Basic Geology of Butcherknife Trail

Butcherknife Trail – a popular short hike accessed from Old Town Steamboat Springs.  Parking is available at Stehley Park.  Alternative access from the North via soccer fields – Strawberry Park Middle School.

The rocks exposed along this hike include igneous, metamorphic, and sedimentary.  The Igneous rocks include quartz and feldspar-rich granite and pegmatites (photo 1, 2, 3).  The Metamorphic rocks include layered/foliated gneiss, schists, and quartzites (photo 4, 5).  The Sedimentary rocks include a well-cemented conglomerate (photo 5, 6) as well as poorly cemented, unsorted reddish mudstone that contains pebble to cobble size angular rock pieces (clasts; not shown in these photos but exposed on the hillside).

 

Photos of the Rocks

1. Igneous and Metamorphic Rocks of Routt County

 

2. Igneous Intrusion – Quartz and Feldspar minerals – Granite

 

3. Igneous Rock – Pegmatite, large feldspar mineral

 

4. Igneous – Metamorphic Rocks of Routt County. Vertical (standing up on end) bedding or foliation.

 

5. Igneous – Metamorphic Rocks below on the left. Unconformity and Conglomeratic rocks on the upper right.

 

6. Conglomerate – rounded pebbles to cobble size clasts (broken pieces of rock) that are cemented together (Sedimentary Rock)

 

7. Close up view of the Conglomerate Sedimentary Rock – Rounded Clasts of various types of rock including red (feldspathic) granite.

 

Geological Map with Butcherknife Trail

Butcherknife trail can be overlain on the highest resolution geological map available (Snyder, 1980).  Thanks to the federally-funded USGS for mapping and making publicly available this dataset.  Another great resource for Colorado-specific geological information is the Colorado Geological Survey.  

Unfortunately, the 1:68,000 scale map resolution (the scale that the map was drawn), does not capture the details of the rocks observable on the hike.  But, this means that the hike observations can be a discovery and should fit into the general framework provided by the geological map.  I would not expect to find a lot of fossils on this hike (for fossils, I would look for carbonate rocks, for example).  I also don’t expect to find ancient lava flows on this hike.  There are other hikes and locales around Steamboat Springs and Routt County to find ancient lava flows.

Butcherknife Trail

Geological Map overlain on Topography with Butcherknife trail (yellow and black path), modified from Snyder, 1980.

 

Rock Descriptions from Snyder, 1980

Qgd: Terrace Gravel (Pleistocene) – Alluvial gravel, including alluvial fans, in terraces 3-18 m above modern flood plains

Tbp: Browns Park Formation (Miocene) – Buff calcareous to noncalcareous, silt to siltstone, sand to sandstone, and gravel to conglomerate with minor chert beds, rare porcellanite beds or fragments of silicic tuff beds.  Conglomerate cobbles are predominantly Precambrian rock, but locally between Oak Creek drainage and the Hayden-Archer powerline, conglomerate includes 1-5 percent trachybasalt cobbles.  May include Tc conglomerate near base 152-244 m thick.

Xgn:  Felsic Gneiss to Amphibolite Metavolcanics (Precambrian X) – Mainly prominently to faintly layered feldspar – quartz- biotite gneiss, feldspar- quartz- biotite-hornblende gneiss, and many varieties of amphibolite.  Includes biotite schist and small lenses of all other Precambrian rocks not otherwise mapped.

YXp Pegmatite (Precambrian Y and X) – Generally small (only larger bodies mapped) coarse-grained irregular bodies of granitic composition that were emplaced over a long span of geologic time.   Age ranges from angular pegmatitic includsions in 1.7 b.y. quartz monzonite intrusives west of Fish Creek Reservoir to segregations which crosscut rocks of the 1.4 b.y. Mount Ethel pluton north of this area.

 

 

 

Geological Map Source – USGS (United States Geological Survey)

—Snyder, G.L., 1980, Geologic map of the northernmost Gore Range and southernmost Northern Park Range, Grand, Jackson, Routt Counties, Colorado; USGS  Miscellaneous Investigations Series Map I-1114. Map Scale 1:48,000.  http://ngmdb.usgs.gov/Prodesc/proddesc_8974.htm

 

 

 

 

 

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