Observations on Structures in the Slates of Northampton County, Pennsylvania

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Abstract
The observations which follow deal chiefly with the secondary structures in the slates of Northampton County, Pennsylvania, a portion of the Appalachian Valley. The rocks of the region include limestones of Cambrian and Ordovician age, the Ordo-vician Martinsburg slate, and the Silurian (?) Medina conglomerate. The general structure is a northward-dipping monocline, intensely folded and showing some faulting. Studied in detail this structure yields observations and conclusions of two types: (i) those that have a regional bearing upon Appalachian geology, and (2) those of more general interest to geologists engaged in structural work, not necessarily in the area discussed. To the former type belong the general northward recumbent attitude of the minor folds, the very common parallelism between bedding and cleavage strikes, and the evidences for an important diastrophic epoch between Martinsburg and Medina sedimentation. Of more general interest are studies relating to the effect of pitch of folds upon dip and strike; description of the appearance of recumbent folds in horizontal and vertical sections; an exposition of a method for restoring structure at depth when bedding and cleavage are seen at the surface; and descriptions and explanations of false cleavage and grain in slate.

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