This guidebook for a 1-day drive in the Front Range west of Denver, Colorado, includes commentary on mountain landscapes, notes on the colorful mining history of the area, and discussions on the geology and some miningrelated environmental hazards in the Idaho Springs area. It was originally prepared as U.S. Geological Survey OpenFile Report 91-426 for a field trip sponsored by Division A-5 (Environmental Quality) of the Soil Science Society of America on October 10, 1991. That original report has been revised and expanded to make this circular more useful as a self-contained guide. A pamphlet entitled "Mountains and Plains: Denver's Geologic Setting" is a complimentary general interest publication that is available free of charge from map sales in building 810 on the Denver Federal Center, Lakewood, Colorado. The pamphlet contains useful background information on the geologic setting of Denver and nearby portions of the Front Range.

A few precautions and suggestions are offered to make the trip safe and enjoyable. Idaho Springs is about 7,500 feet above sea level and the lower oxygen content of the air (about 50 percent that of sea level) may adversely affect people with heart or breathing problems. The wetlands are "wet" so use suitable footwear. The daily weather patterns are changeable. Mornings in Denver may be sunny and warm, but afternoons in Idaho Springs may be cloudy, windy, and cool with rain or snow; be prepared for all types of mountain weather.

As you travel westward from Denver into the mountains, successively older rocks are crossed. By studying the descriptions at the various mileage points and by taking time at each described STOP, you will get an excellent overview of the geologic events that shaped the Denver Basin and the mountains to the west. After traveling some 18 miles into the mountains, you will reach the old mining town of Idaho Springs. Here, you can learn about the two longest tunnels in the area-the Argo and Big Five Tunnels-and how the mine drainage affects the water of Clear Creek.

The Argo Tunnel near the east end of Idaho Springs drains part of the large Central City and Idaho Springs hardrock mining districts and is a source of acid water (which contains arsenic and other heavy metals) that drains into Clear Creek. At the west end ofldaho Springs is the Big Five Tunnel. Near the tunnel entrance an experimental constructed wetland demonstrates technology for the remediation of acid-mine drainage and removal of arsenic and heavy metals. Take an opportunity to view this experimental method for treatment of mine drainage. For access to the wetlands, prior arrangements must be made with the Department of Environmental Sciences and Engineering Ecology or Department of Chemistry and Geochemistry, Colorado School of Mines in Golden, Colorado. Otherwise, the wetlands may be viewed from the road. Permission to enter the Argo Tunnel property may be obtained from Jim Maxwell of Idaho Springs, who owns the hot springs resort. The tunnel may also be viewed from public roads.

After you leave Idaho Springs, the road log guides you down Clear Creek canyon, providing an opportunity to get a closer look at the various Precambrian rocks while traveling through one of Colorado's picturesque canyons.