The day dawned warm and sunny. We left the Eagle River Inn at 9:17am with our Brunton compasses and topographic maps of Minturn, CO in hand and headed to our first stop called the Holy Cross outlook. We parked by the train tracks nearby and began the 0.60 miles round trip up a steep slope by following the deer trails left in the grass.
We reached a flat ledge of the outcrop where we sat down and listened to Stewart Williams, PhD student from Rice University, talk about the geology of the area and how to map it. He began by showing the group how to orient themselves on a topographic map using the compass and the importance of doing so when mapping.
He also introduced the terms strike (line of constant elevation on a horizontal plane), dip (angle of maximum elevation from the strike), apparent dip (less than the true dip), and how to take those measurements directly from distinct bedding planes that we could touch with our hands and by using the sight method for estimating the strike of bedding at a distance.
The visible units dipping to the northeast from our perch were the Pennsylvanian Minturn Middle and the Pennsylvanian Minturn Lower formations. These units were composed of alternating sandstone and shale with variations in bed thickness. Using the sight method, the apparent dip we found for both units was around 18°.
While looking across the valley we discussed the reasons for differences in unit thicknesses, possible erosional processes, tectonic regime, and how the composition of the rocks could affect the present day landforms we see today, which can also give us an idea of the past depositional environments. The rock types were sandy siltstone and mudstone, likely deposited in marine environments with transgressing and regressing seas. Given their variable thicknesses, there must have also been variable energy levels associated with their deposition.
As we began the descent down to the train tracks we made a quick stop at an outcrop to practice taking strikes and dips on rock that we could put our hands on. The lower portion of the outcrop was thin and platy and broke very easily (mudstone) covering the ground below while the layers above were thicker and harder to break (sand and siltstone). We labeled them on our maps and drew in our contacts between units making sure to follow the rule of V’s using the topography.
At around 12:10 pm we reach the trailhead of the next destination at Cross Creek. This trail, for the 3.60 miles roundtrip, started out (along the river valley, through the Eagle Creek mine reclamation area) and ended several hundred feet higher at a ridge of igneous basement. Here, Stewart revealed that we would map the remaining three units; the Leadville Limestone (source of many of the area mines), an undifferentiated unit, and the crystalline basement rock (1.7 Ga) we were standing on. An unconformity between the crystalline basement and the undifferentiated unit indicated that there is missing units from the geological record.
The ridge we mapped from is a rugged grey crystalline basement (granite) composed of Potassium feldspar(1-3mm long crystals), quartz(1-7mm), biotite( 1-3mm), and feldspars (1-10mm). This same unit could be seen exposed to the west but was cut by the river flowing through it. The granite rocks also contained intrusions of a combination of lighter colored minerals identified as quartz and feldspar. Here we fueled ourselves up with lunch and sky mapped the units exposed to the west and north.
At 2:00pm we headed back to the parking lot and back into the van, made a quick stop at the Inn for a break, and then headed back out at 2:30pm for the third and final stop called the Flag Pole. Although this hike was shorter than the rest, 0.7 miles, it was the steepest and most challenging. It took a lot more energy to reach the top and took approximately an hour. But hey what doesn’t kill you makes you stronger! From there we completed the rest of our map and explored the exposed rocks of the Pennsylvanian Minturn Middle sandstone which exhibited mudcracks along the bedding surface.
Finally, back at the Eagle River Inn we sat down to color our maps and produce a cross section of the mapped region to help interpret the local geologic history. The sedimentary units decrease in age from northwest to southeast and dip toward the northeast. The stratigraphy we observed-from oldest (1.7 billion years ago) to youngest (~300 million years ago): crystalline basement, undifferentiated unit, Leadville Limestone, Pennsylvanian Minturn Lower, and Pennsylvanian Minturn Middle.
All the units had very similar strike and dips, varying with plus or minus 5°, indicating that the sedimentary units likely became tilted from a single tectonic event. The large mineral grains in the crystalline basement indicate that it formed at depth at the right temperature and pressure conditions where the minerals had a lot of time to grow. To draw a more complete picture of the geologic and tectonic history of the area, we would need to measure more strike and dips as well as visit more sites to allow for more detailed mapping to be done. By the end of the day everyone understood the basics of mapping, but more realistically, what it takes to be a field geologist… which of course involves a lot of hiking. Up hill!
