lowest part of its host. In the case of a buried prehistoric artifact, pedogenic carbonate coats the lowest portion of the artifact first. When found, the presence of pedogenic carbonate establishes the "stratigraphic up" position for that artifact. Figure five shows the "stratigraphic up" for that ultrathin knife form. 

Where will pedogenic carbonate most likely occur? Low rainfall is the single most important factor for developing pedogenic carbonate. Low rainfall allows the formation of pedogenic carbonate deposits near the surface of the ground while heavy rainfall flushes the water-soluble salts down into the groundwater table, removing them from the sediments where we oftentimes find prehistoric artifacts.

How long does pedogenic carbonate take to affect a prehistoric artifact? That all depends on the soil and the rainfall conditions. I have Late Prehistoric artifacts that have the beginnings of pedogenic carbonate while some of my Paleoindian artifacts show no sign of pedogenic carbonate.  

Figure Six - 3.8-inch-long Clovis spearpoint completely blanketed in pedogenic carbonate on
both sides. Surface found on private land in the high desert of northern New Mexico.
The original host material is unknown. John Bradford Branney Collection.      

I have read comments from people on social media sites asking for advice on how to remove pedogenic carbonate from artifacts. Most knowledgeable collectors will reply something like, "Leave the artifact alone." Occasionally, you will find someone who believes they are smarter than Mother Nature, and he or she recommends some concoction to remove the natural deposit. As far as I know, no one has ever been successful in removing pedogenic carbonate without damaging the artifact. Pedogenic carbonate chemically bonds to the soil, sediment, and rocks. It may look like it is only a surface coating, but it is chemically bound to the crystal lattice of the rock. 

What's my bottom line? Leave pedogenic carbonate alone! If you cannot learn to love the pedogenic carbonate on your artifacts, at least respect it! Like a good marriage, pedogenic carbonate is with your artifact forever.  

  

Reflections...

I found my first "perfect arrow point" when I was nine years old in good ole Wyoming. At the time I was playing third base on a Little League baseball team called the Merchants. I looked down at the ground between second and third bases and there was a jasper arrow point lying at my feet. Even at the ripe old age of nine, I was hooked on artifacts. Actually, I was hooked on artifacts much earlier than even nine years old. My mother inherited her father's extensive artifact collection and from age six on up, I was fascinated with those artifacts. That was six decades ago and my fascination with Prehistoric America has never left me.  

If you get a chance, read my prehistoric adventure series titled SHADOWS on the TRAIL Pentalogy. I think you will enjoy the book series. The adventures will transport you to a happy place, thousands of years in the past.