Arkose is a coarse-grained sedimentary rock that is rich in quartz and feldspar. It is unusual in that feldspar weathers rapidly at the earth's surface, and therefore seldom survives long enough to accumulate without alteration or disintegration. Arkose forms as a result of very rapid weathering of granite or gneiss, and rapid accumulation and compaction in situ of the weathering products, before they can alter or decompose. This rock from Mt. Tom, Massachusetts, consists of angular and sub angular grains of quartz and feldspar in a dense matrix of iron-rich clay.