The speaker, who has photographed every legally accessible tor on Dartmoor, gave members a very interesting overview of its origins.
Dartmoor is one of several granite intrusions in the Southwest Peninsula. The molten rock underlying the crust was less dense than its surroundings, so rose and intruded into the weaker areas of the overlying rocks about 280 million years ago, when the future Devon was close to the equator, heating the closest of these to form an aureole of metamorphic rocks. Sometimes the molten granite incorporated chunks of the country rock, which remain visible and are known as xenoliths (foreign stones). In 3 million years three to five kilometres of the rocks above the intrusion had been eroded by weathering and the granite had cooled from 700°C to 400°C.
As the magma cooled, different minerals crystallised out at different temperatures, tin first, nearest the granite, followed in turn by copper, zinc and iron. Metal ores were deposited in joints running east-west for tin and copper and north-south for lead and silver.
Granite consists of quartz, feldspar and biotite, a form of mica. Tor granite is coarse-grained, with large feldspar crystals, blue or quarry granite is medium-grained and aplite is fine-grained. Of the three constituents, feldspar is the most susceptible to chemical attack and breaks down to form kaolin (china clay). The residual quartz forms growan, a coarse sandy material which is easily removed by weathering. By 50,000 years ago, due to continuing erosion, the granite was exposed at the surface, surrounded by Devonian rocks in the south and Carboniferous in the north.
Dartmoor is, of course, famous for its tors and the speaker described their formation, adding that visitors sometimes enquired who had built them! As the still-buried granite cooled it contracted and became fractured by joints, first vertically and then horizontally. The reduction of pressure caused by the loss of overlying material released the water that formed 4% of the magma and contained dissolved chemicals. This widened the joints and also led to the deposition of the minerals. After the jointed granite was finally exposed, the more elevated sections were left standing as tors when the surrounding growan was eroded.
On the western fringe of Dartmoor, tors like Cox Tor and White Tor are made of dolerite, not granite, and Brentor is the remains of a volcanic plug.
600,000 years ago the Ice Ages began. The main ice sheet never came south of the Bristol Channel, but recent evidence has emerged of a small glacier on NW Dartmoor. Most of the moor would have been tundra. With the seasonal freezing and thawing, solifluction (the movement of material downslope) occurred, leading to the clitter which surrounds so many of the tors.
10,000 years ago the climate warmed, leading to the first settlement of the moor, forest clearance and farming, but around 6,000 years ago the climate deteriorated, becoming cooler and wetter. Peat bogs started to form and the farming was marginalised.