Historical Geology Field Trip: Dan Fosha, GEY 121 Fall 207; Prof. Trina Riegel

This is the first post for my field trip for historical geology for the fall 2007 semester. The sites we will see will be from the Front Range of Colorado, and will focus on the layers that edge up against the mountain front. It is tightly based on a trip found in the book "Field Trips in the Southern Rocky Mountains, USA" published by the GSA. The trip starts in Boulder, at the intersection of Baseline Road and Highway 36. Exit Highway 36 and at the light, set your odometer to 0. Take a left at the light on Baseline. Here we go!

2.1 Miles: Here we have the first view of the Fountain Formation on the side of Flagstaff Mountain. The Fountain Formation is widespread in Colorado, and was formed from the erosion of the Ancestral Rocky Mountains. The Fountain Formation is made up of weathered granite from the easterly mountain range of the Ancestral Rockies, Frontrangea. The depositional environment was one of massive alluvial fans, perhaps like what we might see in Nevada today. The Fountain Formation is responsible for the major geological tourist attractions on the Front Range: Garden of the Gods, Roxborough State Park, Red Rocks Amphitheater and the Flatirons in Boulder.

It is composed of red sandstone and conglomerate, and some thin beds of mudstone. Within the rock, we find quartz, potassium feldspar, granite, and mudstone. The reddish color is particularly pronounced in the finer grained sediments, which we will see in detail in a few posts.

2.4 There is a panoramic viewpoint to the right. You might be able to make out some of the fault zones here. This is looking to the north. The hills in this picture are the tips of the broken rock layers peaking out from the surface, where they lay up against granite underneath.


3.6 First outcrop of weathered granite.



4.1 Stop here to your right at the bend. This is an interesting cross section. We'll scroll from the right to the left, from the parking area. The tilted layers are readily accessible here. To the right (east) are the youngest layers. We are facing north in this picture. This is the Fountain Formation. Look at the dip – it's about 50⁰ here.



This is pretty neat. Here may be an ancient soil, or a paleosol. Others think it is just Boulder Creek Granodiorite that weathered some before it was buried. The circle part that is red crumbles very easily. I found it difficult to call it rock. More importantly, look at the vein right in the middle. This is a quartz vein.


A close up of the weathered granite.

Here is another shot further to the southwest of more veins.



The Boulder Creek Granodiorite is in full force here, showing typical granitic weathering patterns. The brown color is from modern day weathering.


A close up of unweathered (relatively) Boulder Creek Granodiorite.

Here we are looking at the far southwestern end of the outcrop. The Granodiorite is to the left and to the right, the rock becomes a bleached red color and crumbles easily. Scientists think that this is an indication of Paleozoic weathering.

Go back down the hill. You can stop at the panoramic viewpoint if you like again. It is at mile 5.8.

7.5 Turn left on 6^th street. Follow it through the neighborhood to Canyon and take a left.

7.7 Turn right here after the Watershed School, and then an immediate left into the parking lot of Settler's Park. Cross the bridge and walk up to the rock formations here. There is supposed to be a small quarry here, but I didn't find it. I did look at rocks which I think are interesting. Look at the dip in this next picture. It's 90⁰ or more. It is a reverse fault. In some places along the Front Range, older granitic rocks have been pushed over younger sedimentary rocks.

In this park we see the Fountain Formation start to intergrade with the Lyons Sandstone. I think this pink layer here is the Lyons Sandstone.

Here is a close up of the pink sandstone.

After looking around, go back to the parking lot.

7.7 Leave the parking lot, and then turn left on Canyon.

9.4 At 28^th street, turn right. It becomes Highway 36. Keep going

20.9 At CO 121 (Wadsworth Boulevard), exit and go south.

31.1 At I-70, go westbound.

41.1 Take the Morrison exit. Then turn left onto Hogback Road, all the while watching for the signs to Red Rocks Park.

42.9 Take the first entrance to Red Rocks Park. Turn right and go all the way up to the Top Circle parking lot for Red Rocks and the Visitors Center.

43.7 Keep going straight past the parking lot on the left for Red Rocks Trail.

44.1 The road goes through a fun arch tunnel cut.

44.4 Park here.

To the northwest look for a marker stuck in the rocks. It indicates the contact boundary between the Idaho Springs Formation and the Fountain Formation. First notice that the Granite here, to the right, doesn't show as much weathering as the Boulder Creek Granodiorite did earlier in our trip. Then to the left, see the obvious conglomeritic nature of the Fountain Formation.

