Complex sentence: The following data comes from tombstones at the actually churchyard and thus in some cases dates are illegible and have been replaced with question marks, or in cases where only some numbers are legible, with underscores to represent the missing number.
Split into simpler sentences: The following data in most cases comes from actual tombstones at the churchyard and thus in some cases dates are illegible. Missing dates have been replaced with question marks, or in cases where only some numbers are legible, with underscores to represent the missing number.