Complex sentence: This definition includes West Bromwich and Oldbury, which had many deep pits, and Smethwick, which underlies the thick coal but wasn't mined under 1870s and has retained more Victorian character than most West Midland areas.
Split into simpler sentences: This definition includes West Bromwich and Oldbury, which had many deep pits, and Smethwick. The thick coal that underlies Smethwick wasn't mined until the 1870s and Smethwick has retained more Victorian character than most West Midland areas.