Ripples in Time

The Little Yampa Canyon Interpretive Hike comprising this EarthCache is in a scenic area of the Axial Basin about 40 minutes southwest of Craig, Colorado.

Located at the base of Duffy Mountain, the half-mile trail is at an elevation around 6,100 feet. It is an easy hike on gently sloping terrain.

Located on public land administered by the Bureau of Land Management (BLM), this EarthCache can be reached from CO Hwy 13 between Craig and Meeker. Turn west from CO Hwy 13 onto Moffat County Road 17 and proceed for 9.5 miles. Cross the cattle guard and turn right (you will see a sign for Public River Access – 1 mile). About 0.3 miles farther, turn right onto BLM-1593 and go about a half mile to the Little Yampa Canyon Interpretive Hike, which will be on your right just beyond the fence after you cross the creek.

This trail was made possible by the volunteers who participated in the 15th Annual National Public Lands Day on September 27, 2008. Please enjoy using this trail, but keep in mind that it is illegal to collect vertebrate fossils, including bones, teeth, footprints, burrows, and other traces of activity.

The hiking trail begins at N 40°25.182, W -107°51.724. There are four stations comprising the EarthCache and each describes an interesting geologic feature encountered along the hike. The geologic processes you will observe and learn about are 1) weathering, 2) an ancient sea, 3) concretions, and 4) trace fossils.

Station 1: Weathering

As you begin the hike from the road east toward the cliff base, off to your left (north) you will see at least four very large blocks of rock up to six feet high or more. Viewed from N 40°25.202, W -107°51.682, you will notice that these square or angular boulders came to rest here as a result of mechanical or physical weathering of the cliff ahead of you. This type of weathering process happens as continual exposure to conditions such as heat, ice, water, and pressure gradually breaks down rocks and soils.

These rock blocks at the base of Duffy Mountain were most likely formed through frost wedging. This type of weathering is common in areas where the temperature fluctuates above and below the freezing point. Water enters cracks in the rock and freezes, which causes the cracks to deepen and widen as the ice expands. As temperatures rise, the ice thaws and the water flows deeper into the rock crevices. When the water freezes again, the ice enlarges the cracks. This cycle of freezing and thawing eventually weakens the rock until it breaks along the cracks and splits into angular pieces, or blocks, in a process called block disintegration.

These large blocks will continue to erode and break down into smaller rocks. Smaller rock fragments you see along the bottom of the cliff are called talus. As these rock piles accumulate over time, they are referred to as a talus slope.

Station 2: Ancient Sea

Continue along the hiking trail to N 40°25.217, W -107°51.663, where you will find a large boulder containing fossilized ripples that were created by waves when this area was completely under water. During the Upper Cretaceous period, from 65 to 85 million years ago, North America was divided by a huge body of water known as the Western Interior Seaway. This inland waterway was created when the Pacific and North American tectonic plates collided, causing the Rocky Mountains to rise from the surrounding lands. This uplift allowed the continent's central lowlands to become flooded by combined waters from the Arctic Ocean in the north and the Gulf of Mexico in the south, forming a vast sea.

The Western Interior Seaway was a warm, tropical and shallow sea filled with abundant marine life. Throughout the Cretaceous period, the water level repeatedly rose and receded. At its deepest, the sea was probably never more than 900 feet deep. The oxygen-starved environment of the relatively flat and muddy bottom of this shallow sea provided favorable conditions for fossils to form.

The ripple marks you see in this rock are fossilized evidence that this area was once under water. These ripples were formed by shallow water waves moving nutrient-rich sediments that made up the gently sloping continental shelf of the Western Interior Seaway. Wave-formed ripples provide clues to the water environment that was here millions of years ago because wavelengths are influenced by sediment grain size, water depth, and circulation of the water particles in the waves. If the water here had been deep, waves on the surface would not influence the seafloor or create ripple marks.

