Open Spaces in Jefferson County
some county-owned and some otherwiseIncluding basic trail-side geology
by Dick Gibson
Green Mountain is one of Dick Gibson's favorite areas for running. The terrain is a popular mountain-biking area as well, with perhaps 20 miles of trails. In addition, the paved bike-path along C-470 is accessible from the Rooney Road parking lot. Passers-by probably view Green Mountain as a fairly non-descript, almost treeless, rounded hill, but it has some very interesting trail-side geology.
The top of Green Mountain is at 6856 feet, rising about 800 feet above the highway to the west. Fundamentally it is a remnant mass of Green Mountain Conglomerate, a pile of syntectonic gravels that developed adjacent to the rising Front Range about 55-60 million years ago (Eocene time). Syntectonic means that the sediments were laid down at the same time as mountain-building was occurring. The stonger the uplift, the more intense the erosion, and the greater the pile of gravels on the lower slopes of the uplift.
If you enter the park from the west by crossing the footbridge, the first rocks you see, on your left (north), are pinkish to whitish sandstones in a huge, roughly rectangular mass. This is part of the lower sandstone of the Laramie Formation, of Upper Cretaceous age (about 70-75 million years ago). These rocks are nearly vertical, tilted up from their original horizontal orientation by the uplift of the Front Range. The main road-like trail curves up to the north, and you can reach these rocks easily from this direction.
The lower slopes of Green Mountain, on the west side (second trail to the right off the main entrance trail crossing the footbridge), contain outcrops of dark conglomerate. Almost all of the clasts (the pieces of rock) in these 15-foot-high outcrops are large chunks of volcanic rocks like basalt, and they can be as coarse as 8 inches across. The volcanic rocks that were the source for the conglomerate were probably equivalents of the basaltic rocks at North and South Table Mountains, east of downtown Golden. The Coors brewery nestles between these two mesas, which are capped with columnar-jointed basalt flows. They have been dated at 65-63 million years, so the Green Mountain Conglomerate must be younger than that, in order to incorporate the volcanics into the sediment pile.
The conglomerate in this lower section is called volcaniclastic, because it contains so many volcanic rock fragments. Clastic is from a word meaning "break," and refers to the fragmental nature of the materials. You can see many features in the outcrop that illustrate how the material was laid down, in most cases by swift-flowing streams. You can find channels, cross-bedding (which amounts to small dunes on the stream bed), variability in grain size reflecting sorting by the stream currents, and other fluvial (river) features.
Just across the trail from the northernmost outcrop of volcaniclastic conglomerate, the finer sediments contain small chips of petrified wood that are weathering out of the rock. Most of the chips are white to buff, hacky-fracturing to square and rectangular pieces. Often you can still see the tree rings excellently preserved, as well as occasional evidence of bark.
Higher up the mountain, the conglomerate consists mostly of large rounded boulders of pinkish granitic rock. These higher deposits are younger than the lower volcaniclastics, and indicate that older and older rocks were being exposed in the adjacent mountains, to be eroded and re-deposited as the Green Mountain Conglomerate. Most of these clasts are Precambrian in age, here, about 1.7 billion years old. This is the age of the rocks in the next range of high hills to the west, where Interstate 70 enters the foothills in Mt. Vernon Canyon.
If you cross the bridge over C-470 and turn right along the paved bike path, you'll see a lot of Precambrian rocks that have been brought in to fill the ditch along the left side of the path. Go a few hundred steps south, till the rock-filled ditch ends, and you will see (above the ditch) some thin coal beds in the pre-Fox Hills rocks. In the few hundred feet from the near-vertical Fox Hills outcrop NE of the east end of the footbriged, the dip has changed to around 50 degrees -- a significant change in such a short distance. Further south, I believe the quarry in the flank of Green Mountain is in the Fox Hills Sandstone.
As you return to the parking lot, look west across Rooney Road to the slopes of the Dakota Hogback. To the south, below the trees, you may see part of the Hogback Deer Herd -- I've seen more than 15 deer at a time in that area, more than 20 times.
Red Rocks Park
Denver Mountain Parks System
An excellent reference to the geology of this park is Guide Book to the Geology of Red Rocks Park and Vicinity, by Andrew M. Taylor, published in 1992 (Fourth Revised Edition, 1995, 91 pages) and usually available at the Tattered Cover Book Store. It provides a good overview of the geology of the Front Range in the Denver Metro area. Price, about $9.00. See also our short article on the Fountain Formation. Dr. Taylor has generously provided some corrections to the information presented here -- but any remaining errors are my own!Apex Park
The trail described here is accessed from the north side of the lower parking lot of Heritage Square, off Jeffco Hwy 93 and near West Colfax, south of Golden and about 1 mile north of the I-70-Morrison-Colfax interchange. Check our Glossary of Geologic Terms for unfamiliar words in this guide, or contact Dick Gibson if you have questions. Feel free to print out this guide to take on your hike in Apex Park. A detailed map to accompany this guide will be attatched soon! Location Map [36K]
APEX GULCH TRAIL-SIDE GEOLOGY
Please note that collection of minerals as well as other natural resources upon Jefferson County Open Space lands is prohibited.