A close up of the Fountain Formation's conglomerate tendencies. I think this is at the contact point too.


Now walk over to the signs for the visitor center, between the two giant rocks sticking out of the ground. To your left as you walk will be Creation Rock, and then farther to the south, forming the south side of Red Rocks Amphitheater will be Ship Rock. Both of these rocks show the characteristic alternating pattern of the Fountain Formation between chunky paler, conglomerate and dark red finer grained laminated redbeds. Here is Ship Rock.

And here is Creation Rock, the other frame of the Amphitheater. Notice the alternating layers?

Here is a close up look at the layers.

Inside the visitor's center are some neat displays. This one is of a fossilized palm frond from the Cretaceous Era. They also had some Dinosaur tracks, but I'll show you some better ones later on.

Of course, you have to look at the 'natural' Amphitheater that was built here. Where the stage is now was a pile of boulders that had to be blasted away. This picture is interesting too in that it shows the other layers of sedimentary rocks that are tilted up against the Front Range. As we look east here, the younger rocks are farther away. The big ridge dotted with trees is the Dakota Hogback. To the left in the back of the picture is Green Mountain, a very young pile of conglomerate. You can just make out the Denver skyline to the right of Green Mountain.

Go back to your car when you are finished looking around.

44.4 Go back down the hill.

44.9 Turn left where it says 'Geologic Marker'.

45.2 There is a cul-de-sac here. Park in the lot, and go up the concrete stairs to the southeast. There is a neat sign here that explains the geology of this area. It has a similar view to that of the last picture. Here is a close up of the diagram.

45.5 Go back to the road out of the park and take a left.

45.9 Watch for a weathered old sign indicating the Fountain Formation (how convenient!)

46.1 Pull off here, where there is a sign for the Lyons Sandstone. Here is a close up of it. The sign is to the right of this picture. It's taken looking north. Supposedly this is the coarse grained version of the Lyons Sandstone. Bigger grains, that are almost conglomerate sized, indicate that this rock can not be the result of Aeolian, or wind blown sedimentation. I only saw some fine grained sandstone with some neat cross bedding.

Continue down the hill.

46.2 Another weathered sign, this time for the Lykins Formation.

46.5 Junction of West Alameda (Red Rocks entry road) and Hogback Road. Go straight across.

46.7 Stop here; just pass the small metal shelter. Walk back down the road to the shelter and display. The display features fossilized dinosaur bones in the famous Morrison Formation. The darker color makes them fairly obvious. Many of these bones were destroyed when Alameda Parkway was built, but it is likely that there are more bones farther into the hillside. All in all, there are 109 bones visible along this outcrop. The dark stained spot in the center is a pelvis bone, I think.

The Morrison Formation is mostly quartz and a little feldspar. The cement is "ferroan dolomite" with some kaolinite mixed in for good measure. The direction of 'transport' or deposition of the sandstone is to the right side of this photo. There was probably a river near here and the bones, from a dinosaur already dead, were moved about in the water during a flood, and later deposited in a floodplain. So we are probably looking at a sandy streamside. Supposedly, to the left of the bones, the sand is a little coarser, as they stopped bigger sand grains from moving along. The bones are not cracked on their surface, so they were probably buried quickly, probably by sand dunes.

Here is a photo of the upper part of the Morrison Formation. This part is not as resistant to weathering as the lower part is where the bones are located.

Walk farther up the road to the next interpretative sign, the 'Brontosaurus Bulges'. The only way geologists have found to explain these small bulges in the rock layers is that they are the tracks of Apatosaurus (aka Brontosaurus). These dinosaurs were so heavy that they compressed the soil, making a deep impression, and the track was subsequently lithified.

At point you can continue walking up the hill to see all the signs, or drive to the parking area on the other side and walk back up the hill. I will continue in sequence up Alameda Parkway.

As we walk up the hill, we meet younger rock layers. Here is a layer of ash that fell when a Cretaceous volcano erupted in central Colorado. It is somewhat protected in this case. You can see the thin layer go up to the right of the case.

Here is a close up of the ash layer.

Going over the hill, there is a neat sign that shows how the Denver basin was formed and how petroleum was trapped in it.

And here is another one that has a good diagram of reverse faulting along the Front Range.

