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?