Mass wasting events such as the 1925 Gros Ventre landslide continue to change the area. On June 22, 1925 an earthquake with an estimated magnitude of 4 weakened the side of a mountain located three miles (4.8 km) outside of the current park's southeastern border. The next day, 50 million cubic yards (38 million cubic meters) of water-saturated Pennsylvanian-aged Tensleep Sandstone slid 1.5 miles (2.4 km) from its source on Sheep Mountain and into the Gros Ventre River valley 2,100 feet (640 m) below, damming the river. Stressed by snow melt, the resulting 5 mile (8 km) long and 200 feet (60 m) deep lake breached the debris dam on May 18, 1927 and flooded the town of Kelly, Wyoming, killing six.