As you will notice, these wave-formed ripple marks are symmetrical, or appear to have a regular pattern, but ripples formed in other ways will look different. When the water current is weak, waves provide a rolling motion that creates balanced sediment structures. If these ripples had been formed by water currents instead of waves, they would be asymmetrical, or uneven, and would show the direction of the water flow.

Station 3: Concretions

Continue along the trail to N 40°25.207, W -107°51.626. Here you will encounter a large boulder containing concretions. The word "concretion" is derived from the Latin con, meaning "together," and cresco, meaning "to grow." Thus, concretions are accumulations of minerals that grow inside sedimentary rocks.

Concretions form different shapes depending upon the initial weathering process. As you saw at the first station on this trail, weathering weakens the sandstone until it breaks up into blocks of different sizes. As rocks weaken and fracture, they also become more porous, which means they develop a lot of small spaces that air and water can get into. As this weathering process gradually continues, ground water is able to seep in and circulate through the rock.

When rocks are submerged under water, as they were when the Western Interior Seaway covered this region millions of years ago, the ground water dissolves minerals, such as iron compounds, from the inner portion of the sedimentary rock and deposits it again in the outer parts, creating a hard outer zone. Sand that had been cemented together by the iron oxide loosens into powdery particles that can escape through small cracks in the rock. As the sand particles slip away, a cavity is created inside and this forms a hollow concretion. When the water level later drops, the exposed rock dries out and oxidizes, causing the concretion to darken in color.

In the boulder at this trail stop, the ear-like projection on the right side as you are facing the rock is an interesting iron oxide concretion. You can easily see how it is a dark rusty color in contrast to the lighter color of the surrounding sedimentary rock.

Station 4: Trace Fossils

Proceed to N 40°25.191, W -107°51.586. This final stop along the Little Yampa Canyon Interpretive Hike displays evidence that marine organisms were active here millions of years ago. Facing east toward this large boulder, you will be looking at the bottom of one of the layers of a prehistoric bedding plane. A bedding plane is the surface that separates distinct layers or beds of sedimentary rock. The bedding plane in this boulder is easily seen by looking at the left side of the boulder. Each layer represents different deposits of sediment, so naturally the bottom layers are older since they were deposited first.

Petrified within this sedimentary layer in front of you are trace fossils. Trace fossils are impressions or other preserved signs of biological activity such as scratching, burrowing, or walking. The trace fossils you see here are branching burrows appropriately called Ophiomorpha, which translates from the Greek as "snake form". Common in Coastal Plain sediments, these burrows were probably made by shrimp-like creatures and used as dwelling and feeding places. The burrows here are easily identifiable since they are stained and cemented by iron oxide, making them appear darker than the surrounding rock..

Trace fossils are also called ichnofossils, from the Greek word ikhnos, meaning "trace" or "track." In addition to tracks and burrows, trace fossils include fossilized droppings (known as coprolites) and chemical markers (such as stromatolites, which are structured communities of microorganisms found in sedimentary materials). Trace fossils are distinguished from body fossils in that they are preserved signs of biological activity during an organism's lifetime, whereas body fossils are the remains of actual body parts of an organism.

Trace fossils can be exogenic (from exo- meaning "outside" or "external") or they can be endogenic (from endo- meaning "inside" or "within"). Exogenic traces, such as tracks, are made on sediment surfaces. In shallow marine environments, surface trails are subjected to wave and current motions and are therefore less likely to become fossilized. Endogenic traces, such as burrows, are made within the sediment layers. The protection of the surrounding layers allows these trace fossils a better opportunity for preservation, especially when they occur in calm deep water environments.

To claim credit for this Earth Cache:

Email me the answers to the following questions based on your observations and information provided for the Little Yampa Canyon Interpretive Hike.

1. All of the geological processes you observed on this hike occur in what type of rock?

2. What key component influenced the formation of all four processes?

3. At Station 2, measure the distances between 3 or 4 ripple marks, from the top of one ripple to the top of the next one. Are the distances fairly consistent? What can this measurement tell us about their formation?

4. Of the geologic evidence you saw at the 4 viewing stations, which one occurred most recently? How can you tell?