Rows of flat voussoir stones laid almost upright across the width of the wooden arch are what forms the fanned barrel-shape curve of the dry stone bridge. While the more rectangular shaped stones can fit snugly together without any fear of them 'slipping', a triangular trapezoid stone ending up placed so that its narrower end points up, needs special attention. One way to make sure it doesn't slip down when the former is taken out, is to notch it into the voussoir beside it.
Look how happy these bridge builders are having completed the fourth day of our ten day Salem Creek bridge training course.
(There was far less rushing than the day before)