Elgin Sandstone: Depositional Features EarthCache

Note: no "armchair caching", please: we believe a player must visit a cache site in person in order for it to count as a find.

In banking, “deposits” are something left behind for safekeeping. In tv police dramas a “deposition” is a witness’ statement; the heroes examine the statement for clues to solve the mystery. In geology, “depositional features” are clues in sedimentary rock left behind by wind or water that can make statements about ancient environments, telling us what it was like ages ago.

The section of stone exposed along this roadcut showcases many depositional features. Some of them, like ripple marks, are common; others, like load casts, are unusual. Having so many different kinds of depositional features in one small stretch of roadside makes this a great place to learn about the patterns that the environment can leave in sediments.

In 2000, Neil H. Suneson wrote a guidebook for a geological tour of the Tallgrass Prairie Preserve. This is the last stop on the tour. Drive east from the Preserve’s south entrance and down into Sand Creek’s valley. The rocks exposed just east of Sand Creek are older than all the other rock outcrops found in the rest of the Preserve. The geologist J.A. Carter decided in 1954 that the Elgin Sandstone is part of the Vamoosa Formation. It is named for the town of Elgin in Chautauqua Co., KS. Like most of the surface rocks of n.e. Oklahoma, it is Pennsylvanian in age, meaning that the events that formed this stone took place long before any dinosaur ever lived. There are not many fossils here. If you do find any, please remember the Tallgrass Prairie Preserve regulations require that you take only photographs and memories. Samples or specimens should not go home with you, because collecting is not allowed. That’s what the word “Preserve” means- this place is set aside to keep everything here where it belongs: rocks and soil for habitat foundations, flowers and their seeds for food, lizards and other animals for the roles they play, bits of bison hair for nesting birds to use, and so on… Help protect this special place by putting rocks back where you found them if you pick them up to look at them, and maybe walking around a wildflower instead of on it. You will not need a rock hammer or a pick.

Here is the story of an ancient river, preserved in stone. That’s what makes the Elgin Sandstone special. Most sedimentary rocks in n.e. Oklahoma are marine deposits. Looking at Oklahoma’s sedimentary rocks, beach-combers and scuba divers would recognize the basic mud and sand designs caused by tides and waves, and the dune and sand bar shapes found in beaches, shallow sea beds and deeper ocean bottoms. Local fossil hunters are used to finding evidence of sea creatures like crinoids and gastropods, brachiopod shells, horn corals, and even an occasional shark’s tooth. It’s odd to think that Oklahoma’s rocks are mostly marine sediments, when you realize how far away the ocean is, today. However, there just aren’t many places to see fossilized river deposits around here.

Even those of us who live beside a living river may be more familiar with the kinds of sediment deposits found along a sea shore, than we are with those of a river channel. The shape of a river in its bed is often hidden by vegetation, to say nothing of the muddy water flowing over the alluvial deposits. We don’t have an aquarium-window side-view slicing downward into the sediments to show us how they are happening year by year. In fact, many of Oklahoma’s rivers have been drowned in their beds, covered over by ponds and lakes and reservoirs. Here is a peek at a long-ago river. It seems to have been busy!

First park carefully on the south side of the road just uphill from the bridge across Sand Creek. Move your car as far off the road as you can, and keep an eye on all the members of your party, especially children.

Walk up the hill to the east, and cross over to the north side. Begin at the top. As you go back down the hill, you will be going back in time, looking at older and older rock layers. Pay attention to the placement of the features you photograph; you will have to know which one is the youngest (found higher in the stack of rock layers) and which one is the oldest (found lower in the layers.) If you find some lying in the ditch, broken off from the hillside above, you’ll have to look for where they came from on the hillside, to find an intact layer of stone.

There are 8 different depositional features you might find at this site. Some are easier to spot than others. We challenge you to find all 8. To log this earthcache, post at least two photos:

• Each photo should include a different depositional feature from the checklist, along with either a member of your party or your GPS receiver.
• Identify the depositional feature in each photo’s caption.
• One of the depositional features should be a load cast.

