There are two types of rocks that are quarried at this site, a dark gray rock and a pink rock. The rocks are mostly metamorphic rocks, with some igneous intrusions. Although these rocks are different in appearance, they are mined and processed together. Both rocks have properties that make them desirable for use aggregate. They are hard, which means they resist breakdown due to handling and abrasion. They are strong, which means they can withstand the weight of the load they must carry. And they are sound, which means they resist wetting and drying, and freezing and thawing. In addition, these rocks are not chemically reactive. Some rocks are reactive when used in Portland cement concrete. Other rocks have electrochemical properties that cause asphalt to strip from the rock surfaces.

Figure 1.   Lafarge Quarry, Golden

Not all rocks in the Front Range can meet the specifications needed for use in cement concrete or asphalt. The USGS is conducting studies to determine what rocks make good aggregate and to determine where they are located. USGS Open File Report 97-477 is a digital map file that contains this information. The Mountain Backdrop Study is collecting resource information to help determine areas of the Front Range to preserve from development. This data file is an important source of information for that effort.

Aggregate quality is extremely important, and so is quality assurance and quality control. However, large amounts of aggregate are produced in a short period of time, making QA/QC a difficult task. The USGS is conducting experiments with a standard concrete testing hammer to determine if the hammer can also be used to rapidly and accurately test rock. Preliminary tests indicate that the concrete testing hammer works best on limestone and trap rock. Those rocks make up about 80 percent of the crushed stone in the US.

The mountains also contain information regarding the composition and quality of the gravel in the streams that drain the mountains. Freezing and thawing, and other geologic forces cause rocks to break apart into rubble. Some of this rubble makes its way into streams where it is carried by the water. As it moves downstream, the weak material is broken up and carried away. Only the strongest material makes its way out onto the plains. By knowing what rocks occur in the mountain drainage areas, and by understanding how resistant the rocks are to erosion, geologists can predict the types of rock that will occur in downstream gravel.

The west wall of this quarry is an example of one type of reclamation. Part of the wall has been reclaimed by filling and grading the benches, and by planting natural grasses, bushes and trees on the slopes. You can also see two benches that have not yet been reclaimed. The reclamation is begun after mining finishes in one part of the quarry, rather than waiting until the entire mining process is completed. Not all reclamation techniques work in all geologic settings. The USGS is studying how natural factors such as geology and climate affect reclamation. Also, a reclamation technique that might appeal to someone from the semiarid parts of the country such as Denver, might not appeal to someone from the humid east. Therefore, the USGS is also studying the human perception of reclamation.

Figure 2.   Quarry Reclamation

U.S. Department of the Interior, U.S. Geological Survey URL: http://rockyweb.cr.usgs.gov/frontrange/virtour/heritsq.htm Contact: Carol Mladinich  mailto:csmladinich@usgs.gov Updated: 05/16/2001