Complex sentence: An archaeological horizon is a widely disseminated period of common art and artifacts at an archaeological site or, more usually, over a larger geographic area, and is a distinctive level in that site's or area's archaeological sequence.
Split into simpler sentences: An archaeological horizon is a widely disseminated level of common art and artifacts at an archaeological site or, more usually, over a larger geographic area. It is a distinctive level in that site's or area's archaeological sequence.