Stop #4: The Burnt-Rock Falls

 

After around 5.5 km (3.5 mi) you will come to a ledge overlooking the Burnt-Rock Falls, a beautiful example of a geomorphic process called knickpoint regression. As the Hellroaring creek cut down through softer strata (in this case Grinnell and Appekunny argillites) , it encountered a harder layer (Altyn limestones) which slowed its rate of downcutting until it contacted the softer strata again downstream. The erosional rate in the softer strata is much faster that in the harder cap rock, eventually causing the creek to begin to fall over the cap rock and creating a waterfall. The waterfall moves backward through the valley over time due to undercutting in the plunge pool below, and the weathering away of the cap rock above. The Altyn Formation is one of the older formations in the Purcell Supergroup, yet has been jammed inbetween layers of the younger Appekunny and Grinnell Formations near the edges of the thrust sheet created by the Lewis Thrust in the East. Evidence of this deformation lies in the rock walls just to your left, where the limestone blocks show evidence of massive folds and faults. Also take notice of the Hellroaring Creek as it progresses down the upper valley that flattens out near the bottom into a U shape and sharply dips into a V shape as it nears the creek. The U shaped valley is a distinct landform left behind by a glacier plowing down a valley and the V shape is where the river has cut down through the valley post-glaciation, and dominates the valleys shape after the waterfall. From here prepare for the hardest part of the hike, a series of switchbacks that are completely open to the sun, climbing 425 m (1400 ft) in less than 1.6 km (1 mi).