Complex sentence: The Dam was started in 1881 and completed seven years later in 1888, it was the first large stone-built dam in the United Kingdom, and it built partly out of great blocks of Welsh Slate.
Split into simpler sentences: The Dam was started in 1881 and completed seven years later in 1888. It was the first large stone-built dam in the United Kingdom, and is built partly out of great blocks of Welsh Slate.