Boulder fields consist of broken, jagged rock. Some are old lava flows long since cooled, and others are valleys choked with rockslides or slopes of scree. They usually lie near mountains, and most are no larger than a few miles across. Boulder fields are formidable obstacles since they lack water, vegetation, and shade, and if travelers do not have sturdy boots or sandals, the sharp rocks can cut their feet to ribbons. Deep gulches and crevices crisscross boulder fields, offering plenty of hiding places.