For more information on floating the river and the first EarthCache in this series, please see this link.
For the next EarthCache in the trail, click here.
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Gastropods – Shells in the Hills
At this site you will be looking for a fossil bed containing ancient snail shells. It is a tricky find, even with the GPS (since they’re not always 100% accurate), so get ready for a scavenger hunt! The coordinates should lead you to the upriver end of a cottonwood stand on river left. A good place to park your boat is on a small landing where a drainage empties into the Missouri.
Walking towards the given coordinates should lead you past the bank vegetation to the first valley upstream from the cottonwoods. It’s only about 300 feet from the bank of the river to the fossil site, which is on the right side, on one of the first few “knobs” coming off the valley wall. The kind of fossils you are looking for are shown in the photo below, though they are quite small. Essentially, once you walk past the vegetation, look on the right side of the valley and start combing the knobs for shells. Some will be incorporated into the rock itself, while others will be loose on mudstone slopes, weathered out of the source rock above.
Gastropod fossils found in marine sediments in Utah
Source: A. Brayard and J. Thomas, National Center for Scientific Research
The fossils you are looking for here are called gastropods. Gastropods are a class of mollusks that have an asymmetrical, spiraled shell, and include the snails and slugs that are around today. Gastropods are an extremely diverse class, with over 60,000 to 80,000 species. Furthermore, these species have adapted to live in an extraordinary variety of habitats: from land-based gardens, woodlands, deserts, and mountains; to freshwater rivers and lakes; to marine estuaries, mudflats, beaches, and the great ocean depths.
Variation in today’s gastropod shells, and these are just marine ones!
Source: www.jaxshells.org
The gastropods at this particular site are around 70 million years old. They were a terrestrial species, probably living in or near a river system. What’s fun about paleontology is that we get to speculate: Why is there a concentration of gastropods at this particular site? What brought them together, and subsequently brought about their mass death?
Some of the fossils may have traces of a whitish surface on the shells. This is the original shell coating. On many shells this layer has probably dissolved away. In some cases, the shell may have completely vanished, leaving only a cast of sediment from the inside of the shell behind.
A gastropod cast, which sometimes can reveal details of the shell interior
Source: Pangaea Industries, FossilPlus.com
As you clamber over the rocks looking for gastropods, you may notice that the mudstone slopes are filled with cracks. This is known as popcorn texture. It is a common feature in mudstones or shales in arid environments. Popcorn texture is similar to typical mud cracks on the ground, only popcorn texture forms on slopes. It forms when rain causes slopes of mudstones to swell, and then subsequently crack as they contract upon drying.
Popcorn texture in the Morrison Formation, Colorado (Wikipedia Commons)
Please remember that since this is a National Monument, fossil hunting is illegal. You need an official permit from BLM to collect anything. Microfossils have no significant monetary value, but they are important to researchers, so please put them back after making your measurements for the questions below.
To claim this cache: Answer the following questions and send the answers using Geocache's messaging tool.
Q. What appears to be the source rock for these fossils? Where are they coming from? If you cannot identify it, describe it.
Q. How big are the fossils at this site? They can vary a bit so find some bigger and smaller samples and provide a range.