___ Ripple marks. These are probably the easiest to find. You’ve seen these in soft mud. How long does it take for mud or sand particles to stick together and become stone? While chemical sedimentary rock formation can take place in hours or days or weeks or years (think about crystals forming as salt water dries up or minerals collecting on the insides of household plumbing), clastic sedimentary rock formation takes long slow millennia as the particles gradually bind themselves together.

___ Massive and unstratified sandstone blocks. The Elgin Sandstone is a mixture of sandstone, shale, and siltstone. Most of the neighboring rock formations are limestone. While limestone can also occur in massive and unstratified deposits (big blocks with no apparent layering), limestone is a chemical sedimentary rock which usually occurs in marine environments. The limestone layers above and below the Elgin Sandstone tells us the ocean was here before the river, and returned afterwards.

___ Parallel-stratified bedding planes. Some of the sandstone deposits show distinct layers, although the layers do not break apart easily. The layers could indicate periods of time when the river put down sediments and then paused, put down some more sediments and paused again. We need more clues to know how many years it took to form each layer.

Geology is the job of assembling the evidence to tell the earth’s story. In Hawai’i we can see lava turning into igneous rock before our eyes. Since no one was around to see the Elgin Sandstone actually form, anyone’s story is valid, as long as it includes all the evidence. Earth scientists are constantly revising their stories as new discoveries provide more information about the earth’s history.

___ Cross bedding. The light has to be just right to spot this. Over the years, seasonal patterns result in layers of deposits, showing up in the sandstone as streaks or faint parallel ridges. Years of west winds might blow sand grains into a dune on the bank of the river. Then a change in the depositional environment happens. Perhaps the river cut a new channel. Maybe a hundred-year flood stripped away layers of sediments and then formed a blockage with them downstream to create a backwater. It could even be that the underlying rock tilted in a cataclysmic event like an earthquake. Maybe the wind began to blow mostly from the south, piling up sand grains from a new angle. For whatever reason, the newer sediments are laid down on top of the older sediments, in a different direction.

___ Pebble hollows. Look for holes in the sandstone where shale pebbles collected and formed. The more loosely cemented shale eroded away, leaving hollow places in the more tightly bound together sandstone.

Here is a mystery: did the shale pebbles form at the same time as the surrounding sandstone? Or did the shale fill in holes left by something else, and then wear away to leave the holes again? Or were the shale pebbles there on the river bottom already, and the sand came along and filled in all around them to encase them in sandstone? What do you think?

___ Discontinuous layers of deposits. Sections of a layer either got eroded away later, or never got put down in the first place. Imagine a single section of the river, for example. Deep pools could acquire significant layers of sand or mud, while an island sticking up out of the water between the pools might not gain any deposits at all during the same years.

___ Load casts. This is the depositional feature you won’t find in many other places, so this is the one you must include as one of your two photos. Load casts look like sandbags minus the burlap sacks, except instead of piling up, they pushed down into the sediments below. It is thought that they represent heavy blobs of deposits from a single event, like a catastrophic flood that carried a tremendous amount of sediment downstream and dumped it suddenly along the way

___ Shale and siltstone with carbonized organic deposits. Sedimentary rocks are always recycling back and forth, turning from sediments into stone, and then eroding back into tiny particles again. In fact, shale is an excellent soil-forming rock. The amount of carbonized organic deposits you find here probably used to be a lot of swamp vegetation: plants not likely to be found in an ocean. No specimens survived intact as identifiable fossilized species. They may have been well decomposed into the black ooze of a river delta, before being preserved as a mineral deposit. Now the material looks like thin black smears on the paler shale and siltstone.

References Cited:
Carter, J.A., Jr., 1954, Geology of the Pearsonia area, Osage County, Oklahoma. University of Oklahoma unpublished M.S. thesis, 114p.
Suneson, Neil H., 2000, The Geology of the Tallgrass Prairie Preserve, Osage County, Oklahoma, An Introduction and Field-Trip Guide, Oklahoma Geological Survey, Open-File Report OF1-2000, 36p.