Complex sentence: The fighting there was heavy, but by 7 p.m. the superior numbers of Union troops had managed to flank the Southern positions causing them to abandon the depot as well as the inner line of works.
Split into simpler sentences: The fighting there was heavy, but by 7 p.m. the superior numbers of Union troops had managed to flank the Southern positions. The Confederates abandoned the depot as well as the inner line of works.