Tors, relics of former landscapes May 2003

Jumbo Rock in California’s Joshua Tree National Park is my favorite campground. Tucked away snugly between huge granite boulders and monoliths, the place has an almost otherworldly appearance. A good place to rest - and to reflect on the geology, as this is tor country.

Tors can be found all over the world, and there is evidence for their formation under a wide range of climatic conditions, from polar climates to arid deserts.

Although the general processes that control tor formation are fairly well established, figuring out the exact sequence of events and the relative importance of different geological processes for any particular location is not as easy.

Tors on the northern flank of Ben Rinnes, NE Scotland.
Tors on the northern flank of Ben Rinnes, NE Scotland.

In essence, tors are the remnants of former landscape surfaces and result from long-term differential weathering and erosion of the bedrock which, after the removal of the weathered material, leads to the emergence of the resistant rocks as tors.

Historically, two different models have been proposed for tor formation based mainly on studies of tors in the UK, especially Dartmoor.

The first is David Linton’s (1955) Theory of Tertiary Chemical Weathering. Linton proposed that tor formation started with sub-surface chemical weathering of the bedrock by acidic groundwater percolating along joints. Some areas would have been more resistant, forming largely undecayed corestones embedded in the weathered rock. Subsequent rejuvenation removed the weathered debris, leaving behind the core-stones as tors.

Palmer et al. (1962) subsequently proposed the Theory of Periglacial Weathering, based on the periglacial cycle, where freeze-thaw activity breaks up the rock followed by solifuction which removes the debris.

However, as more tors in a wider variety of settings were studied, it became clear that both these early theories can provide at best a partial explanation, especially in the case of the periglacial theory as tor formation certainly isn’t restricted to periglacial environments (and neither do tors occur in all periglacial environments) .

It seems that tor formation depends on the presence of a suitable geologic structure, in combination with a climatic environment which promotes the removal of the weathered debris, such as solifluction in periglacial environments, or wind and flash-floods in arid climates.

Tors can form in a wide variety of geological settings, but the three most common types of tor are the ‘echelon’ tor formed from escarpements associated with monoclines, ‘residual’ tors which are left behind after denudation of granitic intrusions (such as Dartmoor), and ‘outlying’ tors which are remnants of a retreating free-face along an anticline or a periclinal dome.
A basic model for the evolution of the residual Dartmoor tors starts with initial exhumation of the granite batholith during the Lower Cretaceous, causing dilatation jointing, followed by reburial during the Eocene, when the main sub-surface chemical weathering occurred. The batholith was then finally exhumed during the Miocene when fluvial erosion started to carry off the weathered granite, leaving behind the more intact granite, first as inselbergs and finally as massive tors. Finally, in the periglacial environment of the Pleistocene, the tors were further reworked by frost shattering, and the debris removed by solifluction.

A generally similar process probably created the California tors, although the actual weathering processes during the Pleistocene would have been rather different.

Paul De Schutter

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