When plates and the continents riding on them collide , the accumulated layers of rock may crumple and fold like a tablecloth that is pushed across a table , particularly if there is a mechanically weak layer such as salt .
When plates and the continents riding on them collide , the accumulated layers of rock may crumple and fold like a tablecloth that is pushed across a table , particularly if there is a mechanically weak basal layer such as salt .