Geologically the present-day Arroyo Trabuco basin did not exist as early as the end of the most recent Ice Age. The Santa Ana Mountains, forming the northern, eastern and southeastern boundaries of the entire San Juan Creek watershed, did not begin to form until roughly 5.5 million years ago (MYA). The Santa Ana Mountains at the headwaters of San Juan Creek are composed primarily of Jurassic-age (196.6-145.5 MYA) igneous and sedimentary rock overlain by Cretaceous-age (145.5-66 MYA) granite, gabbro, tolamite, siltstone, sandstone and conglomerates. The underlying rock of the Santa Ana foothills are Paleogene-age (66-2.59 MYA) sandstone.