Complex sentence: The United States Coast Guard took control of the Cape Mendocino Lighthouse in 1939 when the United States Lighthouse Service merged with it, and the light was automated by the Coast Guard in 1951 and then abandoned in the 1970's.
Split into simpler sentences: The United States Coast Guard took control of the Cape Mendocino Lighthouse in 1939 when the United States Lighthouse Service merged with it. The light was automated by the Coast Guard in 1951 and then abandoned in the 1970's.