Complex sentence: Hetch Hetchy was sculpted by glaciers as recently as 10,000 years ago (like nearby Yosemite Valley); though the Hetch Hetchy glacier was more recent and larger than the one in the paleo-Yosemite Valley.
Split into simpler sentences: Hetch Hetchy was sculpted by glaciers as recently as 10,000 years ago (like nearby Yosemite Valley). The Hetch Hetchy glacier was more recent and larger than the one in the paleo-Yosemite Valley.