Between 1840 and 1870, as many as 350,000 people and tens of thousands of covered wagons traveled the Oregon Trail, and countless feet, hooves, and iron-rimmed wheels cut and compacted the ground, leaving long-lasting traces still visible on many western landscapes. Emigrants preferred sturdy farm wagons, roughly half the size of a Conestoga, to carry the possessions and supplies that would fit in a wagon box about four feet wide, three feet deep, and 10 to 12 feet long. The covered box sat over two pairs of wheels of different sizes, with front wheels about 44 inches in diameter and rear wheels about 50 inches in diameter. The narrow iron rims, pressed into the earth by the weight of the wagon and its 1600 to 1800 pound load, left a visible legacy across the landscape. In some places, faint two-track depressions mark where wagons crossed mostly level ground, while deeper depressions with sloping sides, called swales, often formed on slopes and at the crests of hills where oxen dug in their hooves and strained to pull heavy loads. In wide open country, two, three, or more side-by-side lines of depressions, swales, ruts, tracks, or scars formed where wagons spread out to pass other parties or escape the choking dust kicked up by those ahead. The best wagon wheels were made from durable hardwoods such as white oak or Osage orange, with wooden rims shaped into circles and fitted with heated iron hoops that cooled, shrank, held the wheel securely together, and protected it from rocks, grinding sand, and other hazards of the road.