Complex sentence: Historically, the sand was and to a lesser degree still is replaced by new sand generated by the erosion of the Scarborough Bluffs to the east, this source of sand is itself diminished due to municipal efforts to reduce erosion of the bluffs.
Split into simpler sentences: Historically, the sand was, and to a lesser degree still is, replaced by new sand generated by the erosion of the Scarborough Bluffs to the east. This source of sand has been diminished by municipal efforts to reduce erosion of the bluffs.