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Home , Dakota Hogback, Denver Formation, Dinosaur Ridge, Fountain Formation, Hogback (geology), Lytle Formation

RMAG Geohike 2021 Guidebook

Ben Burke, PhD

June 2021

Looking Southwest from Dakota to Red Rocks. Photo by Anne Steptoe

1 Geohike 2021

Thank you for supporting the Rocky Mountain Association of Geologists by signing up for the Geohike 2021 Challenge! This guidebook seeks to pique your geologic curiosity as you hike the Red Rocks to Dakota Ridge trail. This guidebook is not written with any particular starting or stopping point in mind, nor with specific directionality in mind. You may hike the trail clockwise or counter-clockwise. You may park at any of the trail access points. Some of these parking locations are directly on the trail; others are a short walk from the trail.

Logistical Details

Parking Locations

Lot Address Latitude Longitude Lot County Rd 93 & I-70 (southeast corner), 39.69805 105.2034 Golden, CO 80401 Ridge (east side) S Rooney Rd & W Alameda Pkwy, Morrison 39.68632 105.1931 CO 80465 (west side) County Rd 93 & W Alameda Parkway, 39.67998 105.1969 Morrison CO 80465 at Red Rocks Park Rd * Jurassic Rd, Morrison 39.67093 105.1949 Road CO 80465 Red Rocks Trail Rd & Red Rocks Trail Rd & W Alameda Parkway, 39.67269 105.2016 Alameda Parkway Morrison CO 80465 Matthews Winters Park 1103 County Rd 93, Golden CO 80401 39.69592 105.2038

Public Transit & Alternate Transportation Details The region is served by the Regional Transit District (RTD) bus and rail service. Nearest bus and rail station: From downtown Denver, take the W line west from Union Station to the end of the line at the JeffCo-Government Center station. Take a rideshare or cab from the light rail stationto the Dinosaur Parking Lot. There is no bus service convenient to Dinosaur Ridge.

Digital Accessibility In addition to this document in PDF or paper format, consider downloading the Mancos app from your favorite app store. This app allows you to see a geologic map at your current location, custom set the transparency of the geologic map, and read a short description of the lithology.

Geographic & Geologic Summary

This hike traverses both the ridgeline of Dinosaur Ridge, also known as the Dakota , and provides expansive views of the to the east, and the variable alluvium of the Mount Vernon creek valley. The ridgeline is on the youngest rocks of the hike, the various sands and of the lower Dakota Group. The western side of the ridge has the lowestmost Cretaceous rocks in the Denver Basin exposed, as well as the Jurassic . You are in the type locality of the Morrison Formation; the town of Morrison is located a few miles south of the trail. The terrain of the valley varies between various alluvium and colluvium and the . The 1967 report by the US Geological Survey titled “Mountains & Plains: Denver’s Geologic Setting” summarizes the geologic setting you will see throughout this hike:

2 Figure 1: Parking Lot Location Map

3 Figure 2: Geologic Map of the Dinosaur Ridge Area from Scott, 1972

4 Figure 3: Geologic Map Descriptions of the Dinosaur Ridge Area from Scott, 1972 5 Figure 4: Hike Segments and Route Map

6 A slice of geologic history is exposed to view in the Denver, , area. Denver is situated on the High Plains near the east front of the . As one travels westward from Denver towards the mountains, successively older rocks are crossed–from the geologically younger rocks of the High Plains and the South Platte River valley to the older rocks of the foothills and the ancient rocks of the mountains. As one approaches the mountain front, an array of hills, , and mesas partly hides the high mountains further west. One of these is Green Mountain, a large, rounded hill between Alameda Parkway and US Highway 6. Conglomerate, or naturally cemented gravel, hundreds of feet thick caps the hulking mass of Green Mountain. The gravel was deposited at a much higher level than the pediments by a still older steam, and is all that remains of a formerly more extensive deposit. The gravel, later cemented to form conglomerate, lies on slightly older volcanic mudflows and stream deposits known as the , the bedrock that underlies most of Denver at shallow depth. The most prominent hogback is formed of the Dakota , a beach deposit of an ancient sea. Abundant ripple marks, still preserved, formed in the shallow water. The footprints of that walked the sandy beaches can still be seen on the sandstone layers. Older than the Dakota and farther to the west, upturned against the ancient rocks of the mountains are the colorful red sandstone monuments of the , which frame Red Rocks Amphitheater and for which the amphitheater is named.

