Geology
The Rock was formed, more or less in the shape we see it today, by a massive upheaval of the earth about 200 million years ago. The earth's plates which formed Africa and Europe collided and a massive lump of Jurassic limestone was forced up from the sea and flipped over. The top ridge of Gibraltar was once far below the sea and is made from billions of compressed seashells. Limestone is made up of millions of small shelled animals which have died and settled in the sea bed; slowly these shells harden and become rock. So another point to remember is that when you walk on the Rock you are stepping on an ancient sea bed.

The African Plate again collided tightly with Europe some 55 million years ago. The Mediterranean became a lake which, in the course of time, dried up until 5 million years ago when the Atlantic broke through the Strait of Gibraltar and flooded it again, isolating the Rock of Jurassic limestone.

It is easy to think of Gibraltar as an island although it is not. However at times between Ice Ages, it was an island due to rising sea levels. The last time this happened was possibly 120 thousand years ago at about the same time prehistoric man arrived on the scene. Since then Gibraltar has become a peninsula connected to the mainland by a narrow isthmus.

Rainwater filtering through cracks, fissures, caused huge caverns including St Michael's Cave to be formed. Over 140 caves have been discovered so far.

Geologically, Gibraltar may be divided into two main parts. The first is the plain to the north which consists of sand to a depth of some 30 feet, followed by some 4 feet of clay, a bed of coarse sand 21/4 feet thick and lime-stone. The second is a mass of the Rock itself southwards from the north cliff to Europa Point consisting of compact grey limestone probably of Jurassic age, overlaid in parts with dark shale, limestone breccia or sands.

 There are over 140 documented caves in Gibraltar, these fall into four categories;

Show Caves
In Gibraltar the majority of caves were formed by the separation of the Bedding Planes in the Jurassic Limestone. The passage of rainwater over tens of thousands of years has produced the fabulous formations which can be seen in all of these caves, some of which are open to the public, such as St Michael's Cave and Lower St Michael's Cave Another example which is less accessible is Martins Cave at Mediterranean Steps.

Sterile Caves
These caves are those which have been found by accident whilst tunnelling. There may be others around the world discovered in similar fashion, but not in such profusion as those in Gibraltar.

Pot Holes
When you have rivers flowing on the surface it is quite common to have them gauging their way deep underground. The water will actually cut its way through bedrock. This will therefore create what is termed as a Phreatic Cave and commonly known as Pot Holes or Swallow Holes. These Pot Holes look very much like giant drainpipes, very smooth, very deep and perpendicular. Remember the fast flow of water, which is making them, does not allow any formations to grow because of the constant erosion.

Marine Caves
Within the last two million years, Gibraltar was subjected to four Glacial Periods. With so much water trapped in the thick ice fields covering the Northern Hemisphere, the sea levels were affected by actually going down approximately 200 meters to their present level. When the ice melted and we entered an Interglacial Period the opposite happened, the sea level rose to over 200 meters to what it is today. These fluctuations in sea level, not only created new sea caves by the action of the waves against the sea cliffs, but also flooded existing ones. Today there are some of these caves which are high and dry, whilst others still remain underwater. An interesting fact about these caves is the absence of formations and the smoothness of their walls, which have been smoothed by the action of waves and sand.

Gorhams Cave is Gibraltar's most famous Marine Cave due to the two major excavations which have yielded valuable information and artefacts. Stone tools, and other human deposits were found dating to Neanderthal (40,000 BC) and Neolithic (6,000 BC) periods. The small beach in front of the cave has allowed Romans, Carthaginians, Phoenicians and Moors to bring boats ashore and enter the cave, this has lead to finds of campfires, glass beads, broaches and lamps. Bones of animals that no longer live in the area, such as hyaenas, lion and grey seals have been found.

It must be said that the first inhabitants of the Rock were animals, since the bottom layers in the stratigraphy of the many caves excavated, usually show animal remains before human. From the different animal remains we can easily work out what kind of Period and Climate we are looking at. The finding of Deer remains is a very good example. If they happen to be Red Deer it is a temperate climate. If it happens to be Reindeer then it is a cold arctic condition.

Prehistoric Man occupied many of Gibraltar's caves. Although there are conflicting arguments as to whether there was a land bridge exposed across the straights or not, during the Glacial Periods. The fact remains that prehistoric man, in the form of the Neanderthals, roamed and lived in our caves. He was later followed by the more intelligent and versatile Cro-Magnon, our distant relative who left much evidence of his culture in the layers of our caves.

As already mentioned, at one time some caves, which were on dry land, are now under the sea. Nevertheless, they are still considered to have been possible human shelters. Some have already been excavated by underwater teams, but unfortunately have not yielded anything so far. It seems they have been completely washed out of any remains. However, the expectation of finding something one day still exists.