Thursday, May 26, 2011

Catskill Delta bluestone and a thought about Charles Lyell (updated)

There is a good deal of bluestone in Fort Tryon Park and in the neighborhood around Pratt Institute in Brooklyn.  Most bluestone, which is not necessarily blue, comes from an area that stretches from central Pennsylvania to the Catskills of New York.  It is a thinly laminated sandstone, the remains of the Catskill Delta of the Devonian era (roughly 350-400 million years ago).  Because it splits into interesting patterns and it fairly resistant to wear, bluestone is often used for sidewalks, as they are here and in Fort Green, and garden paths.  bluestone quarries can be found throughout the Catskills, where it was once an important industry.  However,  most Bluestone today originates in Pennsylvania.
There are two features to look for.  While some say that they can be seen more easily on a rainy night, I think that they can be appreciated no matter the weather or time of day.  The first feature is ripple marks.  The movement of the water left ripple marks in the sculpted sand of the river delta.  The intervals between them can be quite long, which I suppose indicates their origin in a beach rather than river environment.  The larger the slab the better to get the full effect.  There are some very large sidewalk pieces near Pratt and the entrance to Fort Tryon Park has many as well.  Some of which match up to adjacent slabs and so allow one to view several feet of Devonian era river delta. 

Walking through Fort Tryon, I happened upon the same process at work in the present day that, as Lyell showed us, has been amongst those at work on the earth for millions of years.  This is the Gradualism that would so profoundly influence Darwin's thinking.
The second feature to look for is called "flow lineation."  As the water flowed over the sands, it lined the grains up in subtle lineations that  become clear once the stone has been cut.  One can literally see the direction of the flow of the water in the stone itself.  I don't have a good picture, yet, but one can also find indentations from raindrops. If one finds gets lucky, you can see the results of a 350 million year old rainstorm..

    [Click on an image to view it full-size.]
A few words from Charles Lyell:
All are aware that the solid parts of the earth consist of distinct substances, such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but previously to observation it is commonly imagined that all these had remained from the first in the state in which we now see them, — that they were created in their present form, and in their present position. Geologists have come to a different conclusion. They have discovered proofs that the external parts of the earth were not all produced in the beginning of things, in the state in which we now behold them, nor in an instant of time. On the contrary, they have acquired their actual configuration and condition gradually, under a great variety of circumstances, and at successive periods, during each of which distinct races of living beings have flourished on the land and in the waters, the remains of these creatures still lying buried in the crust of the earth. --Charles Lyell, Principles of Geology
from Lyell, Student's Elements of Geology