Ore carts moved rocks and minerals from a mine interior to the mine opening, or portal. Shaped like large rectangular buckets, they rode on metal tracks and were pushed or pulled by men, animals, or rope-haulage systems. End dump carts evolved from the simple boxes on wheels that miners first used, with later advancements including an end gate added to the box and a hinge between the box and the wheel frame that allowed one person to dump a heavy load easily. In the mid-20th century, ore carts were replaced by a more efficient belt conveyor system using longwall shearer machines. As mines increased in size and output, ore carts became impractical because of the distances traveled in the mines and the great weight of the minerals to be transported. The ore carts are similar to those used in the Colorado Fuel and Iron Company mines.