Complex sentence: Because of the construction of Routes I-95 and I-295, the lake basin was excavated at no cost to the county as the gravel was sold to the highway contractors, which in turn helped excavate the basin now known as Lake Mercer.
Split into simpler sentences: Because of the construction of Routes I-95 and I-295, the lake basin was excavated at no cost to the county. The county sold the gravel to the highway contractors, which in turn helped excavate the basin now known as Lake Mercer.