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Water Worked N.H. Cobbles

Last updated on February 18, 2022

I have taught a Natural Hazards Geology Course for many years at Plymouth State University and lead the students on fieldtrips to examine glacially carved features, including streams where large volumes of glacial meltwater rounded rocks, specifically boulders into cobbles. Many non-geology majors have no idea that the Wentworth scale exists which is a chart used to describe the size of particles in sedimentary rock.  The scale uses diameters to classify sediments; particles larger than 64 mm in diameter are classified as cobbles; smaller particles are pebbles, and larger particles are boulders. The smaller the material, the longer the particle has been worked or polished or the greater the volume of water the more scouring power.  Rock particle sizes tell a story about the environment. The fieldtrips could be called “reading the water landscape.”  

Using pebbles and cobbles on cyano-treated paper in my mind seems perfect to illustrate water worked material.  When the paper is exposed to sunlight the chemicals turn the paper blue, except where there are obstacles blocking the sun, in this case cobbles or pebbles.  


Fragrance Lake geology guide | Northwest Geology Field Trips (

Fragrance Lake Geology Guide, Tucker, Dave, Dec. 14, 2012.

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