Complex sentence: To obtain this land their agent John Plympton signed a treaty with some Pocumtucks, including one named Chaulk--who had no authority to deed over the land, and only a rough idea of what he was signing.
Split into simpler sentences: To obtain this land, their agent John Plympton signed a treaty with some Pocumtuck men, including one named Chaulk. He had no authority to deed the land to the colonists, and appeared to have only a rough idea of what he was signing.