Complex sentence: When Georgia seceded from the United States, it was a native son of Columbia County who presided over the Secession Convention, George Walker Crawford, who had previously been the only Whig governor of the State.
Split into simpler sentences: When Georgia seceded from the United States, George Walker Crawford, a native son of Columbia County, presided over the Secession Convention. He had previously been elected as the only Whig governor of the State.