The term ‘mudrock’ is used to describe all non-metamorphic argillaceous deposits. A three-fold classification of mudrocks is proposed: siltstone comprising less than 25% clay fraction, claystone containing more than 40% clay fraction and mudstone restricted to the intermediate materials. ‘Metamud-rock’ is used to describe weakly metamorphosed argillaceous rocks whose strength and geomechanical behaviour has been affected by the albeit limited re-crystallization.
The paper provides clarification of the terms rock material weathering, rock mass weathering, weathering grades and weathering zones, lithorelicts and apparent lithorelicts. The criteria separating moderately and highly weathered rock are reviewed and, for mudrocks, a recommendation made that in engineering geological descriptions the boundary should be amended to 200f the material having soil characteristics rather than 50%. The term ‘completely weathered’ should be replaced by ‘extremely weathered’.
A consistent word order for soil and rock descriptions is advocated, beginning with the material strength. Recommendations are made to resolve the present ambiguities in the use of ‘to/and’ in strength descriptions. An aide memoire for the identification, description and field assessment of strength of argillaceous rocks is provided.