Blackburn Park and Jensen Point Preserve 470-million-year-old Dune
Story and Photos by Jo Schaper
When I moved to The Cedars 30 years ago, I knew we were buying beachfront property.
The center park is a sandy high point on the Tippecanoe Sea beach. The white St. Peter Sandstone is one of the Four Rs that define our town: rocks, river, railroad, and Route 66.
Everyone sees our big white cliffs. On old U.S. 66, (Osage Blvd.) it’s Adam’s Garden and Blackburn Park. It’s the U.S. Silica plant on the east end of town. It’s the rock upon which Jensen Point is perched. To the east-southeast, this sand forms the La Barque Hills, lines Highway NN to Sand Cut Road, and leads to the sandy trails of Don Robinson State Park.
The best place you can get up close and personal is driving into town through the white roadcut on Hwy. F just south of town. At the high point, you are surrounded by white sand, while looking out on the Meramec Palisades, where the river cut the 80-foot sand bluffs over millennia.
A couple weeks ago, Pauline Masson, blog owner of Hometown Matters, asked me to write an explanation of our white silica sand. She had no idea that the Association of Missouri Geologists was on the verge of having their annual field trip here: Barnhart, Eureka, House Springs, Pacific and Wildwood the first weekend of October. I’m a member. We spent the first Friday and Saturday of October on a big travel bus looking at roadcuts, including our very own Blackburn Park. The guidebook had a write up about U.S. Silica. I gave out a writeup and St. Peter rock samples to 40 visitors to our town and invited them back at their leisure.
We have festivals and rodeos and such, but when was the last time you heard of that many professionals coming to our town to look at our rocks?
So, how did Pacific get in the middle of a big white sandbox?
The short story of Missouri geology is this: two billion years ago, North America was a molten volcanic landscape. As the continent solidified, the Ozarks were one of the last places in the middle to cool. We had volcanoes and exploding calderas intermittently, up to about 900 million years ago. Our igneous rocks like those at Johnsons Shut-Ins and Elephant Rocks were laid down about 1.2-1.4 billion years ago, rising to about 5000 feet above sea level, then wearing down. Geologists really don’t know what happened from 900-600 million years ago. No evidence from that time remains.
Starting about 550 million years ago, the state began being covered with warm shallow seas, less than 500 feet deep. These seas would come and go, laying down sediments for millions of years, then eroding for more millions, leaving gaps in the rocks layer. While this was happening, the continents shifted around the globe—two or three times, forming supercontinents like Gonwandaland and Pangea, then splitting apart again as they floated on the hot rock layers below. The continents moved because of plate convergence (mountain building along the edges) and subduction (where one plate moves beneath another).
Depending how deep the ocean water becomes, different sorts of sedimentary rock form. Rock material consists of dissolved minerals, igneous sand, lime-enriched mud or clay, and hard sea-animal body parts. Coral reefs build rock in shallow water before dying. Sandy beaches become sandstone, from dunes and beach sands over time. Mud flats evaporate to shale, dolomite, or salt. Plant-filled marshes preserved as layers of peat, coal, or oil trapped in sand, shale or salt domes. If limestone is re-immersed in sea water, it becomes dolomite and dolostone.
About 490 to 459 million years ago, Missouri was near the earth’s equator, under a shallow sea. This shore resembled Destin, Florida, before birds or land animals. Wind and water rounded the sand grains. Most softer minerals ground to powder by wind and water.
We’re not sure why the ocean moved inland over the beach and dunes, burying it beneath the waves. The beach around the Tippecanoe Sea eventually sunk, saturated by limey water and mud. As more sediment moved over the sand, and it heated from pressure, chemicals in the water cemented the sand into rock.
For the next hundred million years, more sediments were laid on top of the sand layer—many limestone layers, coal, shale and common brown sand. It repeated this process about 30 times from 490 to about 320 million years ago. Eventually the entire sedimentary rock record in Missouri piled up between 1800 to 3000 feet thick. Much has disappeared over millions of years, including the layers over the St. Peter Sandstone here. Because quartz is resistant to erosion, this old beach sand lends its character to our hometown of Pacific—here on the wide Pacific shore.
I have some Crinoids, Sea Lillys, collected from Antire road area that were eroded out, Missouri’s fossil of note. Many are here but concealed in the local strata. This sand has payed it’s share of tax dollars over the years to support the local needs as well as local jobs, and it provides beautiful local vistas. Pennies from heven from the distant past.
Henry, the fossils are from the Burlington Limestone formation, about 100 million years younger than the St. Peter Sandstone. We looked at it on Oct. 7, too.
Jo , I knew that, but this old man , chemo brain, can’t remember his name some days