Harrow and Hillingdon Geological Society

Previous Meetings

A Geological Traverse of Eastern Morocco

Charlie Underwood
Birkbeck College, London.

This lecture described geologically, a north to south transept of eastern Morocco which had been part of a field trip undertaken by the speaker. He also emphasised that the trip was safe, cheap and easy to organise.

The geology of Morocco of is essentially divided into four areas. In the north are the Riff Mountains which are an extension of the Pyrenees. The Northern Plain which contains Casablanca and Marrakech and the Western Meseta which is comprised mainly of deformed Palaeozoic rocks plus others from the Cretaceous period. The High Atlas region is mainly folded rocks from the Jurassic while the Middle Atlas, Eastern Meseta and high plateau are similar geologically to that of the Anti Atlas region. These are gently folded, Pre-Cambrian and Palaeozoic rocks. From Marrakech eastwards, up to the Atlas Mountains, which are still snow-capped in April, is a flat plateau. Here, there is also evidence of younger geological activity with Quaternary volcanic rocks. The valley floors are covered in black basalt. This area is very cold and very dry and covered in Alpine plants. These valleys also have numerous cinder cones, essentially topographically similar to soon after eruption. There are also basalt lavas plus low domed hills 5-10 metres high. However, the vesicles within the basalt are vertical and are the cores from fire fountains. One of the cinder cones has a quarry cut through it and it is possible to see the volcano-clastic rocks. Small xenoliths from the upper mantle which appear green can be seen amongst the rocks. There is very little vegetation or soil which makes it easy to determine the geological structures and thrust faults of the Mesozoic rocks. The rocky outcrops are from a Jurassic reef but the valley floors are formed essentially by waadis. There is a huge variety of rocks including early Jurassic containing ammonites similar to those seen in Yorkshire. There are also thinly bedded layers from which it is possible to determine the Milankovitch cycles which show the regular repeating time intervals. About 180 million years ago the rocks were in deep water. There is also evidence of shallow water in the middle Jurassic where a lower reef was drowned by muddy sediments and then more reefs formed on top of these.

South from the Atlas the area is crossed by the Atlas thrust fault and at the southern edge of this, the thrust pushes over a horizontal undeformed area of Cretaceous plateau. Cutting through this is a gorge fed by melting snows which gives rise to lush farmed valleys. Here, they grow salad crops, beans dates etc. During the Cretaceous there was a rise in sea levels which produced a non-conformity. There are also deposits of coastal red sandstones and mudstones. Later, however, the sea reached this region and there are fossils in the limestone. These were warm shallow water fossils, bivalves similar to oysters and also some stromatolites.

The Anti-Atlas is effectively on the edge of the Sahara Aeolian sands, but there is not much sand in Morocco. There are mainly star dune formations giving rise to unusual flora and hatching crustaceans attract masses of flamingos. The geology in this desert area is a Cretaceous escarpment which forms a big curved amphitheatre which surrounds the dunes. It is mainly red cross-bedded sandstones. In the conglomerates at the base of these rocks are shafts from which fossils are excavated. These include dinosaur teeth, spines from sword fish and Neolithic remains. The core of the Anti Atlas is Palaeozoic rocks, pebbles sculpted by sand blasting, and shales. The Devonian limestone is well-developed and rich in fossils. There are layers of upper and lower Devonian and between these are the “Hills of Breasts” which are big conical mounds true reef-like structures from the middle Devonian period. Highly crystalline calcite has been precipitated in the presence of methane and there is evidence of tubular fossils similar to those found on ocean floors today.

Fossils are a major local industry, but 80-90% are repaired and 30-40% are fake. There are other geological industries but mining has virtually ceased although lead, ferrous oxides and vanadium are still viable on a small scale.

Politically, Morocco and Algeria are still officially at war and hence maps of the area show no country boundaries. There is a lot of gun smuggling as the Algerian desert is easily accessible. It is also rich in meteorites which are traded with Morocco. The dunes move and expose pieces of rock which because there is no bed rock are meteorites. They also find North West African (NWA) chondrites, and at least one meteorite is of Martian origin.

This was another extremely interesting talk full of facts and well illustrated. It also gave an insight into the possibility of field trips.

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