SUPERFICIAL DETRITUS. 
329 
described. It requires simply the insinuation of sand and pebbles between the layers, and then 
that the upper one be made to move over the lower, to produce all the observed phenomena. 
In the softer shales, where the striae are sometimes preserved, they are deeper than in the 
limestones and other hard rocks. In the black shale about Dunkirk harbor, all the grooves 
are broad and deep, compared with those of Black-Rock or Niagara, and there seems no other 
cause except the soft character of the shale. 
There is a remarkable fact connected with these striated surfaces, which may be noticed 
here. The terrace at Lewiston is formed by the upper part of the Medina sandstone, the 
Clinton group, and the Niagara shale, capped by about twenty feet of limestone as already 
described. The following section illustrates the succession : 
J, 2 & 3. Medina sandstone. 4. Clinton group. 5. Shale of Niagara group. 6. Limestone of Niagara group. 
The top of this terrace is three hundred and fifty feet above Lake Ontario, and more than 
two hundred feet above the plain about Lewiston. The projecting shelf of rock a , is the 
limestone of the Clinton group, about one hundred feet below the top of the Terrace. The 
surface of this projecting mass is deeply grooved and striated, the grooves having a general 
southern tendency, but more irregular than where they are seen upon the limestone on the 
top of the terrace ; and at this place, the surfaces two hundred feet lower and one hundred 
feet higher are scored in like manner. We naturally inquire, what agency could produce 
this effect? Here is an abrupt elevation of one hundred feet above the striated surface ; and 
it seems hardly possible that an island of ice, loaded with granite boulders, could have stranded 
upon this projecting shelf, and produced the scoring, and that at the same time others above 
and below could be made in like manner. 
It may perhaps not be out of place here to consider a few facts connected with these striated 
and polished surfaces, in their relation to the theories of glacial and glacio-aqueous action, as 
the agencies in producing such phenomena, and others connected with superficial deposits. 
It will be borne in mind that the Fourth District, in its greatest elevation of about two thousand 
feet above tide water, descends to the level of Lake Ontario, two hundred and forty feet above 
tide, for the most part in a series of steps or terraces over the successive formations ; the 
[Geol. 4th Dist.] 42 
