628 PROCEEDINGS OF THE BALTIMORE MEETING 



the fissure, while buckling was observed 24 hours afterward. The sinking was 

 observed by means of a horizontal board placed across the cleft at the top of 

 the cliff. At the time the writer first saw the slide, about two weeks from its 

 inception, there was a throw or vertical displacement of about 6 feet. This 

 downward movement continued until December 22, when the displacement was 

 about 20 feet. On February 22, 1909, the brick company officials informed the 

 author that where they are excavating at the base of the cliff near the north- 

 eastern extremity of the crack they are uncovering shale layers for a distance 

 of more than 30 feet, which dip toward the valley. This would seem to indi- 

 cate that the layers of rock at the base of the whole mass have settled down- 

 ward into the valley floor. 



Classification of the Dislocation 



This movement of the rocks has been classified as a landslide, although it 

 might be called a fault, since it has been shown that the dislocated mass had a 

 throw of 20 feet. This slide, however, differs from the usual landslips in 

 respect to the slowness with which most of the attendant phenomena took 

 place. The formation of the fissure was the most rapid, and covered a period 

 of two days, while the buckling and settling have extended over a period of 

 several months. The amount of lateral motion was also very small in this 

 slide, which belongs to the more uncommon type of "rock slides" rather than to 

 the "waste slides" of loose material, which are so common in the springtime 

 and during wet seasons along valley sides. Another point of difference is that 

 the saturation of the ground with percolating water is a very important cause 

 of most landslides, while this one occurred during the dryest summer that 

 Cleveland has experienced for ten years. 



Causes of the Feactube and attendant Phenomena 



It is possible to consider the causes of this landslip and its accompanying 

 features from two different standpoints as follows : either the landslide caused 

 the buckling, or the latter was the cause of the former. The writer can see 

 no very important reason for the second assumption, which requires that the 

 shales of the valley floor should first give way and arch up, thereby causing the 

 cliff wall to sink down into the place vacated by the shale. On the other hand, 

 there are several reasons for the first hypothesis, and it is accordingly pro- 

 posed to consider that the buckling was caused by the landslide. 



No mass under the action of gravity can move with a pure horizontal motion 

 unless it is sliding in an inclined plane. On the other hand, a tilting or tipping 

 motion can be produced by a yielding of the foundations. Settling of the dis- 

 located portion of this slide is quite conclusive proof that the foundations were 

 weak, and there are several factors which may have contributed to the weak- 

 ening of the shales at the base of the cliff. 



One possible cause of weakening is the action of underground water in 

 former seasons moving along the joint and bedding planes. It has been previ- 

 ously shown that these shales were much jointed in their normal condition, 

 and that the surfaces of these joint planes were all weathered. This action 

 must have been performed by water percolating from the surface downward 

 through the joint planes. This water continues its journey downward, and 

 will finally work its way along the bedding planes until it reaches the stream 





