David Pahnstrom, Age 10, Of Boise, Ida., for his question:
What kind of rock is Gibraltar?
The southern tip of spain points a finger of land toward the shores of africa. Here in a narrow, shallow channel of water stands the rock of gibraltar. Its towering cliffs are a natural fortress overlooking the ships passing betweeri the mediterranean sea and the atlantic ocean. Gibraltar has played a dramatic part in human history for hundreds of years, but the history of its rocky cliffs dates back millions of years.
The rock of gibraltar has been used as a fortress since about 700 a.d. It is riddled with natural tunnels and caves. From time to time, man made tunnels have been added to install underground fortifications. Geologists have been ab1e to take samples from deep within the mountain and piece together the long history of the rocky formations.
The rock itself is made mostly of limestone with layers of shale down its western slopes. Through the ages, stones and gravel have chipped off from the lofty peaks and formed piles of loose debris down the slopes. On three sides, the steep mountain swoops down to low, flat lands made from silts and sands.
The limestones and shales of gibraltar were formed perhaps 100 million years ago. At that time, the geologists tell us, most of central europe was under a large ocean. Its waters teemed with plant and. Animal life. Miniature creatures lived and died through countless generations, adding piles of their limy shells to the sea floor. Layers of mud and silt also collected on the floors of the ancient sea.
About 60 million years ago, the ocean bed began to rise and in time became the alps and the rocky uplands of south and central europe. The deposits of limy shells became limestone; the muddy sea floor became hard layers of shale. The rocky minerals of gibraltar are the same age and of the same kind as those which form the lofty alps.
Limestone is a soft mineral., and seeping rain water has nibbled many caves and tunnels into the rock of gibraltar. The limestone is rich with fossils. They tell us that the dry land was inhabited by mammals for the past million years. There are also human fossils in the rock of gibraltar. Our cave man ancestors, it seems, lived there many thousands of years before the famous rock became a fortress.
The mediterranean region is still. A restless area of the earth's crust. It is troubled with earthquakes and volcanoes. The straits of gibraltar is a very recent geological event. The narrow channel which links the mediterranean to the atlantic was formed by upheavals in the earth's crust perhaps a million years ago. The limestones of the towering rock were formed under an ancient sea perhaps 200 million years ago.