Complex sentence: Alexander Thom had been examining stone circles since the 1950s in search of astronomical alignments and the megalithic yard but it was not until 1973 that he turned his attention to Stonehenge.
Split into simpler sentences: Alexander Thom had been examining stone circles since the 1950s in search of astronomical alignments and the megalithic yard. It was not until 1973 that he turned his attention to Stonehenge.