The shape of sedimentary particles may carry important information on their history.
Current approaches to shape classification (e.g. the Zingg or the Sneed and Folk system)
rely on shape indices derived from the measurement of the three principal axes of
the approximating tri-axial ellipsoid. While these systems have undoubtedly proved
to be useful tools, their application inevitably requires tedious and ambiguous measurements,
also classification involves the introduction of arbitrarily chosen constants. Here
we propose an alternative classification system based on the (integer) number of static
equilibria. The latter are points of the surface where the pebble is at rest on a
horizontal, frictionless support. As opposed to the Zingg system, our method relies
on counting rather than measuring. We show that equilibria typically exist on two
well-separated (micro and macro) scales. Equilibria can be readily counted by simple
hand experiments, i.e. the new classification scheme is practically applicable. Based
on statistical results from two different locations we demonstrate that pebbles are
well mixed with respect to the new classes, i.e. the new classification is reliable
and stable in that sense. We also show that the Zingg statistics can be extracted
from the new statistics; however, substantial additional information is also available.
From the practical point of view, E-classification is substantially faster than the
Zingg method.