Sedimentary rock in which less than 30 percent of particles are greater than 2 mm
in diameter (gravel) and the sand to mud ratio is at least 1.
SLTTs 2004; Neuendorf et al. 2005; particle sizes defined from Krumbein phi scale
(W C Krumbein and L L Sloss, Stratigraphy and Sedimentation, 2nd edition, Freeman,
San Francisco, 1963; Krumbein and Pettijohn, 1938, Manual of Sedimentary Petrography:
New York, Appleton Century Co., Inc.)