American Scientist

The classic Egyptian obelisk was a monolith—a single large piece of stone, typically limestone or granite—standing as high as about 100 feet tall and weighing as much as hundreds of tons. An obelisk is square in cross-section, but otherwise its geometric proportions are not set in stone. According to one survey of objects referred to as obelisks, the ratio of base width to height can vary from a relatively stubby 1:6 to a quite slender 1:12.5, with about half of the examples falling between 1:9 and 1:11. The mean of 1:9.4 is close to the commonly stated width-to-height specification of 1:10. Regardless of its slenderness ratio, an obelisk tapers gently as it rises, and its top is usually crowned by a pyramidion, whose sides are commonly inclined at an angle to its base. (A pyramid itself may be thought of as an extremely squat obelisk.)