by Dick Gibson
1. A few feet west of the Apex Toll Road sign, the Precambrian begins. You will see coarse pink quartz-feldspar veins (dikes) cutting across the darker country rock (gneiss). Dikes are 1/4" to 3" across. There may be a bit of Pennsylvanian Fountain Formation in the slopes east of this point, and you can see a large block of banded Fountain Formation isolated among the Heritage Square buildings, southeast of this point. The Precambrian rocks in Apex Park are mostly 1.7 billion years old, though it is possible that some of the granitic injections were associated with the Pike's Peak Granite, which is about 1.0-1.1 billion years old.
2. Where the valley floor of Apex Gulch widens, just before heading uphill, you will find many large loose float blocks. One shows the alternating compositional bands in the gneiss very well. The layers are mostly pink feldspar alternating with black amphibole-bearing material, with some coarse quartz in the middle. We would call the rock amphibolite gneiss or mafic quartzo-feldspathic gneiss. The layers (foliation) are slightly folded. These metamorphic rocks may have originally been impure sandstones interbedded with shales. Intense heat and pressure recrystallized the minerals and aligned them in the gneiss. PHOTO - coming soon!
3. At the Apex Park Sign. A few yards to the east and uphill, you can see very convoluted (ptygmatic) layers of the pink quartzo-feldspathic rock that were probably injected into the gneiss. In autumn, hundreds of box elder beetles, wasps, and other insects can be found warming themselves on these rocks. Right behind the left side of the sign, look for very coarse (to 1/2" or more) flat, pink surfaces. These are the cleavages (preferred planes of fracturing) of feldspar crystals. Accompanying quartz is gray and has no flat cleavage surfaces. You may also find some dark greenish flaky material, which is either chlorite or weathered biotite. Both minerals are micas.
4. 30' past the sign is an outcrop up the hillside to the north (right). This is a beautiful example of folded compositional banding and ptygmatic injection. PHOTO - coming soon!
5. 50 feet further on, in the big rock on the right side of the trail, the thin injections of pink quartzo-feldspathic material look almost like toothpaste. They may have been intruded from the much larger mass at the base of the outcrop, where you can see poorly-formed feldspar crystals up to several inches across. Look for the cleavage surfaces.
6. 80-100 feet further, with two lone pine trees on the slope on the right, you can see more folds, and a gray, flat surface that dips to the west. This is not really a bedding plane, but rather a foliation surface in the gneiss. The fine lines that crisscross the surface are mostly joints, small fractures lined by brownish alteration of the rock where it has been in contact with fluids in the fractures. There is one long, straight quartz vein about 1/2" thick cutting across the rock.
7. Just past the pines you can see how a surface, dipping the same way as at Stop 6, has a much more irregular texture when the surface is developed in the coarse pink quartzo-feldspathic portion of the rock, as compared to the fine-grained darker part of the gneiss.
8. 20' further, the trail crosses a low outcrop. Then you will encounter a big outcrop on the right with a 2-4" thick pink intrusive. It contains mostly feldspar and quartz, but there are also lots of blebs of relatively coarse biotite crystals (black, shiny, flaky material). The non-flaky black mineral in the middle of the vein low on the outcrop is probably hornblende (an amphibole).
9. The trail turns right, away from the stream bed. In the high outcrop above the trail are visible some extremely coarse rocks of the quartzo-feldspathic phase, with poorly-formed crystals apparently as big as your hand. It would be fair to call this rock a pegmatite, a term for extremely coarse-grained intrusive material.
10. 100 yards further on, near a number of yucca plants, you will come to a flat outcrop on the right, sloping into the trail. On the left (western) portion of the exposure, you may find numerous small (to 3/8") reddish, vitreous grains. These are garnets. Be careful not to confuse these or other isolated mineral grains with lichens -- if you can scratch it off easily, it may be a lichen (or mica!). There are also some black blocky crystals in the rock that are probably pyroxene.
11. After you pass the Pick 'n' Sledge Trail Sign, most of the float and outcrops are more of the same -- dark amphibolite gneiss with many pink quartzo-feldspathic bands and dikes. Look for interesting folds and pegmatitic (coarse) zones in the dikes. Think about the development of Apex Stream: do you think it follows any preferred course, set by the underlying geology?
