Digital Mapping Techniques '99 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 99-386

Digital Mapping Techniques '99 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 99-386

Digital Resource Database for Management Decisions in City of Rocks National Reserve

By David R. Bedford and David M. Miller

U.S. Geological Survey
345 Middlefield Rd MS 975
Menlo Park, CA 94025
Telephone: (650) 329- 4924
Fax: (650) 329-4936


The City of Rocks National Reserve, jointly administered by the National Park Service, Idaho Department of Parks and Recreation, and local authorities, received the national attention to designate it as a Reserve largely because of its position on historic emigrant routes, including the California Trail. Emigrants passing through the stunningly scenic landscape of giant rock spires and pinnacles dubbed the area the "Silent City of Rocks" and signed their names on many pinnacles. As a result, geology has a key place in the founding of this Reserve, for the geologic landforms led to the cultural resources. Geology also is a key contributor to land-resource management decisions, because the susceptibility of surface materials to erosion, deposition, and destructive processes such as landsliding require geological study. The surface materials also partly control vegetation distribution and habitats, and may influence invasion of exotic species.

We developed custom geologic databases for the City of Rocks so that they can be used in GIS systems with other databases. The datasets have been organized in an ArcView GIS project, a commercial Geographic Information System (GIS) package (Environmental Systems Research Institute, Redlands, California), that simplifies the loading, display, and use of the databases. The database and ArcView project have been developed with the idea that users are not geologists, but do have uses for scientific geologic information. A geologic map database can be plotted on a contour map base to create a standard geologic map. In this mountainous area geology has a strong correlation to terrain, so the geologic map can also be presented as a composite with shaded relief. Maps created from susceptibility models for many geologic processes in the area display areas with potentially high vulnerability to destructive processes such as erosion and rock falls. Physical inventory databases have also been created, such as an inventory of pinnacles and other granitic landforms, and an associated photographic inventory that documents the present state of granitic and related surficial features. The databases are in the final stages of production, as is the development of the ArcView application. Management staff at the City of Rocks has showed strong interest in the database, to be released later this year, and in the ongoing development of scientific research and development of resource applications in the Reserve. The databases will likely be incorporated into an Internet Map Server application that may include virtual field trips and other interpretive information.


The City of Rocks, located near the town of Almo in the Albion Mountains of south central Idaho, was established as a National Reserve in 1988 to preserve cultural and natural resources that attracted commentary in the journals of emigrants to California over a 50-year period in the 19th century. By far the most important part of the emigration through City of Rocks was caused by the Gold Rush of 1849, which led to pioneer wagon trains for several more decades. In addition, the Salt Lake Alternate Trail passed through the southern part of the Reserve, joining the California Trail there in Emigrant Canyon. This trail also was used for several decades as part of the stagecoach route between Salt Lake City and Boise. The emigrants not only commented on the fantastic pinnacles but also on lush green fields and abundant water for their cattle and oxen. They covered pinnacles near the trail with names of rocks, their own names, and other graffiti. Some of these inscriptions remain as a reminder of the past travelers.

At the City of Rocks, geology is a focal point for cultural and natural resources, as well as for recreation. The cultural resources range from the original emigrant routes through late 19th century settlements and the subsistence lifestyles of the early American west. Emigrants took note of the area because of the pinnacles and abundance of water, both being attributes of the geology that formed the Albion Mountains. Members of early communities adapted their lifestyles to the offerings of the land, whether availability of clay for brick making, stone for building, or water for irrigating. Geology has a fundamental influence on natural resources as well, since the geology controls landforms, and these landforms influence the plant and animal communities. Geological features are also noted for their recreational values. The eroded granitic terrain harbors a maze of spires, which are popular among climbers. However, the granitic sediments of the area are not able to withstand many land-use practices, resulting in eroding sections of trails and roads and diminishing wildlife and vegetation.


In addition to the geologic features that have interested travelers and local communities, City of Rocks has attracted geologists because it is a metamorphic core complex. The region was identified as early as 1968 as an area of at least three phases of early-mid Cenozoic regional deformation events which included denudation by crustal extension accompanied by emplacement of granite (Armstrong, 1982). The topic of metamorphic core complexes was widely debated in the 1970's and still attracts many researchers to the western United States, and to the core complexes in and around the City of Rocks. Contrasted to the mid-Cenozoic deformation and accompanying mountain building, research for this study suggests that the three main upland basins in the City of Rocks are relatively stable. For instance, they accumulate alluvium and colluvium very slowly. However, the area is not necessarily stable. Processes such as soil creep, landsliding, and debris flows can impact areas that appear to be quiescent.

