original: At that time, the area contained the Greta Swamp, which was later drained.
simplifications: The area contained the Greta Swamp, which was later drained. The Greta Swamp used to be part of the area, but was later drained. The area had Greta Swamp before it was drained. At that time, the area contained the Greta Swamp. It was later drained. At that time, the area contained the Greta Swamp. It was later drained. At that time, the area had Greta Swamp. It was later drained. At the time, the Greta Swamp was part of the area. At that time, the area included the Greta Swamp. The Greta Swamp was later drained. Then, the area held the Greta Swamp, which was later emptied. The area contained the Greta Swamp, which was later drained.