12. Soon you will pass a major side drainage to the north (right). The next outcrop at trail level has some examples of graphic granite [SKETCH - coming soon!], with small gray quartz grains in more abundant pink feldspar. Graphic granite is a regular intergrowth of quartz and feldspar that gives an angular appearance suggestive of cuneiform writing (whence "graphic," which means "writing"). Chunks of float may show this texture better than most of the outcrop (suggesting that the graphic granite crops out somewhere up the slope). Yuccas and sumac are common in this area. On the outcrop, especially associated with the quartz, you can find lots of gold-colored material. You should be able to flake it off with your fingernail -- it is mica (either phlogopite, a brown mica, or weathered biotite, the black mica).
13. You will pass a second, smaller draw off to the north (right), and you are approaching a thick stand of pines on the other side of the creek to the left. At the next large outcrop, you will find more of the dark gneiss and apparently less of the quartzo-feldspathic material. This is a normal and expectable variation within such rock types, and is not a really major change.
14. Continue around the corner, about 100 yards. Watch for a large (3 foot) isolated float block at about shoulder level on the slope to the right. It contains many prominent pink and white feldspar crystals, less obvious but abundant gray quartz, and dark mica and a few other dark minerals. This is practically the definition of granite, and if you look carefully you can see how the intrusive granite cuts across the grain (foliation) in the more thinly-layered, black-and-white fine-grained gneiss. In the middle of the rock is an elongate blob about 6" long of the fine-grained, thinly layered black-and-white gneiss, completely surrounded by granite. The blob is also surrounded by a black zone that is probably a reaction rim, suggesting that the gneiss was an older piece that "fell into" the granite while it was still molten. We call such an exotic block a xenolith. PHOTO - coming soon!
15. More gneiss and quartzo-feldspathic material is found in the float and outcrops as you continue to the junction with Sluicebox Trail. Continue west toward the Lookout Mtn Road entrance to the park.
16. You will pass several large outcrops at trail level. Notice that the gneiss appears to be lighter in many places, with thin alternating bands of quartz- and feldspar-rich (light) and mafic, amphibloite-rich layers. You can also find many areas of gold-brown and black mica, some of which is quite coarse grained. Garnets are present at the second large outcrop, but they are mostly in the darker groundmass, which makes them hard to find and easy to confuse with the red-brown lichen. Coarse (to several inches) feldspar crystals display millimeter-scale fractures like little steps. These are probably cleavage surfaces, but some may reflect twinning, or crystal intergrowth, in the feldspar. A few nearly pure quartz veins are present.
17. About 100 feet beyond the outcrop with pine tree roots entwined about it, look for a float block at trail level on the left side of the trail. It contains a good example of a quartz vein.
18. Just after the trail opens up, and you have passed a large pine tree on the left side of the trail, look for a light-colored float block (about 2 feet wide) in the left flank of the trail. The quartzo-feldspathic groundmass contains many nice black crystals scattered in it. Their square outline indicates that they are pyroxenes. These crystals are called phenocrysts, and they formed along with the quartzo-feldspathic material, rather than earlier like the xenolith seen at Stop 14. This is also suggested by the well-formed nature of the pyroxene crystals.
19. After you pass a man-made stone retaining wall, above the trail on the right, and the trail enters an open area, watch for a small (1 foot) grayish rock in the side of the trail on the right. It contains a clear example of folded foliation, indicating that these rocks underwent at least two periods of severe deformation before the Paleozoic sedimentary rocks were deposited on top of them. PHOTO/SKETCH - coming soon!. This rock is small enough that it may have been moved, so check other nearby rocks for this structure. Soon after you pass this rock, a small stream crosses the trail from the north (right).
20. Soon after crossing the tributary stream, you will enounter a small outcrop or large float block that has an excellent example of the alternating coarse, pink quartzo-feldspathic layers with fine grained darker mafic layers in the gneiss.
All of the Elk Meadow Open Space is underlain by Precambrian rocks, mostly metamorphic gneisses and schists about 1.7 billion years old. There may be some non-metamorphosed granitic intrusions, associated with the Pike's Peak Batholith, intruded about 1.1 billion years ago. Most of the rocks you see look like granite, but they have a planar texture that indicates that they are metamorphosed, and should be called granite gneiss. Along the Sleepy "S" trail, from the Lewis Ridge Road entrance, you will find some isolated knobs that consist almost entirely of white quartz. These could be wide quartz veins, but if you look closely you can find coarse micas and amphiboles associated with the quarts. This would suggest that the bodies may be pegmatites -- very coarse igneous intrusives. There is additional micaceous mineral development in places, suggesting that the rocks have been metamorphosed (as expected).
copyright © 1996 Gibson Consulting
Elk Meadow Open Space
Please note that collection of minerals as well as other natural resources upon Jefferson County Open Space lands is prohibited.
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