Looking back (east) at St Mary's Lake. The turquoise is caused by glacial silt. At an altitude of 4,484 feet, St. Mary Lake's waters are colder and lie almost 1,500 feet higher in altitude than Lake McDonald. Here, the great plains end and the Rocky Mountains begin in an abrupt five thousand foot altitude change, with Little Chief Mountain posing a formidable southern flank above the west end of the lake. St. Mary lake has a surface area of 3,923 acres. The lake is 9.9 miles long and three hundred feet deep. The waters of the lake rarely rise above 50 °F . During the winter, the lake often is frozen completely over with ice up to four feet thick.