|
click on the red circle to go to images of that stop |
The Virtual Field Trip you are about to go on takes you from the edge of the Great Plains 65 million years ago (mya) and then travels farther back in time as you head to points west and ultimately 1700 mya (1.7 bya), a time span covering more than a third of the Earth's existence. The rock types will vary from sedimentary deposits on the plains and lead to regional and contact metamorphic rocks as well as plutonic igneous rocks, the heart of Horsetooth Rock itself.
The actual field trip will take you approximately 3 hours to complete if you follow the route and spend a minimum of time at each location. The field trip could easily be a full day if time is permitted for rock and fossil hunting, as well as the ultimate conclusion, climbing Horsetooth Rock. It is possible for someone to observe 5 of the 7 stops directly from a car, but not all stops have pulloffs and none of the stops is wheelchair accessible for field study. Parking is currently restricted at Spring Canyon Dam due to security concerns - you should park at Stop 3 and walk to Stop 4 or beyond Stop 4 about a half mile and hike back. Extreme caution should be exercised for Stops 3 and 4 along W. County Road 38E as it is heavily traveled.
The start of this field trip (Stop1) is at the corner of S. Overland Trail and Skimmerhorn St., 1/4 mile south of the intersection of S. Overland Trail and W. Drake Road. Turn west on Skimmerhorn, drive 50 yards and park at the base of the incline. The Stop 1 outcrop is across the street on the south side.
The end of the field trip (Stop 7) is after a 30 minute hike into Horsetooth Park from Stop 6, the parking area. Persons who wind easily should proceed cautiously.The parking area is often crowded and requires a daily use fee of $6.00 / vehicle for Larimer County residents.
One suggestion is to do each stop on the field trip backward, starting at Horsetooth Park (Stop 6 and 7) early in the morning. This often helps to beat the crowd and should provide a parking spot and less traffic on the trails.
Round trip distance via the roads is approximately 12 miles. The hike from Stop 6 to Stop 7 is about 30 minutes, uphill (1 hour total), and about 2 miles round trip. Restrooms are available at the parking lot, Stop 6.
A VERY IMPORTANT REMINDER: Larimer County's Horsetooth Park and the City of Fort Collins Natural Areas prohibit the removal of any flora or fauna and rocks and minerals, even though the geologist in all of us wants to pick up some samples! Please respect these restrictions and help preserve this area for others to enjoy. Remember, take nothing but pictures, leave nothing but footprints . . .
Stop 1 - At the start of Skimmerhorn Street is an excellent road cut exposing the Smokey Hill Shale member of the Niobrara Formation, Upper Cretaceous Period (87-82 mya), just west of the contact between the Niobrara and Pierre Shale.
The Pierre Shale (also Upper Cretaceous) underlies most of Fort Collins and thickens across the northern edge of the Denver Basin.and surface exposures grow younger as you head east across the plains. Capping the Pierre Shale near Wyoming and also just east of Greeley, CO are an Upper Cretaceous Sandstone and then a younger Teriary gravel deposit which makes up the Ogallala Aquifer is added in far Eastern Colorado.
Included in the Smokey Hill Shale outcrop here at Stop 1 are some sandstone and limestone layers as well. This is an excellent place to take a strike and dip measurement of the Niobrara. The dip is 21 degrees to the east and the strike is nearly north - south. The Niobrara is most famous in Kansas, where outcroppings preserve some of the finest fish and Pterodactyl fossils ever found. Within the middle of the Smokey Hill member in the Fort Collins area can be found Pseudoperna congesta, small bivalve shells, similar to the oysters found in the ocean today. The exposed outcrop at Stop 1 is near the top of the Smokey Hill member and Pseudoperna congesta have not been observed here to date.
|
click on the red circle to go to images of that stop |
Stop 2 - Drive to the end of Skimmerhorn Street (2900 block) and directly south of the cul-de-sac is an access trail to the City of Fort Collins Pineridge Natural Area. Be sure to pay attention to parking restrictions and also trail closures in this area.
