The Lake Agnes landslide is a large block of displaced Dakota Formation (Cretaceous) at the southern end of North Park, Colorado. The rocks in the area range from Precambrian to Recent in age, with the Dakota Formation underlain by Morrison Formation (upper Jurassic) and overlain by Benton Shale (upper Cretaceous). The entire area is intruded by Tertiary dikes and laccoliths of basalt and andesite. From considerations of the physical properties of the Dakota Formation in the Lake Agnes area, especially angle of internal friction of wet and dry samples, it is concluded that the slide was caused by reduction of mechanical stability attributable to an 18 degrees dip, a fracture system, and partial floating of the mass by water pressure. The fracture system may have been generated by a nearby buried intrusion.