As we continue down the hill, we get to a younger rock group called the Dakota Group. It is mostly sandstone. Here is a picture of Dakota sandstone, the rock that forms the well known hogback that runs from Colorado Springs to Fort Collins. If this doesn't just say to you "the West", what else do you need, a cowboy hat?


7.1 Here is a bedding plane in the Dakota Group with ripple marks. It's on the east side of the hill, continuing down from where we were. It was likely formed in a 'quiet, intertidal zone'.

Next are some trace fossils, which are burrows and tracks of worms and crustaceans. These are very similar to what we see in modern day tidal flats.

47.3 The grand finale, the Dinosaur Ridge Track site. These dinosaur tracks are located in the Dakota Group. They are very famous, and I think they are better than the ones on the Comanche National Grassland in Southeastern Colorado. In this next photo we are looking up at a 30⁰ angle to the west.

Note that these tracks are colored with charcoal so that they are easily visible. They were discovered in 1937, but not seriously examined until the 1980s (What were those people at Mines doing all that time?) There are two track types here. Both of them are tridactyl, meaning that they were three-toed. The larger footprint was made by a larger dinosaur, something like an iguanodon, which is an ornithopod dinosaur. In the photo above you can see two different sizes of the same type of track. The small ones were probably made by juvenile iguanodons. This picture below, from the website: HUhttp://www.duiops.net/dinos/iguanodon.htmlUH, looks like it could be right from this site in Colorado.

The smaller tracks were made by an ornithomimid dinosaur, maybe ornithomimus, which looked similar to a modern day ostrich.

At the bottom of the hill is a gift shop that has many books and materials about the track site. It also has a lot of dinosaur paraphernalia and other educational materials. If you are interested, there is another dinosaur track site just south of Golden. I didn't have time to make it there, but it is supposed to be very good. Just south of 19^th street and west of US Highway 6, there is a turn off for the 'Triceratops Trail'. I hope you enjoyed this brief look at some of Colorado's geology.

Sources:

Chronic, Halka; Roadside Geology of Colorado; Mountain Press Publishing Co., Missoula, MT 1980

Lockley, M. G.; A Field Guide to Dinosaur Ridge, A Publication of the Friends of Dinosaur Ridge and the University of Colorado at Denver Dinosaur Trackers Research Group, 29 p. 1990

Nelson, Eric P., Erslev Eric A.; Field Trips in the Southern Rocky Mountains, USA; Geological Society of America, 2004.

Weiner, Robert J. and Haun, John D.; Guide to the Geology of Colorado, ed. Geological Society of America, 1960.

Thanks!

That's it! I hope you enjoyed this short tour of the some of the geology around Denver, and thanks for viewing.

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Sources:

Fossils in Schlessman Hall, Pamphlet, Denver Public Library 12/03/5C

Guide to the Geology of Colorado, ed. Weiner, Robert J. and Haun, John D., Geological Society of America, 1960.

Murphy, Jack A., Geology Tour of Denver’s Capitol Hill Stone Buildings, 1997, Historic Denver, Inc., Denver, Colorado, 96 p.

Murphy, Jack A., Geology Tour of Denver’s Buildings and Monuments, 1995, Historic Denver, Inc., Denver, Colorado, 96 p.

Plummer, et al. Physical Geology, 11th edition. McGraw-Hill Higher Education 2007

http://www.cliffshade.com/colorado/index.htm

Roxborough State Park and Environs

On to Roxborough State Park which showcases four important Colorado geological formations. Starting from youngest to oldest they are the Dakota Sandstone, the Lykins Formation, the Lyons Sandstone and the Fountain Formation. A good cross section is found on this page.

Here are a couple pictures of Dakota Sandstone, the first layer in the famous Front Range hogback. Notice in this first picture the oak thickets and juniper on the flank of the hogback. I'm not sure why the plants prefer this environment on the Dakota, but this phenomena seems to be consistent along the Front Range.

The Dakota sandstone is beach sand, and often has ripple marks, and I think the source for the sand was the ancestral Rockies, as massive rivers poured out of them and eroded them away. Dakota Sandstone was formed during the age of Dinosaurs, and so fossils are relatively abundant here. This part of Colorado was a giant shallow inland sea at the time when the sand was deposited.

It was then uplifted during the Laramide Orogeny, which is a technical way of naming the time when the mountains we have today in Colorado were first created. All of these rocks here in the park form the limb of a syncline that dips to the east.

