The Marl Slate and Basal Permian Breccia at Middridge, CO. Durham Bell, J. ; Holden, J. ; Pettigrew, T. H. ; Sedman, K. W. Abstract Publication: Proceedings of the Yorkshire Geological Society Pub Date: March 1979 DOI: 10.1144/pygs.42.3.439 Bibcode: 1979PYGS...42..439B