Between 30,000 and 15,000 years ago , South Sister 's southern flanks were covered with ice streams .
A small amount of ice extended below 3,600 feet ( 1,100 m ) .
Between 30,000 and 15,000 years ago , South Sister 's southern flanks were covered with ice streams , and a small amount of ice extended below 3,600 feet ( 1,100 m ) .