Large red and pink quartzite stones in the park are part of the border marked after North Dakota and South Dakota were admitted to the Union in 1889 as the 39th and 40th states. In 1890, Congress ordered the entire border to be surveyed and delineated with conspicuous monuments of durable material, and quartzite mined in the Sioux River Valley near Sioux Falls, South Dakota, was chosen. Surveyor Charles L. Bates oversaw the project, which took two years and placed about 720 markers at half-mile intervals along the 360-mile border. Each monument weighed 800 pounds and stood 10 feet tall, with carvings indicating location. Markers for the west side of the state were shipped by rail to Dickinson, North Dakota, then carried by horse-drawn carts to their exact sites. Many remained in the wild, though some fell or were removed. The park is also tied to the Yellowstone Trail, a transcontinental northern road used when roads were poorly marked and often in poor condition. Ole Quammen was among the men who gathered in 1912 to form the Yellowstone Trail Association, which promoted road improvements and tourism, and the road crossed the park, where the original pillars still stand at its southeast corner.