Rock creep and the development of the Niagara cuesta

This work addresses the post-glacial development of the Niagara Escarpment, speci?cally the east-facing scarp slopes between Hamilton and Collingwood. The escarpment is one of a series of scarps that shape the Great Lakes Basin. One interpretation suggests that the escarpment has evolved through homoclinal recession to the west. It is therefore viewed as an erosional feature brought about due to ?uvial action on the weaker shale formations that underlie the cap rock. However, the deposition of large amounts of drift during the Pleistocene glaciation has resulted in the disruption of the preglacial drainage pattern within the Great Lakes Basin. Therefore, the current drainage is typically orthogonal to the scarp face with an absence of ?uvial action along the base. Consequently, the cuesta is experiencing an extremely slow rate of retreat. Conventional thinking on the evolution of the present morphology suggests that it developed rapidly following deglaciation due to periglacial processes. The concept of the escarpment as a relict feature during the Holocene therefore pervades the modern literature. However, the cambering of the cap rock towards the scarp face and the apparent motion of individual joint-bounded blocks indicates that in the absence of exogenetic processes, a slow development due to endogenetic processes has dominated the modern development of the escarpment. It has been proposed that the present morphology of the cliffed sections may be due to deformation within the shale layers. In order to test this hypothesis, the rheology of the rocks that form the escarpment was tested and compared to the gravitational stresses that would be expected within the rock mass. The results indicate that the compressive strength of the shale layers of the Cabot Head Formation is lower than the expected effective stresses. It is therefore concluded that in the absence of high con?ning stresses, as would be expected near the cliff face, slow plastic deformation within this formation is occurring. The morphological expression of this deformation is seen in the cambering of the escarpment, the dilation of pre-existing joints, and the rotation of joint-bounded blocks.