The last three eruptions at Mount Hood occurred within the past 1,800 years from vents high on the southwest flank and produced deposits that were distributed primarily to the south and west along the Sandy and Zigzag rivers. The last eruptive period took place around 220 to 170 years ago, when dacitic lava domes, pyroclastic flows and mudflows were produced without major explosive eruptions. The prominent Crater Rock just below the summit is hypothesized to be the remains of one of these now-eroded domes. This period includes the last major eruption of 1781 to 1782 with a slightly more recent episode ending shortly before the arrival of the explorers Lewis and Clark in 1805. The latest minor eruptive event occurred in August 1907.