Page 180 - Laboratory Manual in Physical Geology

TEXTURAL FEATURES OF SEDIMENTARY ROCKS

GRAIN SIZES GRAIN ROUNDNESS GRAIN ARRANGEMENTS

Grains
visible;
larger
than
sand.

Gravel-sized

Grains Very angular grains (VA)
visible;
like in a
sandbox.

Sand-sized Poorly Sorted

Grains
barely
visible;
feels
gritty.
Mud-sized (clay + silt)

Silt-sized Subangular grains (SA)

Grains
not visible;
feels
smooth;
dull luster
on freshly
broken
surface.

Clay-sized Moderately Sorted

Crystals
not visible;
feels
smooth; Subround grains (SR)
shiny luster
on freshly
broken
surface.
Microcrystalline

Aggregate
of visible
crystals.

Crystalline Well-rounded grains (WR) Well Sorted

FIGURE 6.3 Textures of sedimentary rocks. Scale for all images is × 1.

Sedimentary Processes, Rocks, and Environments ■ 159