GABBRO is another intrusive igneous rock, but it is heavier and darker
than granite. It is composed mainly of feldspar and dark iron-bearing
minerals that give the rock a dark color. It is coarse grained and
contains little or no quartz.
Mineral crystals of gabbro are especially tightly interlocked, making
the rock very difficult to break. Weathered gabbro is a rusty color on
the surface, because the iron in gabbro changes color just as a piece of
metallic iron becomes coated with rust when left out of doors.
Like other igneous rocks found near the surface in Illinois, gabbro was
carried into Illinois by the glaciers and deposited as glacial debris.