Segment 1: Northern Parking Lots to Alameda Parkway via Dakota Ridge

This location is a spectacular starting location for the hike. You are parked on the western side of Dinosaur Ridge, the first hogback on the western edge of the Denver Julesberg Basin west of Denver, Colorado. This segment can start either at the Matthews-Winters Open Space parking lot or at the Stegosaur parking lot. The parking lot is situated in the Strain Member of the Lykins Formation.There are two trails away from the parking lot. One departs from the east end of the parking lot and crosses the exposed face of the roadcut for the interstate highway. Although beautiful, this trail is a dead end. The other heads up the hill from the south area of the lot. Quickly, you leave the Lykins Formation behind as you ascend the ridge. Quickly, you move into the younger rocks of the Jurassic Morrison Formation and further upwards in time, passing through the , the oldest Cretaceous rock here. At the ridge, you are in the Cretaceous Dakota Group. The Dakota Group contains several shale and sandstone units that you will see throughout the hike along the ridgeline. This segment ends when you descend east of the ridgeline to the road. Your eastward view from the ridgeline is into the Denver Basin, with the City of Denver as well as the metro region spread out before you. The topographic body in front of you in the foreground is Green Mountain. It is capped by the Green Mountain Conglomerate and the Table Mountain , a -aged potassium rich (Scott, 1972). Green Mountain has many trails on it and is popular with hikers and mountain bikers. In the valley between the ridge you are standing on and Green Mountain Colorado Route 470, a ring road to the City of Denver. The valley is covered to a large extent in the Holocene-aged Piney Creek alluvium, however the underlying upper Cretaceous bedrock peeks out here and there. The uppermost Cretaceous units exposed here are the , a slope former, as well as the Fox Hills Sandstone, a brightly colored set of outcrops seen at the base of Green Mountain. The fact that the Fox Hills, an uppermost Cretaceous fluvial sandstone is juxtaposed near the Dakota Group sandstone and shales, is made possible by an offset on the Golden Fault that is several thousand feet. Your westward view from the ridgeline is the face of the Front Range. Most prominent on that is the reddish of the Fountain Formation, a delta sequence that is exposed in several places along the Front Range. Most prominent among those exposures are the outcrops at Red Rocks Park to the south of your vantage point and further south at and . To the north, the Fountain Formation outcrops near Boulder. In the valley below you to the west is the Morrison Formation–mostly shale with lacustrine sandstone lenses–and the Lyons Sandstone.

7 Figure 5: Parking Lot Locations at the Matthews-Winters Park and Stegasaur Lots

8 As you hike south, you will see Alameda Parkway rising up the east side of the ridge to meet the trail at the ridgeline. There are parking areas at the base of the ridge on either side where the road is closed to vehicular traffic. The trail connects the roadway to the ridgeline via a short jog on the east side of theridgeline.

Alameda Parkway Trail Intersection

This segment of Alameda Parkway is a very popular natural history walk with world-class dinosaur tracksites on view part way down the hill at a small semi-enclosed viewing area. At the top of the hill where Alameda Parkways crests the ridge, a small roadcut reveals spherically weathering nodules, a few oil seeps, and volcanic ash layers.

Figure 6: Alameda Parkway Crest at Dinosaur Ridge. Photo from north side looking southeast. Photo by the author

Be careful to stay in the pedestrian section of the roadway and look both ways for both bicycle and tour bus traffic. The Dinosaur Ridge Visitor’s Center operates a tour bus that traverses the exhibits along theridge.

