Although Gibraltar is abstract, the connection is easily made between its base—a weighty lump of lignum vitae (a tropical hardwood)—and the Mediterranean rock that gives the work its name. This mass of wood is rough and solid, and seemingly unshaped. More delicate, and more clearly marked by human artifice, are the work's sloping plane of walnut, its painted wood ball, and its two steel rods balancing a crescent and a sphere, respectively. Gibraltar recalls the biomorphic forms in Surrealist art, particularly that of Joan Miró, a strong influence on Calder. But there is also a poetic whimsy that is Calder's alone.
The sculpture is contradictory in its qualities. The rods are thin and linear, and express an upward, airborne drive and eccentric balance; the lignum vitae is heavy, earth-hugging, solid. The surfaces, too, show various materials being variously treated, implying methods from machine-making to hand-polishing to leaving well enough alone. These disjunctions have a good-humored wit, which does not disguise the work's grace. Calder once said that "the underlying sense of form" in his work was "the system of the Universe," and Gibraltar, with its sun, moon, and heavy earth, is a solar system in miniature-a system revealed as a fine-tuned balance of opposites.
from The Museum of Modern Art, MoMA Highlights, New York: The Museum of Modern Art, revised 2004, originally published 1999, p. 159