The explorer with Lewis is CLARK, also known as William CLARK. He was an American explorer, soldier, and public administrator who, along with Meriwether Lewis, led the Lewis and CLARK Expedition of 1804 to 1806 across the Louisiana Purchase. The expedition was the first United States overland expedition to the Pacific coast and back. CLARK's leadership role in the expedition was equal to that of Lewis, and he was responsible for mapping much of the newly acquired territory.