thrive when sheltered between the rocks and exposed on the moor. Geology Brimham Rocks are formed from a medium to coarse sandstone known both as the Lower Brimham Grit and also as the Lower Plompton Grit, one of a series of such sandstones laid down in the later part of the Carboniferous period in what is now the Pennine region. In formal terms this particular grit which is between 10 and 30m thick, forms a part of the Hebden Formation, itself a sub-unit of the Millstone Grit Group. It was deposited 318-317 million years ago during the Kinderscoutian substage of the Bashkirian stage. The rock which has traditionally been referred to as Millstone Grit, originated as river-deposited sands in a delta environment and contains both feldspar and quartz pebbles. Deposition from moving water has resulted in the cross-bedding which is very evident in most of the outcrops. Brimham Rocks has been described as "a classic geomorphological site, significant for studies of past and present weathering processes and their contribution to landscape evolution." Although discussion continues around the formation and date of tors such as these throughout Britain, much of the development into the forms displayed at Brimham is likely to have taken place over the last 100,000 years before, during and after the last ice age – the Devensian. Some disintegration of the rock strata may have occurred along weaknesses such as fault and fracture planes whilst still buried. Sub-aerial weathering has continued the process. The outcrops were exposed when glacial action, gelifluction and further weathering and water erosion removed the loose material which separated them. During periods of harsher climate, windblown-ice as well as particles of sand and dust have more effectively eroded weaker layers to give rise to these wind-carved shapes. Such processes have created holes in some rocks, and left dust on the ground below. Erosion continues, caused by weather and tourist numbers. One possibility for the mushroom shape of some stones is that they were exposed to sandblasting at ground level when an ice cap melted 18,000 years ago, narrowing the bases of outcrops, then they were subject to all-over sandblasting when the ice had gone, causing the irregular shapes. Due to this process the Idol rock, for example, will not exist for ever. Druidic theories It was not until the early 20th century that it was understood that the rocks were created by natural forces.