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NEW YORK STATE MUSEUM 



were bent upward until, the elastic limit being exceeded, the fold 

 snapped along the crest, furnishing relief to the bent flanks and per- 

 mitting them to straighten. In the rock here only a single set of 

 good joints appears, and this runs at right angles to the axis of the 

 fold separating it into a series of transverse blocks. On bending, 

 these seem to have fractured individually instead of collectively, so 

 that the axial fracture does not coincide in the different blocks, but 

 departs from the median line, now on one side and again on the 

 other, as illustrated in figure 10, giving rise to the dovetailing of 

 slabs along the crest, so well shown in the photograph. 



Fig. 10 Plan of fold in Potsdam sandstone, j-j-j=joints; f-f=fracture along crest, illus- 

 trating the manner in which the fracture §hifts laterally in the different joint blocks, caus- 

 ing overlap of the rock edges along the crest. 



One view .of one of the folds in the Lowville limestone is shown 

 in plate 30. The greater part of this fold is covered with soil, but 

 centrally it has been stripped and a small amount of rock removed 

 for local use. It seems to have about the same length as the previous 

 one, and to be buckled up about the same amount. Its axis trends 

 to the northwest. The rock is more closely jointed than in the Pots- 

 dam fold, and with two good sets present, one of which trends north- 

 west with the fold, as the view clearly shows. Fracture then was 

 unnecessary in this case and readjustment took place by utilization 

 of these northwest joints, and instead of being actually folded, as 

 might be judged from the photographs, the displacement really has 

 the character shown in figure 1 1 , as sketched on the spot. 



20° sw 



Fig. 11 Diagram to illustrate the arrangement of the joint blocks in the Lowville fold 

 shown in plate 30 The central block lies nearly horizontally, the adjacent ones tipped in 

 the directions and by the amounts indicated. 



Small postglacial folds .of similar type have been described by a 

 number of authors and from various localities, and they have re- 

 sulted from several different causes. Gilbert, after seeing some of 



