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Measurements of ridge-spreading movements at Bald Eagle Mountain in northcentral Colorado were reported in USGS Open-File Report 90-543 for the years 19751989. Measurements were renewed in 1997 and 1999 using the Global Positioning System (GPS). Movements are generally away from a ridge-top graben and appear to be concentrated along 3 or 4 trenches with uphill facing scarps that are parallel with slope contours. A point just below the lowest trench has moved the most a total of 8.3 cm horizontally and slightly downward from 1977 to 1999 relative to an assumed stable point on the periphery of the graben. Movements from 1997 to 1999 are less than 1 cm or within the error of measurement. INTRODUCTION Geologic and geomorphic studies in many of the mountainous parts of the world during the last 30 years have shown that large mountain masses have undergone spreading movements under their own weight. The movements are inferred to be nearly horizontal near the base and both lateral and vertical near the crest of the mass. These displacements commonly produce linear trenches, upward-facing scarps on the flanks of ridges, and grabens along ridge crests. These features resemble those of recent tectonic faults and indeed spreading movements often make use of any preexisting faults or discontinuities that have an appropriate orientation. In areas where cultural activities require that nearby faults be identified and their recurrence intervals determined, careful studies may be necessary to decide whether scarps and trenches are the result of tectonic activity or gravitational spreading. Spreading movements are most common in glaciated regions and may have begun when retreat of the ice from valleys left adjacent slopes oversteepened and unsupported. The ground surface offsets are generally one to a few meters at each scarp, rarely some tens of meters, and the movements at most places appear to have been completed long ago. However, in some localities, closed depressions and holes unfilled with soil or vegetation indicate that slow movements may be continuing. Apparent gravitational spreading has been noted in several geologic settings. Where isolated masses of igneous rock lie on soft shale, as at some laccoliths of central Colorado, the overlying brittle crystalline rocks have broken into slices and spread on the underlying plastic shale. This is the situation at Crested Butte, the locale of a well-known ski resort. However, ridges composed entirely of homogeneous granitic rock also appear to have spread. In all instances that we have observed, the spreading mass of whatever composition is closely divided by well-developed sets of joints. Such is the condition at Bald Eagle Mountain, a broad ridge of gneissic Precambrian granite extending northeastward from Mount Massive, one of the high peaks of the Sawatch Range of westcentral Colorado (fig. 1). The Bald Eagle Mountain study area is one of moderate relief between elevations 3200 and 3660 m (10,500 to 12,000 ft) (See fig. 2). Access to the foot of the slope is by hard-surface road westward from the city of Leadville, past Turquoise Lake, and thence by a gravel road up Busk Creek to the portal of the Carlton Tunnel, a trans-Continental Divide water diversion project (See fig. 3). The unimproved road continues north and westward over Hagerman Pass at the Continental Divide and on to the town of Basalt. Access on foot toward the trenches east of Busk Creek is by a Forest Service trail that begins at the parking lot near the east portal of the Carlton Tunnel and zigzags up the slope to the southeast. Geologic study of the gravitational spreading features at Bald Eagle Mountain and several nearby ridges began in 1975. The relation of these features to local topography and geologic structures here and elsewhere in the western United States has been described by Varnes and others (1989). At the beginning of our study, a few surveyed points were placed to determine if any movements were occurring. In succeeding years, more points were added to create a survey net, which is shown in figure 2, and measurements of the net were discussed in Open File Report OF 90-543 (Varnes and others, 1990). As in all previous surveys, station 4 was assumed to be stable and, in a local coordinate system, to have coordinates 10,000 m N, 10,000 m E, and altitude 3645.415 m (from estimate of 11,960 feet on the topographic sheet). The azimuth of line 4-6 was assumed to be constant at 155.93634 degrees. During surveys in all years, slope distances were converted to horizontal distances at the elevation of station 2, and coordinate positions were determined at that elevation. The surveys made in 1975, 1977, 1982, and 1984 were made with helicopter support to transport heavy electronic distance measuring (EDM) equipment. When the area under survey became included within the Hunter-Fryingpan and Mount Massive Wildernesses, access by helicopter was prohibited. So, in 1989 only tripods, targets, and a 1-second theodolite were carried up to the site to measure horizontal and vertical angles but not distances. Computation of coordinates in 1989 therefore required that one distance and one direction in the net be assumed unchanged since 1984. For this purpose, not only the azimuth of the line between stations 4 and 6 but also the distance between stations 6 and K were assumed to remain unchanged from their 1984 values. The results of recalculating the network were not entirely satisfactory; moreover GPS measurements in 1997 indicated that the distance between stations 6 and K could not be assumed constant. Therefore, 1989 positions are not accepted and reported in the present paper.

u.s. geological survey open file report/open-file report

Measurement of ridge-spreading movements (Sackungen) at Bald Eagle Mountain, Lake County, Colorado, II: Continuation of the 1975-1989 measurements using a Global Positioning System in 1997 and 1999