The glacial drift in Illinois contains many pieces of igneous and metamorphic rocks, but most of the rocks native to Illinois are sedimentary.
Some sedimentary rocks are made up of weathered fragments of other rocks that have been moved by rivers, waves, winds, or glaciers. These sediments have been deposited and later compacted or cemented by the mineral matter carried in water moving through them. Such sedimentary rocks are called clastic (meaning broken pieces) rocks, as opposed to those formed by chemical precipitation from water and those that consist of fossil remains.
Clastic sedimentary rocks are classified first on the basis of the size of the grains of gravel, sand, silt, and clay of which they are composed; on the type of deposition, such as glacial drift and wind-blown silt or loess, that produced them; and then on the basis of their mineral composition. Rocks such as gypsum and some of our limestones were formed by chemical precipitation from sea water.
Metamorphic rock literally means rock that has changed form. The change of form has been caused by heat and pressures that occur below the surface of the earth or by heat from upward moving hot magmas or melted rocks. Examples of metamorphic rocks are found in Illinois as boulders and pebbles in the glacial drift.
Descriptions of the rocks and minerals represented by specimens in the Geological Survey’s rock and mineral sets follow. The numbers preceding the descriptions correspond to the numbers on the specimens in the sets.
Although the set includes the rocks and minerals that are most commonly found in Illinois, many others can be collected. For that reason, following the descriptions is a key for identifying other Illinois minerals and rocks.
As a further aid to the beginner and the amateur, a list of equipment useful in making a rock and mineral collection is given at the end of the book.
GRANITE (1)
GRANITE is one of the most widespread intrusive (originating deep within the earth) igneous rocks. It consists chiefly of feldspar and quartz with small amounts of biotite, muscovite, or hornblende. Most granite is light colored, but it can be white, gray, yellow, pink, or deep red. The texture ranges from medium grained to coarse grained.
Granite pebbles or boulders are the most common igneous rocks found in glacial deposits in Illinois. They are not native to the state but were brought here by the great ice sheets or glaciers that advanced from southern Canada to cover much of northern United States during Pleistocene time.