Complex sentence: Their return journey to North Dakota took them through present day Fort Pierre, South Dakota where they buried a lead plaque which as unearthed in 1913 and is an important artifact of the exploration of the western prairies.
Split into simpler sentences: Their return journey to North Dakota took them through present day Fort Pierre, South Dakota where they buried a lead plaque. The plaque was unearthed in 1913 and is an important artifact of the exploration of the western prairies.