Sod houses spread across Nebraska, Kansas and North and South Dakota. They were common there, because all these areas had the same problem - there were no trees on the great plains, and so no wood to build with. Wood which was needed for the roofs was by far the most expensive part of each house, as it had to be painfully transported by wagon. The problem of building materials was solved by the arrival of the railways which could transport wood and other materials quickly and cheaply. Sod houses were abandoned for more modern buildings, and began to disappear. Now less than 100 remain, but they are carefully preserved as relics of pioneer life on the prairie.