Take the trail a short distance (200 yards) up the hill towards the southwest and you will reach the ridge of the first hogback, the Fort Hays Limestone Member of the Niobrara Formation. You are now standing just shy of 1 mile above sea level.
Here you will easily find abundant Inoceramus mollusk shells, although rarely whole. Also observed are Pseudoperna congesta attached to the Inoceramus. Careful inspection of surrounding layers also reveals small ironstone concretions, many weathered out of the rock and resting on the exposed surface.
Directly below the Fort Hays member of the Niobrara Formation is the suite of shales, sandstones, and limestones commonly refered to as the Bentonite Formation, but referenced on the Geologic Map of the Horsetooth Quadrangle as the Carlisle Shale, Greenhorn Limestone, Graneros Shale, and Momry Shale. The layers begin directly west of the Fort Hays hogback ridge and are exposed along the trail as it continues west across the Pineridge Open Space valley. The Greenhorn Limestone is nicely exposed at the bottom of the hill. Because these rocks erode easier than the Fort Hays limestone, they are considered valley formers along the Front Range. The small Dixon Reservoir, as well as College and Claymore Lakes to the north, lie atop these formations. The Mowry Shale is included in the Lower Cretaceous (145 - 100 mya). The Bentonite Formation lies directly on top of the hogback forming Dakota Group on the west side of Pineridge Open Space. The Dakota Group is explored in more detail at Stop 3.
Return to Skimmerhorn Street and follow the directions to drive to Stop 3.
Stop 3 - Directions: Go north on S. Overland Trail from Skimmerhorn Street (Stop 1) 1/4 mile to W. Drake Road. Turn right (east) onto W.Drake Road and go one mile to S. Taft Hill Road. Turn right (south) onto S. Taft Hill Road and drive 1 1/2 miles to the intersection of S. Taft Hill and W.Harmony/W. County Rd 38E. Turn right (west) onto W. County Rd 38E and drive approximately 1 3/4 miles to Stop 3 at the top of the switchback and the exposed road cut. The parking area is directly in front of you as you climb the hill. Be careful pulling off into this small parking area - you can bottom out low suspension vehicles!
You are now located at the top of the Dakota Group, the most prominent hogback forming ridge west of Fort Collins, and the eastern flank of Horsetooth Reservoir.
Here you can see the impressive Spring Canyon Dam and many of the improvements that were made during reconstruction from 2000-2003, including the large stability berm at the foot. The improvements to the eastern dams were needed to reduce the chance of dam failure during a seismic event (earthquake). You can read more about this project by going to the Bureau of Reclamation website or abouthorsetooth.com.
The main hogback is formed by the South Platte Formation of the Dakota Group and consists of a well sorted, fine to medium grainer sandstone which is well cemented. The outcropping at the Spot 3 parking and across the soad on the south are excellent examples. The cementation is so good that many locals use this sandstone for boldering practice.
Below the first sandstone member of the South Platte Formation lies a middle shale member of dark gray carboneferous shale with thin bentonite, gray siltstones, and sandstones. The shales erode easily and form a minor valley between Spot 3 and Spot 4 on the map.
At the base of the South Platte Formation is the Plainview Sandstone Member which lies directly on the Lytle Formation. The contact between the Plainview and Lytle marks what geologists call an unconformity - a break in the sequence of rock deposition. The Lytle is still older than the Plainview, but erosion exposed the top of the Lytle Formation before the Plainview was deposited.
The Lytle formation is very coarse grained with numerous levels of conglomeratic sandstone. It is well cemented and forms a very rigid hogback. Just 9 miles to the south the Lytle is exposed on the east limb of a syncline as the verticle and impressive Devil's Backbone west of Loveland. This syncline is formed by the Milner Mt. Fault. Devil's Backbone is also another wonderful day hike and is a Larimer County Park.
