The firth extends from the junction of the Sound of Mull and the Lynn of Lorn in the north-east, in a south-westerly direction towards
Colonsay. The narrower north-eastern part is the Inner Firth, and the wider south-western part is the Outer Firth. The firth takes its name from the historic province of
Lorne on the adjacent mainland. The Firth of Lorn is a continuation of the
Great Glen - the
Great Glen Fault runs along the north-western side of the firth. A conspicuous feature of the coasts of the firth is the presence of rock platforms and raised cliffs. These can be seen on the coast of Mull, on the Island of
Kerrera and on the mainland. They are believed to be the result of marine erosion at a time when sea levels were higher than at present, probably during a late interglacial period. The nature of the seabed in the firth has been studied by several methods, including echo-sounding, sampling with grabs, and surveys using submersible vessels. The bedrock is mainly precambrian of the
Dalradian Supergroup overlain
uncomformably by rocks of the
Old Red Sandstone. A notable feature is the Insh fault which forms a submarine cliff up to 150m high, passing to the west of
Insh Island. The Firth of Lorn is noted for its strong tidal currents, particularly in the channels between the islands. In the
Gulf of Corryvreckan, between
Jura and
Scarba, tidal flows can reach 8.5 knots, and form a famous whirlpool. When the west-going flood tide emerges into the main part of the firth it can form
overfalls, dangerous breaking steep seas, particularly if it meets a swell from the west or south-west. This is known as the "Great Race". Bealach a'Choin Ghlais (the channel of the "Grey Dogs"), between Scarba and
Lunga being narrower, is if anything more hazardous. ==Habitats and conservation==