This cache will teach you about the geology behind the preserved dinosaur tracks and fossilized dinosaur bones here in Mill Canyon. While the fossils have been known since the 1980s, the dinosaur trackway was only discovered in 2009. There is no physical cache container to find; to log this earthcache, you will need to email or message us the answers to the questions below. The questions are repeated in the unencrypted hint for your convenience.
Stage one of this earthcache is an easy drive for most vehicles and an easy walk for most geocachers. While we hope you also make it out to stage two to see the dinosaur fossils on the Dinosaur Trail, that stage is optional, as it is a rougher drive along a wash, followed by a more rugged trail. The terrain rating is based on stage 1 only.
The Mill Canyon trackway is a unique site, both for the period it preserves and the creatures that are represented here. While there are other dinosaur track sites in the area, such as the Copper Ridge site a few miles north, those other sites are from the Jurassic era and only contain tracks from one or two different species. The Mill Canyon site is unique both for the quantity of tracks preserved here and for the quality and diversity of tracks. there are as many as ten different tracks, known as "ichnotaxa," from the Lower Cretaceous period, representing dinosaurs that were lizard-like, bird-like, and crocodilian.
Up the road at the Dinosaur Trail, Jurassic artifacts are left behind in the form of fossilized bones.
There are no fences or guards here at Mill Canyon. You, the visitor, are entrusted with their care. Please look but DO NOT TOUCH. Your care today will keep these historic relics intact for tomorrow.
STAGE 1: THE TRACKWAY
Based on current scientific theory, 112 million years ago, this area was much wetter than it is today, and the land was covered by a shallow lake. The tracks you see at stage 1 were formed when these dinosaurs walked on a mat of algae exposed when the lake water receded. After the algae was eventually buried under other layers of sand and mud, it eventually became limestone, which then became chert as microcrystals of silicon dioxide grew within the rock. The hardness of this chert (5.5 on the Mohs scale, meaning it could scratch glass) is likely what preserved the tracks so well. The tracks are incredibly intact, having resisted the weathering processes that stripped away the softer layers above.
For these ancient tracks to have survived, several specific steps had to happen. The sediment the dinosaurs walked through needed to be just the right texture -- not too soft and not too hard. Very wet soil would collapse on the tracks, erasing them. Very dry soil would simply blow away. If the soil was hard, the dinosaurs would not have left enough of an impression to be preserved. It also helps when the later sediment that filled the tracks did so slowly and was a different type than the one on the ground, especially if that later layer was less resistant to weathering than the layer preserving the tracks.
However, even though the stone is hard, the tracks are not impervious. There are signs of at least one track being damaged by casting. When plaster, silicone, or other substances are poured on rock like limestone, sandstone, or chert, it can penetrate the surface, potentially staining the rock permanently and potentially destroying the track. Please keep in mind that making replicas of tracks such as these is illegal without a permit; again, please look but do not touch.
Trackways can tell us much about how dinosaurs lived and moved (did they walk with their tail up or drag it? did they splay their legs out and drag them, or walk with their legs under their bodies?), how quickly or slowly a particular dinosaur was moving (based on the distance between tracks), and social behavior (how they moved together). Different tracks in the same area indicate that, while different dinosaur species may not have been present at the exact same time, they were in the same area as other species. Follow the trail around the boardwalk. There are a number of interpretive signs, including a large information board at the sheltered bench. These signs will answer questions 2a and 2e and can help you with 2c and 2d.
Spring 2022 update: unfortunately, question 2d may be much easier to answer once the site opens back up, thanks to some careless contractors.
Fall 2022 update: while contractors damaged some of the tracks that were outside the main track site while replacing the boardwalk, fortunately most of the tracks in the area were not damaged.
STAGE 2 (OPTIONAL): THE DINOSAUR TRAIL
As you drive uphill from the trackway to the Dinosaur Trail, you may assume that the fossils at the trail are newer than the dinosaur tracks below. After all, sedimentary rock is formed as layers of sand, rock, detritus, animal remains, and other material, and the deeper a sedimentary rock layer is, the older it is than the rock above it. But here, appearances are misleading: as you drive, you are actually crossing a part of the Moab Fault, which uplifted the older layers of sediment here higher than the newer layers down at the trackway. When the Cretaceous creatures were making tracks in the algae mat down at the trackway, these Jurassic dinosaur remains were already approximately 40 million years old and had already been fossilized and preserved in the rock.
The rocks at the Dinosaur Trail are part of the Late Jurassic-age Morrison Formation, a rock formation that contains the most abundant number of dinosaur fossils in North America. Because the areas where these fossils are found are so widespread, radiometric dating is used to determine the age of similar fossils found in different areas. Radioactive material goes through a constant state of decay; as the particular material radiates atomic particles, it changes into a decay product that is chemically different. Radiometric dating looks for the ratio between the naturally occurring radioactive isotopes in a layer and their decay products, then uses the known rate of decay to determine how old the layer is.
The dinosaur bones in the rock here are not actually bones, but have been geologically preserved. A fossil is any remains of an ancient living organism preserved in the earth. They are usually found in sedimentary formations, and like the tracks down the road, conditions have to be just right to preserve them. When the dinosaurs here died, they were buried in sediment. The softer organic material decayed, but the calcium bones were left behind. Over time, mineral-rich water seeped into the sediment around the bones, cementing and hardening it into rock. Sometimes the same minerals either seeped into the spaces and formed crystals to preserve them, or the water dissolved the bones and the minerals in the water replaced the bones. In other cases, fossils form from molds and casts. If a dinosaur's remains completely dissolve in sedimentary rock, it can leave an impression of its exterior in the rock, called an external mold. If that mold gets filled with other minerals, it becomes a cast. Or, an internal mold can form when sediments or minerals fill the internal cavity, such as a shell or skull, of an organism, and the remains dissolve.
The wash you see today has been around for millions of years, but just like at the trackway, it was much wetter then. In Jurassic times, this was once a lush river valley. The rock face here not only preserves dinosaur remains, it also contains preserved mud deposits (mudstone) and even petrified trees. Follow the loop trail to the right of the parking area, cross the wash, and follow the signs along the rock face. The Dinosaur Trail will eventually loop back around to the parking area.
LOGGING THIS EARTHCACHE
To log this earthcache, send us a message or an email and copy and paste these questions, along with your answers. Please *DO NOT* post the answers in your log, even if encrypted. There's no need to wait for confirmation from us before you log, but we will reply to messages and to emails if you include your email address in the message. Group answers are fine; just let us know who was with you.
1. The name of this earthcache: Dinosaurs in Mill Canyon
2. At the trackway (stage 1)
a. What is the name of the rock formation that preserves the tracks?
b. Describe the color and appearance of the rock in which the tracks appear.
c. In terms of weathering, based on the cache description, signs, and your observations, how do you think this rock layer compared to the next overlying layer?
d. Can you see any evidence of damage to the dinosaur tracks? Describe what you see.
e. The dromaeosaur tracks only show two toes, when we know this dinosaur had three. Why isn't the third toe showing in the tracks?
3. The dinosaur trail (optional stage 2)
a. Describe the appearance of the fossils. How are they different from the rock around it?
b. How does this layer of sediment compare to the layer at the trackway?
Photos of your visit are not required but are always appreciated.
SOURCES
Bureau of Land Management (Please note, some BLM sources used to write this earthcache are no longer available online.)
Wikipedia
Utah Friends of Paleology: Mid-Mesozoic Proceedings
Live Science: How do fossils form rocks?
This earthcache was created with the permission of the Bureau of Land Management.