Sangay developed in three distinct phases. Its oldest edifice, formed between 500,000 and 250,000 years ago, is evidenced today by a wide scattering of material opening to the east, defined by a crest about high. This first Sangay, pockmarked by secondary ridges, is thought to have been in diameter, with a summit located southeast of the present summit. The curved shape of the remnants of this first structure indicates that it suffered a massive flank collapse, scattering the nearby forest lowlands with debris and causing a large part of its southern caldera wall to slide off the mountain, forming an embayment lower on its slopes. This thick block, the best preserved specimen of Sangay's early construction, consists of sequentially layered breccias, pyroclastic flows, and lahar deposits. Acidic andesites with just under 60% silicon dioxide dominate these flows, but more basic andesites can be found as well.