Complex sentence: Two Leggings was born between the years of 1846 and 1851 this date isn't quite known, because many Native Americans living free in the American west during the mid- 19th century had no need to keep accurate records.
Split into simpler sentences: Two Leggings was born between the years of 1846 and 1851. The actual date is not known because many Native Americans living free in the American west during the mid- 19th century had no need to keep accurate records.