Large blocks of the Lytle scatter the eastern shore of Horsetooth Reservoir all the way to the hogback ridge. and help to obscure the Jurassic age rocks beneath.
click on the red circle to go to images of that stop |
Stop 4 - Directly under the Lytle Member of the Dakota Formation is another unconformity and then the Morrison Formation of the Upper Jurassic Period (161-145 mya). This famous formation has yielded numerous dinosaur fossils up and down the Front Range and a few years ago a full Stegosaraus fossil was unearthed nearby in Masonville.
The road cut here is particulary dangerous, as the road shoulders are narrow and you may need to cross at a blind corner for both directions of traffic. The rock is interbedded green, red, yellow, white, and even purple weathered claystone and siltstone which is poorly cemented. The rock crumbles easily and is difficult to sample. The Morrison Formation forms the bulk of the slope below the Lytle Conglomerate on the east side of Horsetooth Reservoir.
Also exposed on the east side of the reservoir are the Sundance Formation, Upper and Middle Jurassic (175-161 mya) and the Jelm Formation of Upper Triassic age ( 228-200 mya) which contain several unconfomities in the suite of rocks. There is also an unconformity at the base of the Sundance and Jelm. The Canyon Springs Member of the Sundance, a reddish-brown, orange-pink sandstone is also expressed in western Colorado as the Entrada Formation.
Below the Sundance and Jelm Formation is the Lykins Formation of the Lower Triassic and Upper Permian age (260-245 mya). This reddish-brown siltstone and fine-grained sandstone, with some carbonates, forms the valley where the bulk of Horsetooth Reservoir rests.
One limestone layer, the Forelle Limestone member, strikes along the entire reservoir and under the north Horsetooth Dam. This layer had always allowed some water to pass beneath the north dam, but by 1998 concern was mounting as the flow increased dramatically. The reservoir was lowered and engineers built a cutoff wall and new filter for Horsetooth Dam. The east facing dams, Soldier Canyon, Dixon Canyon, and Spring Canyon, were all modernized to minimize seismic event risks. Work was completed in the Fall 2003 and the Reservoir was brought to full capacity in April 2004 at a pool height of 5430 feet - the most water ever held in Horsetooth Reservoir.
The Lykins formation is one of the thickest units in the strategraphic sequence at over 600 feet.
Stop 5 - Directions: Return to W. County Rd 38E and head south toward the South Bay and the former town of Stout, CO. An alternative Stop is the South Bay area which has restrooms and a very nice exposure of the Lyons Sandstone, but there is a user fee for this area. Continue up the west side of Horsetooth to the top of the hill and pull off at "The Perch" parking area. This is Stop 5.
click on the red circle to go to images of that stop |
You are now sitting on top off the Lyons Sandstone, Lower Permian period (299-270 mya). This is one of the most important economic rocks along the Front Range. The quartz cement of this sandstone makes this a particularly strong rock, forming the predominant hogback on the western side of Horsetooth Reservoir. Rocks quarred below "The Perch" were used for construction in nearby Fort Collins and also shipped around the country. Quarrying operations were recently started in the Lyons Sandstone south of Masonville.
The Lyons Formation shows massive wind swept dune formations, leaning toward a terrestrial origin rather than aquatic like so many of the other formations.
Below the Lyons is the valley forming Owl Canyon Formation, also of the Lower Permian period (299-270 mya). The rock is mainly a red silt and sandstone interbedding. You will see this layer better when you proceed to Stop 6.
Also at Stop 5 is an excellent view of the beginning of the Buckhorn Creek Fault in the Precambrian metamorphic and igneous rock, resulting in the faulting of the overlying Lyons down through Fountain Formations. Just over the hill to the north is a second fault which parallels the Buckhorn Creek fault and creates Spring Canyon in the Horsetooth Mtn. Park. The rocks across from "The Perch" have been thrust upward by nearly vertical faulting, resulting in the offset of the formations.
