Complex sentence: The basin is an embayment of low-lying land on the western margins of the Fens, and, the level of inundation by 1300 B.C., led to the construction of a timber causeway along the trackway route.
Split into simpler sentences: The basin is an embayment of low-lying land on the western margins of the Fens. The level of inundation by 1300 B.C. led to the people constructing a timber causeway along the trackway route.