Next over is the Lykins and the Morrison Formation. The Lykins formation is just by the parking lot, and is composed of colonies of fossilized blue-green algae, probably similar to the health food supplement. These fossils are called stromatolites, and they form thin beds in the rock.

During the time when the Lykins Formation was formed, Colorado looked like Iraq, an area of shallow coastal mud flats and arid low lying inland areas. I didn't get a good photo of the Lykins, so I will offer you this one instead from the website of the Denver Museum of Nature and Science. Click on Lykins to see what the stromatolite layers look like. Here's an even better one.

Here in Roxborough, the Morrison Formation supports a healthy grass population, and isn't easily seen. I think the Morrison Formation is underneath this meadow here, as shown in the photo above.

The Lyons sandstone has grains that are more rounded compared to the Fountain Formation. Supposedly Ponderosa Pine and Kinniknnik are common here. This is just behind the upper parking lot.

Lyons Sandstone was formed during a time when the area was covered by coastal sand dunes but still had major rivers flowing through, perhaps like Egypt with the Nile or Iraq with Euphrates/Tigris system.

The Fountain Formation is easily identified by the relatively large, sharp grains of sandstone mixed with pebbles of quartz and feldspar. Also unique are the lichens that seem to prefer it as a substrate, shown in the second photo.

During the formation of the Fountain Formation, there was a lot of erosion from rivers that flowed out of the Ancestral Rockies, the mountain range that was here before the Rockies we know of.

Directions (from Highway 85/Louviers):

32.5 take ramp to Titan Parkway. Go west. Stay on road towards Roxborough (follow signs)

39.2 Dakota hogback approaches.

39.6 Road goes through Dakota Sandstone.

42.1 Roxborough State Park.

Across the Alluviums

From here, it’s time to go on an excursion. I like to hike in Roxborough State Park, so we’ll travel out there and look at the geology along the way. As we go, we'll drive across the alluviums that sit underneath the metro area. All the alluviums that form the surface geology of the Denver area are outwash from the Colorado Rockies that still rise above the plains, not the ancestral Rockies, which are long gone.

This diagram, copied from "Guide to the Geology of Colorado" shows a cross section of the alluvial terraces in the Denver area. I-25 and the South Platte, for example run in the little trough on the far left of the diagram, so as we view the cross section we are facing south. The type is too small to be scanned legibly, but to get the idea, know that each terrace is an alluvium from a different time era. In general, the younger alluviums are to the left, and the older ones are to the right. Bedrock (not shown on the diagram) can be found peaking through the alluviums, like at Roxborough, or to the far right against the mountains, like the Flatirons in Boulder.

This first picture is the corner of Kalamath and Colfax. Piney Creek Alluvium(1500 years old or so) is underneath the street here.

This next picture is at 2nd and Kalamath. A mammoth tooth was found here. Fossils of extinct mammals are often found in alluviums in the Denver area.

This picture is perhaps what the alluvium looks like, a mix of gravel and sand.

Here is the South Platte, taken from Santa Fe going south in the Littleton area. Post-Piney Creek alluvium is along the main channel of the river. Post-Piney Creek alluvium is an alluvium similar to Broadway alluvium, but Broadway alluvium is much older, about 10,000 years old or so.

Here is some more Eolian sand blown from the South Platte River. Look at the rabbitbrush and sandsage in the second picture, telltale signs of sand beneath the soil.

This picture, looking to the west of Highway 85 (Santa Fe), shows terraces of the South Platte.

This is a picture of Broadway alluvium on the road to Louviers. We are getting closer to East Plum Creek.

This alluvium here is the Louviers alluvium, and comes from weathered Pikes Peak Granite.

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Directions (from Library):

Continue on 14th Avenue east.

5.2 left on Lincoln

5.3 left on Colfax

6.1 left on Kalamath: Post-Piney Creek Alluvium

Continue south on Kalamath.

7.7 2nd and Kalamath. Mammoth tooth found underneath.

Continue on Kalamath. Kalamath becomes Santa Fe (Highway 85)

12.8 South Platte on right. Continue on Santa Fe.

20.9 Sand dunes and shrubs on left.

21.2 Terraces of the South Platte on left.

25.7 turn right to Louviers

26.9 Broadway Alluvium on right. Follow road across bridge and into Louviers. Stay on road to the left out of town. Road goes to left again back towards creek.

28.4 Louviers alluvium. U-turn. Go back to Highway 85

30.3 turn left on Highway 85.