Looking back at Gunsight, Fullisade and Reynolds and Heavy Runner Mountains on a blustery day. The sedimentary rock of Fusillade Mountain was shaped by 1.6 billion years of geologic processes from sediment deposition, uplift, thrust faulting, and erosion by glaciers. The major geologic event that sculpted the landscape of Fusillade Mountain and the other peaks in Glacier National Park began approximately 2 million years ago when large ice sheets of the Pleistocene Ice Age repeatedly advanced and retreated until about 12,000 years ago