The predominant rock type in Borrego is granite.  Cougar has some interesting examples of granite formations, such as this one.  Cougar creek has polished these rocks to a slippery shine, exposing veins of infused minerals.  You can tell how high the water gets during the Spring runoff by how far up the right side of the hillside the polished rock surface extends.  High water would make this hike significantly harder, if not impossible.  Vegetation and rock walls make it difficult to make alternate routes up-canyon.