The Pigtail Bridge is located 3.2 miles (5.1 km) north of the visitor center on S.D. 87. This structure, supported by ponderosa pine logs, follows a style of architecture commonly used in the Black Hills in the 1930's. The purpose of building such a bridge is to allow the road to negotiate sharp changes in topography in limited space. Several similar bridges are located on U.S. Highway 16 (Iron Mountain Road) near Mt. Rushmore.
    The Pigtail Bridge is built in an area that is underlain by extremely old rocks. The granites and schists near the bridge are approximately two billion years old. The schists just below and to the north of the bridge are recognizable by their dark color and vertical lineations.
    The bridge goes through an area of ponderosa pine and hardwood trees. The hardwood trees may be the result of more water runoff, less browsing by deer and elk, or a combination of both. Elk are very timid and may be reluctant to browse on hardwood trees along the roadside.

HABS Report on the Pigtail Bridge:

The Pigtail Bridge and its looped segment of SH 87 occupy approximately an acre and a half of Ponderosa pine forest in the heart of the granite mountains of Wind Cave National Park. A granite outcrop just to the north of the bridge rises to 4523', while a bison watering hole east of the bridge has an elevation of 4465'.The bridge's elevation of 4492' is halfway between these high and low points of the immediate environment. The bridge spans a 75-foot gap in a granite stratum that runs northwest to southeast.
    The loop lies at the confluence of two ravines, one from the northwest, the other from the southwest. The ravines join and drain eastward 1000 feet into Reaves Gulch. The construction of this raised asphalt loop at the center of a drainage confluence has helped establish moist ecosystems in the ditches around the loop and in the depression in the center of the loop. Twenty-four-inch steel pipes with concrete culverts funnel rainfall from the ravines into the center of the loop, which is drained by a 30-inch steel pipe within a stone culvert. This drainage system slows water runoff so that the site hosts an array of moisture-loving plants not frequently found in the area, such as Docks (Rumex sp.) and Autumn willow (Salix serissima). The 30-inch pipe empties alongside the road 200 feet to the east, where it is partially dammed, providing a watering hole for bison. Several times a day bison from the southwest ravine will travel down the road to water here. They also feed on grasses to the immediate east, south, and west of the loop.
    The bridge itself has created a cool, shady microclimate in which seedlings of American elm (Ulmus americana), Bur oak (Quercus macrocarpa) and Green ash (Fraxinus pennsylvania) establish themselves, enhancing the natural setting of the bridge. Birds nest between the bridge's steel I-beams, and a Cordillaren flycatcher (Empidonax occidentalis) summers within several feet of the bridge.
    The loop winds 773 feet from the beginning of the south-approach guardrail, to a point 140 feet east of the bridge. Along the loop, the width of the road surface varies between 19 feet and 37 feet. At station point (SP) 0 + 00' (where the south approach guardrail begins), the road is 25 feet wide. At SP 1 + 40', the road narrows to 22 feet. At SP 3 + 53', at the beginning of the north-approach guardrail, the road widens to 30 feet. It reaches its maximum width of 37 feet at SP 4 + 35', then narrows to 19 feet at SP 6 + 22', directly below the bridge's centerline. At SP 7 + 02', the road widens to 27 feet before descending into Reaves Gulch.
    Turning radii also vary throughout the loop. The south approach begins on a 294-foot radius, but meets the bridge on a 287-foot radius. Likewise, the north approach, which meets the bridge on an 86-foot radius, begins its approach 115 feet earlier on a 66-foot radius. Continuing along the loop, the southwestern portion curves evenly on a 46-foot radius, while the segment of road that passes beneath the bridge (between SP's 5 + 54' and 7 + 02') tightens from a 211-foot radius to a 206-foot radius.