an isolated mountain in the Northern Caucasus, in the Caucasian Mineral Waters region (Stavropol’ Krai, RSFSR). Elevation, 993 m. It is a laccolith with a crystalline core hidden under sedimentary rocks—Upper Cretaceous and Paleogene limestones and marls. There are springs of sulfurous carbonated water. The peak is smooth and dome-shaped. The Proval karst channel (depth, 20 m) is on the southern slope. The slopes are covered with broad-leaved forests of ash, hornbeam, oak, and beech. The Perkal’skii Arboretum with approximately 800 species and varieties of trees and shrubs is located on the mountain. At the foot of Mashuk is the site of M. Iu. Lermontov’s duel (there is an obelisk with a high relief of the poet), and on the southern slope is the grave of the Russian military topographer A. V. Pastukhov. The balneological health resort and city of Piatigorsk are located at the foot and on the slopes of the mountain.