FLINT ROCKIN’ IN SOUTH-CENTRAL TEXAS
by Jim Miller, B.Sc., M.Sc.
Being a knapper and rockhound in the Pacific Northwest has its plusses and minuses. On the plus side, large pieces of beautiful obsidian and dacite (both of volcanic origin) are available with a little hard work. Colorful jasper, agate, silicified volcanic ash and petrified wood are also present in localized areas. On the down side, the nonvolcanic rock types are pretty tough to collect in volume and unfractured pieces tend to be fairly small. Furthermore, chert and flint are virtually nonexistent in the Northwest.
I had heard rumors of abundant flint and chert in Texas, so I recently ventured to the Lone Star State to see for myself. In short, I was simply amazed. Quality knapping material was so abundant that I was in silica shock! This story talks about the origins of flint in south-central Texas and recommends collecting strategies based on my direct (but limited) personal experience. Suggestions are also made for heat-treating the knappable stone from this part of Texas.
Almost all of the land in Texas is privately owned and fenced. Be aware that permission is needed to access private properties. However, I understand that the riverbeds below the normal high water line are considered public land. So access along the floodways of active rivers and creeks is generally allowed.
Limestone is present at or near the ground surface across much of central and south-central Texas. This thick sequence of limestone (several thousand feet or more) was deposited in shallow seas at the time dinosaurs walked on exposed land areas. Various horizons within the limestone contain layers and nodules of flint, chert and occasional chalcedony. The varieties with dark gray to dark brown coloration look, feel and behave like the “true” flints of Europe. I will refer to these nodules or layers of nearly pure silica as “flint” even though some geologists may argue with that description.
About 10 million years ago the limestone deposits of central Texas were uplifted to about 2,000 feet above sea level. This formed the extensive “Edwards Plateau” area and exposed the limestone to erosion. Modern streams that drain the Edwards Plateau cut through the limestone deposits and carry flint nodules and tabular (flat) pieces of flint into the rivers. Flint is much harder and more durable than limestone, so it wears away fairly slowly as the rivers and streams transport it downstream.
While in Texas, I collected flint from three types of geologic deposits: 1) ancient gravel units that now occupy the crests of ridges, 2) modern river deposits, and 3) limestone outcrops. Each of these collecting areas is described below.
Flint in Ancient Stream Deposits
Many of the hills and ridges south and west of San Antonio are capped with a few feet to ten feet of gravel and cobbles. These gravel deposits give evidence of long-gone streams that drained the Edwards Plateau. The ancient streambeds were left stranded from deeper erosion by nearby drainages. The ancient gravel deposits are exposed in road cuts and gravel pits, where collection of flint cobbles is feasible in some areas. I’d pick a less traveled gravel road rather than a major highway, for safety if nothing else.
Based on my observations, almost half of the ancient gravel consists of flint or chert, with lesser amounts of limestone. These gravel deposits are quite old, and enough time has passed for brown iron oxide coatings or white cortex zones to form on the outside of most of the gravel and cobbles. The color and quality of the interior of a cobble is pretty much a guess. Bring a rock hammer to spall off a corner so that the interior quality of the stone can be determined (Photo 1).
I found some excellent material in the ancient gravel, including flint with colors of gray, tan and dark brown. Some even qualified as “root beer” flint. The largest cobbles I found were about 10 inches in diameter. It took about an hour of searching and testing to fill a bucket with quality knapping rock.
Flint in Modern Stream Deposits
I examined gravel bars in several active riverbeds between the communities of San Antonio, Uvalde, Junction, San Angelo and Glen Rose. All of these rivers and streams drain the Edwards Plateau, and each stream was found to contain abundant cobbles of chert and flint. The neat thing about looking for flint on a modern gravel bar is that all of the varieties of flint within the upstream drainage basin are present in the downstream gravel bars. In addition, the river does a good job of breaking up the fractured materials, leaving intact only the strongest and most durable cobbles of flint and chert. About five percent of the cobbles on the gravel bars consisted of chert or flint, with the remainder being limestone (Photo 2). The largest cobbles of quality material that I found were about 12 inches in longest dimension.
