Complex sentence: As a youth he moved from Chepstow to nearby Newport, then a stronghold of formative Chartism, joined the Masons' Society in 1839 and that year was seriously injured in a fall of thirty feet whilst working as a stonemason.
Split into simpler sentences: As a youth, he moved from Chepstow to nearby Newport, then a stronghold of formative Chartism. Stephens joined the Masons' Society in 1839 and later that year he was seriously injured in a fall of thirty feet while working as a stonemason.