BGS Rock Description Look-up
The British Geological Survey offer an excellent service for looking up rock descriptions based on the LEX part of the LEX-ROCK descriptions found both on CENTREMAPSlive PDF geological mapping and the associated Legend/Key.
The following link will take you to the British Geological Survey pages:
The British Geological Survey Lexicon of Named Rock Units
Simply 'Enter the Lexicon of Named Rock Units' and replace the '%' symbol in the 'computer code' field with the LEX code.
On submitting the query a full rock description including rock thickness, top and bottom boundary definitions and much more can be found.