Tom Schweich  

Rocky Flats

Rocky Flats National Wildlife Refuge

• Wyoming Circle, east to N. Ford Street.
• Pine Ridge Road, west and north, ultimately to White Ranch Open Space Park, east entrance.

Somewhere in this quarter-quarter section are the locations of two trenches dug to explore a small graben of Quaternary age, thought to be related to the Golden fault. Excavations revealed a complex geologic picture suggesting two periods of fault rupture with a total of 5.5 m of displacement. Dating of zircons and glass shards from an ash deposited in the graben and disrupted by faulting yielded dates of 0.6 to 0.7 million years, suggesting faulting occurred more recently than those dates (Kirkham, 1977).

Stabilized landslide near CO Hwys 58 and 93.	Magpie Gulch.

Field Trip Stop 14 — Highway 6-58-93 Junction — of Noe, et al. (1999) illustrates the Golden Fault, associated landslide hazards, mitigation of landslide hazards, and the short public memory.

Looking to the north and south, we can see the distinctive hogback of the Dakota Sandstone at both ends of the valley. But why is this hogback missing at the valley center? The answer is that the Golden Fault, a large, Laramide thrust fault, weaves along the valley just to the east of the mountain front. Here, the fault has displaced nearly 8,000 feet of the Paleozoic and Mesozoic, sedimentary section (Van Horn, 1976). The trace of the fault can be seen as it cuts up the side of the first large alluvial terrace to the south of Clear Creek. There is a distinct break in the vegetation between the Fountain Formation to the west (covered with mountain mahogany shrubs) and the Pierre Shale to the east (covered with grasses).

Looking directly west, we see a road cut for State Highway 93. This portion of the highway was first constructed in 1991. A small landslide formed in the west (opposite) face of the cut shortly thereafter, and the resulting toe bulge closed the southbound lanes of the new highway. Within a year, the landslide had developed a rim of head scarps with up to 12 feet of vertical slippage, and had captured the surface flow of a small stream in Magpie Gulch. Early efforts to drain the landslide were unsuccessful. A full geologic investigation subsequently revealed that the landslide sits directly atop the Golden Fault, with low-permeability Pierre Shale on the eastern side and a wedge of fractured, permeable Fountain Formation on the western side. In 1994, three lines of rock anchors (about 40 anchors in total) were installed across the landslide. The Magpie Gulch stream was piped across the head scarps, longer horizontal drains were installed, and a remote data-logging unit was set up. To date, this mitigative effort appears to have been successful. The combined maintenance and mitigation operations for this incident are reported to have cost about 3 million dollars.

The houses to the north of the landslide were built shortly after the landslide was mitigated and all signs of its existence had been “erased” from view. One can only imagine the concern of these residents had the landslide been active a few years later. This is a good illustration of the often short-term public memory of geologic hazards.

• US Highway 6, either east through Denver, or west to US I-70 and Bishop, California.
• State Highway 58, east to US I-70 east and Denver.