Complex sentence: For many decades, it was largely a farming community; when parts of the township blossomed into the more densely-packed neighborhoods of Helmetta, Jamesburg, and Spotswood in the late 19th century, they seceded.
Split into simpler sentences: For many decades, it was largely a farming community. After parts of the township grew into the more densely-packed neighborhoods of Helmetta, Jamesburg, and Spotswood in the late 19th century, they seceded.