BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 10, pp. 131-134, PL. 12 March 23, 1899 



DISLOCATION AT THIRTYMILE POINT, NEW YORK 



BY GROVE KARL GILBERT 



{Presented before the Society December 30, 1898') 

 CONTENTS 



Page 



Observations ■ . 131 



Interpretations 133 



Observations 



On the shore of lake Ontario, 30 miles east of the mouth of the Niagara 

 river, is a bluff point on which a lighthouse stands. The waves are 

 there actively engaged in eating back the shore, and their work has pro- 

 duced a cliff about 20 feet high, exhibiting red shale of the Medina for- 

 mation, with a thin cover of drift. The shale is of variable texture, some 

 of its layers being so arenaceous as almost to deserve the name of sand- 

 stone. The strata lie nearly horizontal, except at one place. At the 

 apex of the cape is a fractured anticline, and this is accompanied by a 

 vertical dislocation of 6 feet, the uplift being on the northeast side. The 

 axis of the disturbance trends northwest and southeast, and the outline 

 of the coast is such that the disturbance reappears in the bluff about 200 

 feet to the eastward. There was also at the time of my visit a third 

 point of exposure, where the waves had reached the zone of fracture and 

 developed a small cave. 



Figure 1 gives a section of the anticline at the point of best exposure, 

 and its character is further shown in plate 12. The disturbed zone di- 

 minishes downward, as though terminating or changing to a simj^le fault 

 a short distance below the exposure. At the top the anticline is over- 

 turned toward the right, or southwest, a feature to be more fully described 

 presently. At the base of the section observation iscutoff by the shingle 

 of the beach, but evidence of disturbance reappears at a distance of a 

 few rods beneath the water of the lake, where beds of the same forma- 

 tion show an anticlinal structure, and this structure was traced as far 



XX— Bull. Geol. Soc. Am., Vol. 10, 1898 (131) 