The geologic database consists of an Arc/Info (Environmental Systems Research Institute, Redlands, California) format dataset that represents bedrock and surficial deposits for the City of Rocks. Geology was mapped at 1:24,000 scale and larger when necessary. Much of the recent mapping was done using GPS and in-field databases to improve accuracy and increase efficiency of mapping. The digital field systems used for much of the mapping consists of a PLGR GPS unit with an average 8m locational accuracy, and a PalmPilot handheld computer running a database package which serves as the geologist's notebook. Because notes and positional information are acquired digitally and accurately, they allowed daily mapping information to be downloaded into a laptop computer at the end of each day and incorporated into a spatial database of observations. Nightly updates to the geologic map were performed using the daily observations and Digital Orthophoto Quadrangles in ArcView GIS, using ArcView's on screen digitizing capabilities for shapefiles. When mapping was completed, the geologic map was imported into an Arc/Info database, and topological errors inherent in shapefiles were corrected. The database consists of a geologic units and faults layer, structural layer, and cross-sections; all are viewable in ArcView or other GIS packages.


Stability models for several types of geologic processes were developed to help land managers at the City of Rocks to understand the interactions of terrain, geologic materials, vegetation and climate. These models can be combined with other survey data such as vegetation assemblages and animal habitat to develop more comprehensive models for determining management alternatives for uses such as: grazing allotment decisions, facilities siting, road and trail siting, fire management, water use, drainage diversion decisions, and developing monitoring strategies. Input data sets for the susceptibility models include data from: 1) U.S. Geological Survey (30 meter Digital Elevation Models, Digital Orthophoto-Quadrangles, and topographic maps): 2) Soil Conservation Service (Soil survey of part of Cassia County, digitized in ArcView); and 3) custom data from field studies (bedrock geology and surficial materials, and landforms).

Figure 1 Figure 1. Flow diagram illustrating the Geologic Susceptibility Models.

The basic form of each model is a series of geologic, terrain, and climatic factors that are numerically modeled to create an output susceptibility map. Input factors are individually weighted to classify each factor into an appropriate scale (for instance, slope is continuously changing data, but there may be threshold slope values that need to be expressed in the models). Individually weighting each factor in the model also puts all the data into the same range of values. This is important in order to numerically compare factors: if a geologic unit is "late Pleistocene alluvium", and you want to model the slopes on that unit, you have to convert a text attribute into a numerical attribute. Similarly, to compare erosion potential between bedrock units and surficial units, the units must be weighted against each other so that surficial units will be more susceptible to erosion. Each factor is classified and assigned numerical values based on theoretical and experimental studies from the literature, from knowledge of geologic processes, and from inferences based on geologic deposits and other information at the Reserve. Once each factor is individually weighted and in the same range of values, an equation can be developed that weights each factor against the others. This equation is then evaluated on a pixel-by-pixel basis to arrive at the final result, which is classified by inspection and calculation into categories of high, medium, and low susceptibility and displayed in map form. Figure 1 illustrates this process. Although the models are essentially empirical, they yield numerical results that can be used to test and calculate statistical measures of suitability, should those approaches be desirable.

Figure 2

Figure 2. Sample susceptibility map showing the reserve boundary and roads in the Reserve.

The models display areas with potentially high vulnerability to destructive processes (Figure 2), particularly if the land is disturbed (such as by wildfire, road building, and drainage diversion). Simple GIS overlays of these maps provide guides to making land-use decisions, such as routing a road. The models are created using Arc/Info's GRID module, which performs raster-based analysis. Results of the models can be viewed as images within ArcView, or converted into a vector format to allow the model results to be queried. The models can be improved by collecting better quality information on soils, more detailed slope maps, and better precipitation data.


The most notable landform features of the Reserve are the prominent, steep-sided, smooth and rounded granitic pinnacles. The name City of Rocks refers to this assemblage that resembles the assortment of tall and short buildings found in a city. These rock features are as high as 500 feet and are found across nearly 2000 feet in elevation from low basins to high ridges. Most pinnacles have formed in the granite of the Tertiary Almo pluton. Joints oriented nearly north-south are the most strongly developed, causing many of the pinnacles to be elongate north-south. Convex upward, dome-shaped joints and nearly horizontal joints are believed to be responsible for the two main types of geomorphic features in the pinnacles: bornhardts and tors respectively (Cunningham, 1971).

