Complex sentence: Once thought to be part of the Lewisian complex, it lies beneath the Colonsay Group of metasedimentary rocks which forms the bedrock at the northern end of the Rinns.
Split into simpler sentences: Once thought to be part of the Lewisian complex, it lies beneath the Colonsay Group of metasedimentary rocks. The Colonsay Group itself forms the bedrock at the northern end of the Rinns.