The summit areas of Sail are composed of the Ordovician Kirkstile Formation. This is the typical rock of the Skiddaw fells and is composed of laminated mudstone and siltstone. Beneath this are the Greywacke sandstone turbidites of the Loweswater Formation. The Causey Pike Fault runs across the southern flanks of the fell, beyond which are the rocks of the Buttermere Formation. This is an olistostrome of disrupted sheared and folded mudstone, siltstone and sandstone.