As you proceed to Stop 6, you will go down a fairly steep hill through the Owl Canyon siltstones and then past the hogback forming Ingleside Formation, also Lower Permian (299-270 mya). This well cemented sandstone is reddish-pink in color and caps the underlying Fountain Formation, the last sedimentary sequence before the basal unconformity and metamorphic and igneous rocks. The Fountain Formation, Lower Permian and Middle to Upper Pennsylvanian (311-299 mya), is the thickest strata of all the sequences and is mainly a conglomerate, medium to coarse feldspar sandstone, and some siltstones. The color of the Fountain Formation is very red, giving rise to the spectacular exposure of this formation at Red Rock Amphitheatre in Morrison, CO, outside Denver. The bottom of the Fountain Formation sits over 1.7 billion year old metamorphosed sediments, representing an unconformity of 1.4 billion years; more than a quarter of Earth's history missing! The contact is difficult to find here, but exposure at Red Rocks Park is very clear, as well as some areas west of Loveland like PineWood Reservoir.
Proceed along W. County Rd 38E past the entrance to Inlet Cove and then up a hill in a narrow valley toward Masonville. At the top of the narrow valley is a parking area on your immediate right. This is Stop 6 and the Horsetooth Mtn Park parking area.
Stop 6 - You should now be in the parking area for Horsetooth Mtn Park. There is a daily use fee of $6 / vehicle if you plan to hike to Stop 7 and Horsetooth Mtn beyond. Your altitude is roughly 5800 feet. It is a 1400 foot verticle climb to the top of Horsetooth Mtn.
The parking lot sits on the Buckhorn Creek Fracture Zone, near where the Milner Mtn Fault comes up from the south. To the south is Milner Mtn. To the north is Horsetooth Mtn. The Buckhorn Creek fault runs east down the valley you just drove up, across Inlet Bay, and then up the road to Stop 5. The fault proceeds westward up Buckhorn Creek and splits to form the Redstone Creek Fault. The faulting is probably inactive, but could have been active as recently as 5 million years ago when the Tertiary uplift was coming to an end (23 -5 mya)
There are two trails you can take - one follows a road and is suggested for bicycles and horses. This trail initially travels west. The second trail is for hikers and starts off by the upper pavillion and heads east, but soon swings back west as it switches back up a small valley. These rocks you are walking (or riding) on are 1.7 billion year old mica schist, composed of quartz and feldspar. They are metasedimentary, having once been layers of sedimentary deposits probably originating from the older (2.5 bya) Wyoming Craton to the north. The metamorphosis resets the atomic clocks, so probably occurred about 1.7 bya. The strike of the foliation is parallel with the Buckhorn Creek fault with a dip of the metasediments close to verticle. Interspersed are small to large pegmatitic dikes also striking west to east.
click on the red circle to go to images of that stop |
Stop 7 - Where the trail splits to Horsetooth Falls or the Soderberg Road, follow the Sodderberg Road to the left and climb to an altitude of almost 6200 feet, where the trail meets the bicycle/horse road. At the junction is a very large pegmatite dike with wonderful quartz, mica, feldspar, and tourmaline crystals. This pegmatite is similar to those pegmatites that make up the core of Horsetooth Mtn and Arthur's Rock to the north. These pegmatites are 1.6 bya and may be associated with the emplacement of the Boulder Creek Granodiorite. The Horsetooth Mtn Pegmatite is the dark blue formation on the map near Stop 6 and Stop 7. To the east of Stop 7 are even younger intrusions, labelled Yd on the map and drawn as red lines, which are 1.4 bya and may be associated with the emplacement of the Silver Plume Granite elsewhere along the Front Range. Farther up the road is another parallel pegmatite dike which includes xenoliths of metamorphic rock
You can continue on the road and up toward Horsetooth Mtn from here or return along the same path. The trip to the top of Horsetooth without too much stopping is probably 1 hour from this point. Round trip is probably 2 hours.
Total distance back to Fort Collins will vary, but is close to 6 miles and will take 15 minutes from the Horsetooth Mtn Park parking lot, Stop 6.
That concludes the actual field trip!
