[Pluton] Igneous intrusion - A sill is a tabular pluton that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorph
ic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes, which do cut across older rocks. [Wikipedia]
A [soil](https://n2t.net/99152/h1280) with 50% of the particles by mass smaller than 75 microns (# 200 sieve) with plasticity characteristics plotting above the A-Line [PI = 0.73(LL-20), where PI = pl
asticity index and LL = liquid limit].
Concentration of silt in bands, zones or layers adhered to coarser grains. This is a cryogenic phenomenon caused by freeze-thaw cycles that are responsible for the downward silt movements. Some author
s explain these conditions as fragipan formations.