The Gibraltar Waterfall 

by Bill Baird

The weather down on the great plains had been unusually warm again as it had been for the last few years. In the late summertime the great salt lake began to shrink and the old shore lines were now miles from where they had been a few decades ago. The freshwater springs continued to flow unabated as they were mainly fed by water from the mountain glaciers that had been melting at an ever increasing rate over the last few years. In the late spring the great herds moved northwards up the slopes out of the great basin following the new grass. As autumn came nearer they moved back down into the basin to their traditional grazing grounds. 

This year the sound of the waves pounding on the western ridge had become very noticeable, especially when strong winds blew consistently from the west. On this particular autumn evening, the strong west wind was accompanied by a low pressure area moving into the gap between the two continents which the ridge connected. Sea level had never been higher and for the first time in millions of years, trickles of water from the highest waves began to find their way through low points on the ridge and flow down its eastern slope. At first the trickles were intermittent and the flow of water was small. However as the wind persisted and massive waves continued to pound onto the beach. One particular spot became eroded by the flow and more sea water started to pour through the breach. Eventually the seawater began to run through the breach even when not driven by the waves. By the time the storm had eased a permanent breach had been made in the ridge. Now each time the wind blew from the west or even when the tide rose, water poured through the breach in a growing river of sea water. On its short eastward journey from the high level Atlantic Ocean to the low lying plains the channel grew wider and more powerful by the day. Descending from the rocky ridge it tore away the soil and soft rocks, forming a cascade that eventually developed into a huge waterfall. Day by day the waterfall grew in volume and width until its thunderous roar disturbed the grazing herds on the plains away. 

At first, apart from the noise near the waterfall, the denizens of the great plains noticed little difference. True a new riverhad appeared, running down to the salt lake from the west, but there was little initial change in their life style. By the spring, however, animals whose winter migration pattern took them south of the lake found they could no longer complete their traditional journey north. In their path was a large and powerful salt water river. The salt lake had grown to such an extent that they could no longer go round it to the east. After milling around the water's edge for a few days, the herds turned south to the great mountains of the southern continent. Those to the north of made for their traditional northern grazing grounds, although some smaller groups occupied some nearer areas of high ground. Little did they realise that their age-old pattern of life had been interrupted irretrievably. 

Within a few decades this great river running from the Atlantic Ocean would fill the whole basin, bringing the level of the Mediterranean Sea up to that of the worlds other oceans of. No longer would there be a land bridge between Europe and Africa allowing migration between the two continents. Those animals that had made for higher ground now found themselves trapped on newly formed islands as the waters rose. Their descendants would become smaller, in the evolutionary struggle to match body size to a limited environment and food supply. 

Evidence for these momentous years is revealed in the great masses of fossils found on the Mediterranean islands. Sometimes, deposits of bones were of such quantity that commercial extraction was possible. The clue as to why the bone assemblages comprised animal groups now mainly known in Africa has only recently come to light. Drilling programmes in several areas of the Mediterranean have revealed the presence of extensive thick deposits of salt laid down during late Tertiary times. In the Upper Miocene period the Mediterranean, once more, became cut off from the Atlantic Ocean as Africa and Europe collided, closing the straights of Gibraltar. During the following centuries, evaporation exceeded inflow from rivers and the Mediterranean Sea was reduced to a series of large saline lakes occupying only the deepest hollows. Great rivers, such as the Nile and the Rhone, continued to feed these lakes. In so doing, they cut a series of great gorges and deep valleys whose existence had previously mystified bathymetric investigators. Life on the low lying plains surrounding these lakes life soon established a regular pattern, with endemic animals and plants soon colonising the new ground. Cycles of birth and death, migration and movement, became re-established and assumed an air of permanency. This continued for hundreds of thousands of years on the great plains below the western ocean. Now the ocean has returned and the cycle has been completed, with the Mediterranean a sea once again. 

Further Reading

Boyd Dawkins, W. 1873. The Physical Geography of the Mediterranean During the Pleistocene Age. Popular Science Review, 159-168. 

Debenedettil, A. 1982. The problem of the origin of the salt deposits in the Mediterranean and of their relations to the other salt occurrences in the Neogene forrnations of the contiguous regions. Marine Geology, 49, 91-114. 

Heft, S. 1977. Gibraltar - ein riesiger Wasserfall (a gigantic waterfall). UMSCHAU 77, 20, 680. (Translated by Mr P. Davidson, N.M.S.) 

Stanley, D J. 1990. Med desert theory is drying up. Oceanus, 33, 1 - 23.