two most important in Chiapas, and one of the most important in Mexico with a total river length of approximately , draining an area of , with an average flow of about forty million cubic meters. In addition to the Grijalva, there are other flows of water in the area in and around the canyon, many of which are seasonal. These consist of streams, some of which form waterfalls on the canyon's sides, and underground movements which have created caves and karst formations. The last important water formation in the area is the manmade reservoir of the Chicoasén Dam. The canyon proper is deep and narrow, characterized by vertical walls. As the gap changes direction as much as 90 degrees in places, it separates the Meseta de las Animas mesa in the west from the Meseta de Ixtapa mesa in the east. The width of the canyon varies from . Most of the canyon's walls are between high, reaching at their highest point. These walls expose a long process of disturbance in the Earth's crust with layers of limestone from the Upper Mesozoic, which contain fossils of marine creatures. During the Mesocretac Period, there was an elevation of ocean floor which formed much of the mountains of the area. Notable features The interior of the canyon has thirty rapids, five waterfalls, three beaches, two freshwater springs and a cofferdam three meters wide. The canyon contains endangered and threatened species such as the Central American river turtle and the American crocodile, which can be seen on the riverbanks. The walls of the canyon contain numerous small caves, rock formations and other notable features. The best known of the area's caves is the Cueva de Colores ("Cave of Colors"). This cave gets its name from the filtration of magnesium, potassium and other minerals which form colors on the walls, especially shades of pink. It contains an image of the Virgin of Guadalupe inside usually surrounded by fresh flowers and burning candles left by visitors. The Cueva de Silencio ("Cave of Silence") is so named because of a lack of echo or any other kind of resonance in its interior. In another small cave, there is a stalactite called the Caballito de Mar or "Seahorse" after its shape. Of the various seasonal waterfalls, the best known is the Árbol de Navidad ("Christmas Tree"). The “branches” of the Árbol are made by deposits from