The Iron Dike of Boulder and Larimer Counties

Iron Dike on summit of Steep Mtn, 09/26/04

Iron Dike on summit of Steep Mtn, 09/26/04

Iron Dike on south side of Steep Mtn, 09/26/04

Iron Dike on south side of Steep Mtn, 09/26/04

Iron Dike at Hidden Valley, 09/26/04

Iron Dike at Hidden Valley, 09/26/04

Iron Dike intrusion into granite on southern end of Ironclads, with Stefan, 06/05/05

Iron Dike intrusion into granite on southern end of Ironclads, with Stefan, 06/05/05

Pasque flower on Iron Dike, 05/12/07

Pasque flower on Iron Dike, 05/12/07

The Iron Dike is one of the natural wonders of Boulder and Larimer Counties. It is an iron rich igneous rock intrusion that is 1.3 billion years old. In other words, it's a wall of rock that was formed by magma squeezing into a crack of the Silver Plume Granite which covers the northern Front Range of Colorado. The Silver Plume Granite is somewhat older, at 1.4 billion years old.

The Iron Dike is more weather resistant than the granite that surrounds it, so it usually extends several inches above the nearby granite. It spans the distance of about 38 miles (61 km) from Magnolia, just south of Boulder Canyon, to Mount Chapin in Rocky Mountain National Park, then picks up again in the Medicine Bow Mountains before disappearing beneath the younger rocks south of the Colorado-Wyoming border. It's width is typically 50 to 100 feet (15-30 meters).

We've made a map of the Iron Dike, created from those points we have visited and recorded with our GPS.

The rocks of the Iron Dike look much different than the neighboring granite. In places, its rock looks like rusted iron, almost black with some red. In other places it's bright red, or brown speckled with beige.

When the Iron Dike was born, the Earth was very different than it is now. It was a time before humans, before dinosaurs, before flowers, before birds, before mammals, before reptiles, before insects, before plants, and before fish. When it was born, multicellular life was just beginning. There was no life on land. In the oceans, the most complex organism was the sponge.

In the table below, are some GPS points where we have found the Iron Dike. Some convenient places to see it are:

The Iron Dike is great for hiking because the vegetation tends to not grow densely on it. Conifers especially don't seem to like the iron rich ground, although aspen grow on it. How about an Iron Dike Trail? 38 miles from Magnolia to Mt. Chapin. It might spur interest in geology, and science in general.

- Stefan Hollos and Richard Hollos

References



The age of the Iron Dike - a distinctive Middle Proterozoic intrusion in the northern Front Range of Colorado, William A. Braddock and Zell E. Peterman, The Mountain Geologist, Vol 26, No 4, Oct 1989, p97-99.

Petrology and weathering of the Iron Dike, Boulder and Larimer Counties, Colorado , Ernest E. Wahlstrom, Bulletin of the Geological Society of America, Vol 67, Feb 1956, p147-163.

GPS data points on Iron Dike
Place nameElev [ft]Accuracy [ft]Lat/Lon
Iron Dike near Longs Peak Trailhead Parking area94354140.27114/105.55592
Iron Dike east of Ferncliff, next to Hwy84441640.18779/105.49952
Iron Dike east of Ferncliff, south of previous point88071040.18438/105.49763
Iron Dike east of Ferncliff, south of previous point90641640.17912/105.49549
Eastern edge of Iron Dike on Storm Pass Trail102741940.29174/105.56863
Western edge of Iron Dike on Storm Pass Trail102472840.29268/105.57246
Peak 9331 (north of Ironclads)9331?40.1753/105.4931
North end of Iron Dike ridge on Peak 933193611240.17600/105.49338
South end of Iron Dike ridge on Peak 9331??40.17437/105.49280
Our most southern point on Iron Dike before going off to Ironclads92661640.17299/105.49223
Iron Dike at southwest corner of Ironclads87391940.16262/105.48882
Eastern edge of Iron Dike near Peak 8312 (Lefthand Canyon)82612440.07390/105.43175
Western edge of Iron Dike near Peak 8312 (Lefthand Canyon)83291740.07370/105.43330
Iron Dike on Sugarloaf Road76742940.02197/105.40012
Iron Dike next to Gold Lake Ranch86691740.08796/105.44756
Iron Dike south of Gold Lake Ranch86983440.08626/105.44576