Tommy Freeman, age 13, of Dallas, Texas, for his question:

How hard is the Rock of Gibraltar?

We have heard so much about this mighty fortress that we assume it to be made of the most durable of rocks. Its eastern slope towers 1,400 feet above the Mediterranean waters and in pictures its steep cliffs certainly seem to be made of granite or perhaps even some harder stone. This, however, is far from true. The mighty Rock of Gibraltar is made mostly of limestone, and limestone is rated as one of the earth's soft or medium soft minerals. The chalky material tends to chip and crack and dissolve in running water. The pounding surf chews chunks from the feet of limestone cliffs and crumbly limestone peaks are eroded by weather.

The processes of earth erosion are relentless, but they are slow. The great Rock of Gibraltar has stood for ages and will lift its proud head for ages to come. It covers about two square miles and a sandy peninsula links it to the mainland of Spain. The fortress is riddled with tunnels and studded with guns to guard the narrow strait between the Atlantic and Mediterranean. The fortifications are strengthened with concrete and no doubt the great Rock will endure as long as men need such structures to post guard over their waterways.