A massive, mainly snow-covered mountain (4,285 m) which is the highest and most imposing of the five extinct volcanic mountains that comprise the Executive Committee Range of Marie Byrd Land. The feature is marked by a spectacular caldera on the southern side and stands NE of Mount Waesche in the southern part of the range. Discovered by R. Admiral Richard E. Byrd on an airplane flight, Nov. 18, 1934, and named by him for Mabelle E. Sidley, the daughter of William Horlick, manufacturer, who was a contributor to the Byrd Antarctic Expedition, 1933-35. Also, A large and prominent mountain (3,290 m) of volcanic origin, standing immediately SW of Mount Sidley and marking the southern end of the Executive Committee Range in Marie Byrd Land. The feature is snow covered except for rock exposures on the S and SW slopes. Discovered by the United States Antarctic Service expedition on a flight, Dec. 15, 1940, and named for V. Admiral Russell R. Waesche, U.S. Coast Guard, member of the Antarctic Service Executive Committee.