Postglacial Geologic Processes

After the Laurentide ice receded north of the St. Lawrence lowland over 12,500 years ago and local ice melted out of the cirques, very cold conditions, along with high winds and permafrost, persisted in the northern Hudson region. Ice still gripped the glacial soils and wedged into joints in the bedrock, loosening shed-sized angular blocks. The homogeneous, anorthosite domes developed curved sheets of rock decimeters thick at the surface. The sheets cracked into large angular plates along joint planes perpendicular to the surface, in a process known as exfoliation. The rock slabs continue to be prime candidates for sudden rockslides, and the ground-levelbases of the domes are surroundedby this very coarse debris strewn over the talus slopes.

Ice-wedging is a common cause but earthquakes are also causative factors in debris slides, and they may be connected to groundwater problems. Earthquakes occur along ancient faultlines and trigger failure of unstable slopes and slumping of coastal plain sediments far from any earthquake's epicenter. Renewed movement and seismic activity in recent times has been linked to postglacial rebound in which stresses in the differentially rising crust are relieved along faults. In addition, industrial and suburban development along fault lines has led to increased use of groundwater and disposal of wastewater into the ground, as well as destabilization of slopes and excavation of bedrock for superhighways and residential and industrial construction sites. All of these factors can have a negative impact on fault line stability and lead to increased activity.

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