Sam Marcos Flint Draft

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san marcos flint

Article and Photos by Patty Duhon and Tom Jones

“I will not live long enough to fully appreciate what is on my property”
- Patty Duhon -

My husband and I moved to San Marcos 10 years ago and purchased property in the hills west of town. I started exploring our property and immediately noticed the rocks that littered the ground.  There was such variety of shapes, colors and forms. 

Growing up in Orange, Texas I was a rock enthusiast but did’t make many discoveries. I still have a couple of rocks from the fourth grade. I painted them with clear fingernail polish to bring out the vivid colors. I got my love of rocks from my Mom. On every trip, she collected a rock and would write the location and date found on the back. When she died, there was a BIG conversation about who would inherit her rocks. ; )

I am outside often working the garden and doing chores.  While walking, my head is down scanning the ground for anything that catches my eye.  After a heavy rain is always a good time to take a walk and hope to find something special. My efforts are often rewarded.  I collect many beautiful pieces of flint and other mineral specimens, moving them to a pile, rock wall or add to a landscaping project.  Over the years, the many rocks I collected resulted in displays throughout the yard and filled many bins with my favorite collections.   

My early goal with the rock landscaping displays was to have something in the yard to share with my grandkids when they visit.  I wanted them to join me on a walk looking for flint.  I was a little saddened but not surprised when they were not interested.  My favorite flint color is black with a white limestone coating the outside.  Another favorite is blue with specks of white over the surface.  These are hard to find, and it is always a treat to locate one.

Recently my husband and I started clearing a cedar grove which exposed a new area that had not been searched previously.  There was so much beautiful flint, with several pieces showing signs of being shaped into tools.  I sent a photo to Tom and invited him to come to San Marcos to walk the property with me and hopefully explain how these deposits were formed.

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Tom Jones

I first met Patty earlier this year as I was preparing to lead a geology walk on her property for a training class site visit.  She showed me her extensive rock collection and asked me about how they were made and what created the unusual shapes. Patty and I share a love of rocks. Recently She asked me if I wanted to explore an area that was just exposed after clearing the cedar trees.  I quickly accepted. We spent several hours walking her property, making new finds and looking at all of the interesting rocks she collected over the past 10 years.  Patty agreed to collaborate with me on an article for the newsletter.  I could help her understand the formation of flint, its importance in San Marcos and also share this information with the Chapter.

Flint, a variety of chert, is a sedimentary rock that can be found is many areas of Hays County.  It is a hard and durable mineral formed from the silica (quartz) rich skeletal remains of marine organisms.  Flint refers to the type of chert deposited in limestones.  It is formed into nodules or in layers, which are referred to as ledge flint.  It is common to see a thin layer of limestone covering the outside of the nodules, typically white and rough in texture.  Flint varies greatly in color from white to black, but most often manifests as shades of gray, brown, grayish brown, light green and blue.

About 100 million years ago, a shallow inland sea covered much of Texas. Large populations of very small organisms such a diatoms and radiolarians lived in sea creating tiny structures made of biogenic glass or silica. Over long time periods, the silica remains accumulated in thick deposits on the sea floor, forming siliceous oozes.  This process is relatively rare and only makes up about 15% of the sea floor.  Both the limestone and flint were formed concurrently.  That is why flint is found embedded into a limestone matrix. The floor of the shallow sea had marine worm holes and other burrows, creating narrow tubes and cavities in the sediment.  The silica rich ooze filled these openings and hardened into the flint nodules we see today. 

The abundant natural resources in eastern Hays County sustained populations for thousands of years at what is now known as Spring Lake.    A key resource was flint.  Flint was used to fabricate a wide variety of stone projectiles, points, scrapers, knives and other tools.  It is not known if the flint on Patty’s property was quarried by these indigenous peoples. I think it is likely because its location is only a short hike from the springs.  Another notable location is the famous Georgetown flint in Williamson county. Georgetown flint is the name given to an unusually good chert variety that occurs along the eastern fringe of the Edwards Plateau. Prehistoric knappers long ago recognized that Georgetown flint was a truly superior material. Numerous artifacts include large and especially thin bifaces which were made from this material. Some of these tools were found at sites hundreds of miles from the quarry. It is noted that flint deposits in Hays County are regarded as a ‘close cousin’ to Georgetown flint. Some of the above information on Georgetown flint was extracted from this interesting article, which can be viewed at this LINK:

Having the opportunity to visit Patty and explore her property was an incredible experience. Next time you are walking on your property after a good rain, keep your head down and you may be surprised at what you find.