Rocky Flats Alluvium Outcrop Along Ridgeline

Approximately one mile south of the Dinosaur Parking Lot is a patch of Rocky Flats Alluvium. Scott, 1972, describes the Rocky Flats alluvium as: Reddish-brown pebbily silt and clay interlayered with gravel. Stones deeply weathered, and, in the upper part of deposit, covered by calcium carbonate. Inferred Rocky Flats channel preserved in Mount Vernon Canyon and in wind-gap through west of Green Mountain. Thickness of alluvium 10-15 feet.

Segement 2: Dinosaur Ridge to Red Rocks Trail Road Parking Area

This segment begins with a short uphill from Alameda Parkway on Dinosaur Ridge up to the crest of Dinosaur Ridge. You are in the Dakota Group of lower Cretaceous shales and sandstones. The trail has two short switchbacks on the west side of the ridgeline just below the crest before connecting in a straight line with Hog Back Road. You leave the Dakota Group sandstones behind as you leave area containing small trees and enter open country. The trail traverses the lower Cretaceous Lytle and Jurassic Morrison formations. Both are hardly noticeable on the trail since they are slope-forming lithologies that readily erode. The trail crosses Hog Back Road via an unprotected crosswalk and stays on the north side of Red Rocks Park Road between Hog Back Road and Jurassic Road where a small 12 vehicle parking area sits on the south

9 side of the intersection. From this point, the trail is on the north side of Red Rocks Park Road. Through open country, the trail traverses the Piney Creek and Louviers Alluvia to the intersection of Red Rocks Trail Road and Alameda Parkway at a small 6 vehicle parking area. To the north of the trail, two houses and some low, white sandstone outcrops are visible. These are outcrops of the Permian-aged .

Segment 3: Red Rocks Trail Road Parking Area to Matthews-Winters Open Space Parking

This segment of the trail traverses the highest elevations of the hike. Between the Matthews-Winters Open Space parking area on the north end of the segment and the Red Rocks Trail Road parking area on the south end of the segment, the sole bedrock lithology traversed by the trail is the famous Permian and Pennsylvanian-aged Fountain Formation. This formation is informally known as the Red Rocks Formation, for the concert venue south within the park from the trail. The Fountain Formation is described by Scott, 1972, as: Maroon arkosic thick-bedded coarse-grained sandstone and conglomerate containing this layers of dark-maroon micaveous silty fine-grained sandstone that are more abundant in the lower part. Char- acterized by festooned and torrential cross-bedding. Composed primarily of detritus, but contains rare fragments of lower Paleozoic rocks in lower park. Conglomerate near base contains boulders as large as 10 inches in diameter. Thicness [up to] 1650 feet. More recently, an entirely Pennsylvanian age has been proposed by Sweet (2015) based on field work to the south at Manitou Springs, Colorado. The highest elevation section of the segment traverses a flat to gently rolling landscape. This is a Holocene rock slide deposit from the overlying Precambrian gneiss. The southern end of the segment contains two spectacular outcrops of Fountain Formation that the trail crosses between. The trail also traverses several types of alluvia and colluvia many of which are recent ( in age).

References

• Scott, Glenn R., 1972. Geologic Map of the Morrison Quadrangle, Jefferson County, Colorado. IMAP- 790. Scale 1:24,000, doi.org/10.3133/i790A. • Sweet, Dustin E., Carsrud, Corbin R., and Watters, Aaron, 2015. Proposing an entirely Pennsylvanian age for the Fountain Formation through new lithostratigraphic correlation along the Front Range. Mountain Geologist 52 (2) 43-70. • US Geological Survey, 1967. Mountains & Plains: Denver’s Geologic Setting. 23 pp. US Government Printing Office, Washington, DC. doi.org/10.3133/70039249

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