In this image we see the Robert’s Mountain thrust contact up close. Above the contact we see the Early Missisppian conglomerates (but shouldn’t this be the distal sediments) and below we see a thick band of sheared rocks. To constrain the age of a thrust fault like this, you need to know the age of the youngest rocks in the footwall and the oldest age of the sediments produced by the deformation (the oldest age of the foredeep sediments). For the Roberts Mountain thrust, we know that the fault was active over roughly 10 million years. Image courtesy of Professor Burchfiel.