early 1940s and in 1970, respectively. Other moraine-dammed lakes within the wilderness area include Thayer Lake on North Sister's east flank and four members of the Chambers Lakes group between Middle and South Sister. Before settlement of the area at the end of the 19th century, wildfires frequently burned through the local forests, especially the ponderosa pine forests on the eastern slopes. Due to fire suppression over the past century, the forests have become overgrown, and at higher elevations, they are further susceptible to summertime fires, which threaten surrounding life and property. In the 21st century, wildfires have been larger and more common in the Deschutes National Forest. In September 2012, a lightning strike caused a fire that burned in the Pole Creek area within the Three Sisters Wilderness, leaving the area closed until May 2013. In August 2017, officials closed in the western half of the Three Sisters Wilderness, including of the Pacific Crest Trail, to the public because of 11 lightning-caused fires, including the Milli Fire. As a result of the increasing incidence of fires, public officials have factored the role of wildfire into planning, including organizing prescribed fires with scientists to protect habitats at risk while minimizing adverse effects on air quality and environmental health. Geology The Three Sisters join several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends north–south. Constructed towards the end of the Pleistocene Epoch, these mountains are underlain by more ancient volcanoes that subsided due to parallel north–south faulting in the surrounding region. Part of the Cascade Volcanic Arc and the Cascade Range, each of the three volcanoes formed at different times from several variable magmatic sources. The amount of rhyolite present in the lavas of the younger two mountains is unusual relative to nearby peaks. The Three Sisters form the leading edge of a rhyolitic crustal-melting anomaly, which might be explained by a combination of mantle flow (movement of Earth's solid silicate mantle layer caused by convection currents) and decompression that has generated similar melting and rhyolitic volcanism nearby for the past 16 million years. Like other Cascade volcanoes, the Three Sisters were fed by magma chambers produced by the subduction of the Juan de Fuca tectonic plate under the western edge of the North American tectonic plate. The three mountains were also shaped by the changing climate of the Pleistocene epoch, during