I found flint in three general shapes on the modern gravel bars: 1) river-rounded cobbles that had been transported a considerable distance from the source rock, 2) flat “tabs” that originated from thin, horizontal layers of flint, and 3) irregularly shaped nodules (“amoebas”) that probably had not been carried far by the river (Photos 3, 4, 5 and 6). Most of the flint is brown or gray, but occasional pieces of black, pink, purple, burgundy, white, speckled or banded flint also can be found. With experience, you can begin to figure out what type of flint is present by looking at the color and texture of a cobble. But it is best to confirm your guess by spalling off a corner to look inside. It’s kind of like an Easter egg hunt!
The easiest collecting spot I found was under the old steel bridge for Highway 377 in the town of Junction. A passenger car can be driven under the bridge to access a gravel bar of the South Llano River. It took me about an hour to fill a bucket with nodules, spalls and bifaces of super-nice material from this location.
Flint cobbles on modern Texas gravel bars generally do not have a thick cortex because the river wears the soft cortex material away. But the rounded cobbles often have an outer surface zone with closely spaced cone fractures (Photos 7 and 8). These fractures resulted from impacts with other hard rocks during floods. The fractured surface zone is typically about a quarter inch deep and needs to be removed by percussion flaking.
Because flint is so abundant on some of these gravel bars, it’s easy to get greedy! I found myself discarding all but the finest-grained materials that spalled easily (Photo 9). You may want to consider taking home only sizable spalls and bifaces unless you have a lot of room and a heavy-duty suspension!
Flint in Outcrops
After seeing so much flint in the streams, I was very interested to see what the flint looked like in the host limestone. Fortunately, through a connection with a friend, I was able to gain access to a large private ranch north of San Angelo. Two local Texans and I looked for flint on a series of limestone ridges separated by gentle valleys containing intermittent streams. This area was quite scenic, with branching cedar trees on the ridges and oaks in the valleys.
This was a great adventure! Wild turkeys and deer were nearly as abundant as the flint. As near as I could tell, we were the first modern people to search for flint on this ranch. Broken pieces of brown, dark gray and banded flint littered nearly all the slopes (Photo 10). Most of these pieces of flint contained bulbs of percussion, so we knew we were onto something interesting. We followed the spalls uphill and found a couple of horizons that contained abundant nodules of very high quality flint. This flint develops a white patina on the surface of the stone with age and weathering (Photo 11). We found broken bifaces, cores and spalls that had zero patina, opaque white patina, and everything in between. This variety of patina development indicated a wide range of age for the discarded spalls and flakes. Obviously, Native Americans had used this area as a source of quality tool-making material for thousands of years. We were continuing that tradition!
There were other horizons of limestone with chert nodules, but the quality wasn’t as good as the horizons near the tops of the ridges. I filled two buckets with partial nodules and spalls (including some ancient reject material). It was hard to find a loose nodule that had not been tested in the past by some ancient knapper. They had to drag me away from this spot!
Knapping Texas Flint
All of the material I shipped home (350 pounds) can be knapped raw. In the field I was able to take spalls 4 inches long or longer with my solid copper bopper (Photo 12). While I was in Texas, I also made several points as gifts for my San Angelo hosts using the local raw flint. If you like to maintain the original strength and character of the rock you knap, you can have a great time with this fine Texas stone. But as raw rock, it’s a little tough for my tired, old wrists and elbows.
Heat treating definitely makes the Texas flint and chert more brittle and brings out an attractive satin surface on flake scars. My experimentation found that a considerable variation in temperature is needed for treating the various types of flint and chert I collected. All of the rock should be given 24 to 48 hours of drying time at 205° F before raising the temperature above 210° F. In general, the darker the color, the lower the temperature needed for treatment. The smooth, dark gray or dark brown flint cooks nicely at 350° to 400°F. Don’t attempt treating pieces of the dark-colored flint thicker than about 1.5 inches – you’ll just make sharp gravel out of it! Temperatures of 400° to 500°F work well for the tan-colored chert. The white to gray chert requires a temperature of 500° F to 700° F to produce a beautifully smooth, sometimes pink stone that knaps much like treated Burlington flint (Photo 13).
I now know why artifacts are so abundant in Texas. With so much good stone around, knappers could practice their skills without worrying about wasting rock!
If you have the time and interest, I strongly suggest a rock collecting trip to Texas. Besides south-central Texas, quality material is abundant elsewhere in the state. Take your time because there’s plenty of room to explore. Happy rock hunting!