DOI: 10.5066/p91wtclk
title: Rock mass quality and structural geology observations at Bald Eagle ridge and vicinity, Lake County, Colorado

description: Bald Eagle ridge, located about 14 km west of Leadville, Colorado, is slowly spreading and sagging, resulting in sackungen that have been monitored by the U.S. Geological Survey (USGS) since the mid-1970s (for example, Varnes and others, 1989; Varnes and others, 1990; Varnes and others, 2000). The ridge forms the southeastern flank of the glaciated Busk Creek valley and is underlain by Mesoproterozoic St. Kevin Granite (Van Loenen, 1985; Kellogg and others, 2017; Ruleman and others, 2020). Relief from the valley floor to the ridgeline is about 400 m.
 
As part of an effort by the USGS to better understand rock mass quality and structural conditions that promote slow spreading and sagging, we assessed rock mass quality and collected structural geology data at and near the ridge in 2022 and 2023. The quality (strength) of a rock mass depends on the properties of intact rock and the characteristics of discontinuities (for example, bedding, fractures, cleavage) that cut the rock. Rock mass quality can be estimated in the field using a variety of classification schemes.
 
All of the sites where we measured rock mass quality and structural data were accessed by hiking. At each field site, we made our measurements at rock outcrops, which were typically found at the base of cliffs and along ridges. Outcrops ranged in size from about 30 m2 to 100 m2. We visited a total of 20 sites in the field. All of the sites occurred within St. Kevin Granite. At each of the 20 sites, we collected data that we later used to classify rock mass quality according to four commonly used classiﬁcation schemes:
(1) Rock Mass Quality (Q, for example, Barton and others, 1974, Coe and others, 2005);
(2) Rock Mass Rating (RMR, for example, Bieniawski, 1989);
(3) Slope Mass Rating (SMR, for example, Romana, 1995, Moore and others, 2009) and
(4) Geologic Strength Index (GSI, for example, Marinos and Hoek, 2000, Marinos and others, 2005).

We also determined Rock Quality Designation (RQD, for example, Deere and Deere, 1989, Palmström, 1982) and estimated intact rock strength using a Proceq Rock Schmidt Type N hammer (see the RatingsReadMe file (.pdf and .jpg) for details). Schmidt hammer rebound values were converted to Uniaxial Compressive Strength (UCS) using the equation shown in Figure 2 of Katz and others (2000). Additionally, at each field site, we collected strikes and dips of any observed fractures.  At one detailed study location on the southeast side of Bald Eagle ridge, we collected only structural data (no rock mass quality data) from sites spaced from 10-100 m apart. These site names begin with an S or a B to designate if each site was on a scarp or a displaced block, respectively.  
 
All four rock mass quality classification schemes use data from characteristics of discontinuities present in the rock. Discontinuity data that we collected in the field included: total number of discontinuities, roughness of the surface of the discontinuities, number of sets of discontinuities, type of ﬁlling or alteration on the surface of discontinuities, aperture or “openness” of discontinuities, and the amount of water present. A blank field data collection sheet (FieldDataCollectionSheet (.pdf and .jpg)) is included in this data release. Numerical ratings for each of these factors are assigned based on the correlation of ﬁeld measurements and observations with descriptive rankings. The rankings used for Q, RMR, SMR, and GSI classification schemes are shown in Table 1, Table 2, Table 3, and Figure 1. Additional details regarding descriptive rankings and numerical ratings not shown in the tables and figures are given in the RatingsReadMe file (.pdf and .jpg).
 
All field measurements, numerical ranking values, and calculated Q, RMR, SMR, GSI, and RQD values are given in the RMQMeasurements_Ratings_Values file (.csv and .xlsx). Abbreviations of rating parameters (for example, R4e, Jw, etc.) for the RMR, SMR, and Q classification systems used in column headings are defined in more detail in Tables 1 and 2. All structural measurements from sites where both rock mass quality and structural data were collected are given in the StructuralData file (.csv and .xlsx). Structural data from the detailed study location on the southeast side of the ridge are given in the SouthEastSideStructuralData file (.csv and .xlsx). The adjustments for planar and toppling discontinuities used to determine SMR values are given in the SMRCalculationsWorksheet file (.csv and .xlsx). Final Q, RMR, SMR, GSI, and RQD values for each site are presented in a separate file (FinalRockStength_QualityValues (.csv and .xlsx)). All rock mass quality values are positively correlated with rock quality. That is, as Q, RMR, SMR, GSI, and RQD values increase, rock quality increases.

Additional information included in this release includes the following. Photos from each site are included in a separate folder (RMQSitePhotos), organized by the individual site names and the names of the photographers. A Google Earth BaldEagleSiteNameCoords.kml file showing site locations, site names, and geographic coordinates is also included.
 
The survey data included in this data release were used in the following interpretive paper:
 
Coe, J.A., Avdievitch, N.N., Allstadt, K.E., Collins, E.A., Jensen, E.K., Hoch, O.J., Schaefer, L.N., Ruleman, C.A., Godt, J.W., Matthews III, V., in review, Sackung at Bald Eagle ridge, central Colorado: an updated interpretation of ridge-spreading movement, structures, and mechanisms from 50 years (1975-2025) of U.S. Geological Survey research: Engineering Geology.

Disclaimer: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
 
References

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Varnes, D.J., Radbruch-Hall, D.H., Varnes, K.L., Smith, W.K., and Savage, W.Z., 1990, Measurement of ridge-spreading movements (sackungen) at Bald Eagle Mountain, Lake County, Colorado, 1975–1989: U.S. Geological Survey Open-File Report 90–543, 13 p.,
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Varnes, D.J., Coe, J.A., Godt, J.W., Savage, W.Z., and Savage, J.E., 2000, 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: U.S. Geological Survey Open File Report 2000–205, 23 p., https://pubs.er.usgs.gov/publication/ofr00205.