Hidden Valley Creek once flowed directly into Horseshoe Park before an Ice Age glacial moraine diverted its course. Beaver continue to alter the creek by building dams that have created the chain of ponds visible there. A huge, open, light-colored alluvial fan in Horseshoe Park marks a dramatic change caused when the upstream Lawn Lake dam failed on July 15, 1982. Flood debris dammed Fall River and eventually formed a temporary lake. Other evidence of the Lawn Lake Flood can be seen along Roaring River below Mummy Mountain.