Page 37 - Atlas of Sedimentary Rocks Under The Microscope

70, 71 Terrigenous clastic rocks
Si ltst n e s
o
(continued)

Many siltstones show small-scale sedimentary structures.
70 shows a laminated siltstone. the laminae being defined
by changes in grain size. The dark layers, seen near the
base of the photograph, arc composed almost entirely of
clay-sized material, whereas the band just above the
centre is composed of clear fine sand-sized quartz. The
photograph also shows graded layers (below and above
the coarse band). The fining-upwards grading is shown by
a decrease in the clear quartz material and an increase in
the dark-coloured clay.
71 shows a siltstone 111 which ripple cross-lamination
can be seen. indicating a flow from right to left, as seen in
the picture. The ripple structure is picked out by the
alternation of dark clay-nch and pale clay-poor laminae.

70: Coal Measures. Upper Carhoni{erous. Lancashire.
Engla111/; magnifica/ion x 15, PPL
71: Ashgillirm. Uangrano[(. Dy(ed, Wales: magn(fic­
arion x 9 . PPL.