Article from UA History website.
It was a sensational 7-6 Arizona football victory over Pomona College on Thanksgiving Day, November 6, 1914, that led to the building of the "A" on Sentinel Peak, west of Tucson.
In what was doubtless a burst of enthusiastic pride for his alma mater, Albert H. Condron, a member of the 1914 team and a civil engineering student, suggested to one of his professors that a class assignment be made to survey Sentinel Peak for the location of an "A".
The site was cleared of shrubbery and cactus, trenches dug to outline the letter's foundations, rock at hand was mixed with mortar and water hauled up the mountain by six-horse teams. The total cost of materials, equipment, and transportation was $397. The back-breaking work was done by the students themselves, Saturday after Saturday, with many difficulties and discouragements, but the "A" was finally whitewashed on March 4, 1916. No one called it Sentinel Peak anymore. It was known thereafter as "A" Mountain. The "A" is 70 feet wide and 160 feet long (or "tall").
The basalt rock quarried from the construction site was used to build the Rock Wall surrounding most of the university's historic district.