DPS_Stone City Field Trip_May 5th_10am to 3pm
Registration begins April 25, 2018
You will be asked via email to download, fill out, and sign a Holdharmless Agreement to bring with you to the field Trip.
LOCATION: Drive 12 miles west on Route 21 from the Rte 21 and Rte 6 intersection, Byran, Texas. Go past the Little Brazos River and slow down while crossing over the Brazos River Bridge. Move to the right shoulder and turn into parking area just past a telephone pole on the right. Walk toward the river, outcrops of Eocene rocks are to the left.
SAFETY: Age limit: No children under 10 years old due to nearness to moving water.This location is only safely accessible when the river level is at 15 feet or less and has not rained in the previous 3 days. This area can be very muddy and slippery; water levels are too dangerous otherwise. We will monitor the water levels at USGS Website (Bryan Gauge Height in feet). https://waterdata.usgs.gov/tx/nwis/dv?referred_module=sw&site_no=08108700
At the site always look before you pick up rocks; lizards and snakes may be lurking. Stay away from brushy areas and the rivers edge. Do not climb steep slopes with sharp trowels or sharp implements of any kind. Adults are responsible for the safety of their children.
WHAT TO BRING: Bring a hat and sunscreen and plenty of water and energy bars. Be prepared to get dirty. Sturdy shoes are needed for steep slopes. Participates will supply zip lock collection bags and markers and recommendations on how to build your collection. You should bring a gardening trowel or small pick hammer, and gloves. The closest restrooms to the Brazos River are 15 miles away. Fast food restaurants and 2 hotels are at Rte 21 and 6 Intersection in Bryan, TX.
GEOLOGIC OVERVIEW: Geologists study outcrops such as these to understand the environments that caused them. The information gleaned from an outcrop can help in oil exploration, understanding climate change, evolution, hydrology and paleobotany. Studying an outcrop of rock layers is like reading a mystery novel with a few chapters missing. We see the final chapter before us, but the how, whys and when requires a lot of observation and discussion. Geologists continue to study this outcrop today and what you find here today may further our understanding of these Eocene rocks.
Here is some information summarized from studies done on these rocks. These beds were deposited in a marine environment, 35 million years ago. The area at that time was much like a shallow bay, 30 to 50 feet deep, near a delta much like the Mississippi Delta today. The sands layered in the outcrop are coming from a nearby delta. Sometimes tides and currents and hurricanes move the sands around on the ocean bottom, causing hummocky, ripple laminations and graded bedding. The sands are gray to yellowish in color in the outcrop. Do they look like beach sand? During quieter times, in ideal conditions when the salinity, water depth and food source was just right, marine life blossomed with a community of clams, gastropods and corals, covering the sea bed, burrowing and thriving. The most prolific fossil layers are found in the green clay bed and over 250 marine species are found in it. Are all the fossils you find in growth position and unbroken? The preservation of fossils is very rare in geologic history and this site is especially world renown for the preservation of delicate shells, many of which have their original color, sheen and patina. Tread carefully! You will also notice the large blocks of limestone scattered on the slopes. This Moseley limestone layer caps the top of the outcrop and is very hard and resistive to erosion. It also contains many of the fossils seen in the green bed, although, extracting them is impossible! Take a look and see if you can see large horizontal and vertical burrows thought to be made by shrimp, lobsters or crabs. The rocks and fossils that you see today say volumes about their past, all you have to do is read and observe them closely!
Points to Ponder on the Outcrop
- There are 4 basic lithologies seen at this outcrop (see attached stratigraphic column)
- All marine sedimentary layers are deposited as horizontal layers in the ocean or lagoon. Are the layers at this outcrop horizontal or tilted?
- Which layers are slope forming and which have steep faces?
- Which layers are fossiliferous(contain fossils) and which non-fossiliferous?
- Which layers have sharp bottom and top boundaries?
- Note the chaotic and hummocky layering in the sandstones. These can be caused by storms (Ike!), tides and currents. Using your grainsize comparator, are the sands, fine, medium or coarse grained? Poorly sorted or well sorted?
- This outcrop is approximately 35 million years old and deposition of it took over 10’s of thousands of years to accomplish. Which rocks were deposited first/last?
- Have you found some fossils shells with small holes “drilled” in them?
- Preservation is a rare event, we find here only the animals that had hard shells, any evidence for soft bodied species, that is, trace fossils?
- Shell material is made from calcium and carbonate, where did that come from?
- Geologist’s use the phrase “the present is the key to the past” to interpret the geo-history of outcrops. Which key evidence found here today clearly shows that the rocks found here were formed in a marine (ocean) environment?
- As you walk along the outcrop, you may come across a fossil log. How could it have gotten here?
- The green marine clay bed consists primarily of pin head sized pellets. Are the fossils in it unbroken and in growth position?
Flis, J.E., and C.J.Flis, 2014, Stone City Member, Middle Eocene, Claiborne Group,
Stone City Bluff , Burleson Co., Texas, Main Glauconite Bed (MGB) Study Guide,
Flis, J.E, Yancey, T.E., Flis, C.J., 2017, Middle Eocene Storm Deposition in the Northwestern Gulf of Mexico, Burleson County, Texas, U.S.A. http://www.gcags.org/Journal/2017.GCAGS.Journal/2017.GCAGS.Journal.v6.14.p201-225.Flis.et.al.pdf
Yancey,T.E., 1995, Depositional trends in siliciclastic deposits of the Stone City Transgressive Systems tract, Middle Eocene,Texas: Gulf Coast Association of Geologic Societies Transactions, v. XLV, p. 581-586.
Middle Claiborne Group Invertebrate Fossils from Stone City Bluff, Burleson County, Texas , 2001 John and Barbara Emerson, p.138.
Texas Fossils, an amateur collector’s handbook, by William H. Matthews III
Guidebook 2, Bureau of Economic Geology, The University of Texas, 2004.
Geologic Highway Map of Texas, compiled by the American Association of Petroleum Geologists, H.B. Renfro, Chairman, 1979.
Attached: Stone City Stratigraphic column.
Emergency Phone Numbers………911
CHI St. Joseph Health Primary Care West Villa Maria
1612 W Villa Maria Rd Ste. 130, Bryan, TX 77803
Phone: (979) 690-4836
Baylor Scott & White Health Booneville Clinic
748 N Earl Rudder Fwy, Bryan, TX 77802