The pinnacles and granitic landforms database represents outcrops of granite features with abrupt topographic relief on one or more sides, as distinct from exposed pediment and other nearly flat granite features. The location of pinnacles were "heads-up" digitized using ArcView GIS, and then converted into Arc/Info format. Digital Orthophoto Quadrangles of the Reserve were used as a base for digitizing the polygonal outlines of the bases of pinnacles within the Reserve. Stereoscopic viewing of multiple years of aerial photographs (1956 - 1992) aided in distinguishing granite features of raised relief from exposed pediment and rock fall. Digitizing was done at variable scales from 1:3,000 to 1:10,000. Field checking was performed in many areas during the photographic documentation of granitic rock features.

A spatial inventory of location of granite rock outcrops having raised relief can be used for a variety of purposes, including base map applications and spatial analysis in a GIS. Base map applications include: identifying and locating (1) rocks with names, (2) pinnacles used for climbing, (3) rocks bearing inscriptions, and (4) rocks with unique, rare or fragile features. The database can also be used to: (1) prepare thematic maps (or pamphlets) using prominent pinnacles as selected reference points, (2) reference features or landmarks on trail maps or climbing guides, (3) prepare road maps, and (4) aid in search and rescue operations. Below we describe its use in a rock and fragile feature inventory.

This digital database can be used in ArcView, or other GIS packages, to query for relationships with other GIS datasets for interpretation, resource management, maintenance, and design & development applications. This dataset can also serve as a base map for baseline inventory of other resources (e.g. wildlife habitat; pack rat midden localities; inscription rocks; climbing rocks, etc).


The pinnacles database was used to initiate a rock/fragile feature inventory. This database provides photographic documentation of many of the landforms in the Pinnacles database. The photographs document the present condition of features that appear to be fragile, including rock faces undergoing frequent climbing activity and special wildlife habitat situations related to the shapes of landforms. The database contains features such as woodrat middens, natural arches and windows, precariously balanced rocks and rock shelters. The photographic database contains two types of images: photos of a particular feature, and sets of photos that are of vistas or panoramas. Photos of vistas are symbolized and queryable in ArcView by both the location that the photo was taken from, and the general direction the photo is depicting (Figure 3).

Figure 3

Figure 3. Screenshot of photographic database in use.

Photographs were taken using a digital camera and aerial photographs and topographic maps were used to note the location of each photo. A short description of the features seen in the photograph was incorporated into the database, as well as common keywords that can be attributed to many photographs to allow ease of querying the database for a particular theme found in the photos. The photographic database is implemented in ArcView through the use of hotlinking: when a user clicks on a point or points in the photographic database, all photos associated with that point are automatically loaded into separate windows within ArcView. Further modifications to the ArcView project will probably automatically load the description of the photo as text at the bottom of the image. Currently there are over seven hundred photos in the photographic database, including repeat photography from the 1970's and again in the late 1990's. Another research project in the Reserve was the professional photography of nineteenth and early twentieth century rock inscriptions by the many emigrants through the City of Rocks. This database, not presently in a GIS, accentuates the inscriptions and special filtering has allowed the analysis of the inscriptions, some of which aren't visible to the naked eye. The database of inscription photography could also be incorporated into a GIS because we have provided a spatial database of rocks on which these inscriptions are found.

The purpose of the inventory and photographic database of special features is to document the location and current status of features considered to be fragile in the Reserve. By incorporating photography, land managers will be able to visually inspect the current conditions of these features, and in the future, will be able to analyze the changes that have occurred on these landforms. The photographic inventory provides a baseline for analysis, so that qualitative, and possibly quantitative, analysis of the degrees of impact or recoverability to these landforms will be possible.


Geologic oriented databases have been developed to help land managers in the City of Rocks National Reserve make more informed decisions about land use that is affected by geologic processes. Geologic databases, terrain, susceptibility to geologic processes, derivative maps, and inventories of physical resources are included in the database to give access to geologic information on a manager's desktop. A GIS system based on ArcView GIS has also been developed to promote the use of this information by simplifying the management, display, and use of these databases.


Armstrong, R.L., 1982, Cordilleran metamorphic core complexes--From Arizona to southern Canada: Annual Reviews of Earth and Planetary Science, v. 10, p. 129-154.

Cunningham, F.C., 1971, The Silent City of Rocks, a bornhardt landscape in the Cotterrel Range, south Idaho, USA: Zeitschrift fur Geomorphologie, v. 15, p. 404-429.

Return to Table of Contents

This site is
Maintained by the Eastern Publications Group Web Team
Last revised 11-2-99