Part 3

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.

Native granitic rock probably lies very deep beneath the entire state. It has been found in deep oil-test drillings along the western and northern margins of Illinois.

GABBRO (2)

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.

PORPHYRY (3)

PORPHYRY is an igneous rock identified by its texture rather than its mineral content, which is variable. Distinct crystals (phenocrysts) of minerals are embedded in a matrix of fine-grained rock. The phenocrysts formed before the main mass of the rock hardened.

Any igneous rock may have a porphyritic variety, such as granite porphyry and rhyolite porphyry, although porphyries are most likely to form in association with fine-grained igneous rocks.

Porphyry is found in Illinois only in glacial drift.

BASALT (4)

BASALT is the most widely distributed volcanic rock. Pyroxene, feldspar, magnetite (an iron ore), and in some instances olivine, biotite, and hornblende, compose this rock. The dark green, gray, or black color is due to the dark-colored minerals that make up much of the rock. The minerals in basalt are fine grained and are packed closely together. Phenocrysts of olivine, pyroxene, and hornblende may be present. Basalt is easily identified by its color and fine-grained texture.

The glaciers brought basalt into Illinois along with other igneous rocks.

PERIDOTITE (5)

PERIDOTITE is the only igneous rock native to Illinois that crops out at the surface. It is found as dikes (irregular veins) or sills (thin sheets) that were formed when molten rock from deeper in the earth intruded into cracks and fissures in the bedrock of southeastern Illinois (Hardin, Pope, Gallatin, and Saline Counties). Peridotite pebbles and boulders also may be found in the glacial drift.

Peridotite ranges from very fine grained to medium grained and has an even texture. It is dark gray to greenish gray, depending on the minerals present. In general it is composed of olivine, hornblende, pyroxene, and mica, with little or no feldspar or quartz.

GNEISS AND SCHIST (6)

GNEISS is a metamorphic rock composed of roughly parallel bands of minerals. It is medium grained to coarse grained and is generally light in color. The names given to gneiss emphasize a distinctive texture or mineral or indicate composition. For example, biotite gneiss emphasizes a mineral, and granite gneiss indicates the composition of the rock.

SCHIST is much like gneiss but is fine grained and has a thinly layered structure that makes the rock break with a wavy surface. Some common types of schistose rocks are talc schist, chlorite schist, and hornblende schist. As the names indicate, they are characterized by their dominant mineral. Mica schist may be formed by the metamorphism of either sedimentary or igneous rocks.

Gneiss and schist are not native to Illinois but are found in the glacial drift.

CONGLOMERATE (7)

CONGLOMERATE is a sedimentary rock made up of pebbles or other rock fragments cemented in a background or matrix of finer material, generally silica, calcium carbonate, clay, iron oxide, or a mixture of these substances. The rounded rock fragments have been worn by being rolled in streams and along beaches.

If the pebbles embedded in the matrix are sharp and angular, freshly broken and not worn, the rock is called breccia and is generally found near the place where the fragments originated. Conglomerate or breccia may be made up of any type of rock or mineral, most commonly durable material such as chert, quartz, quartzite, granite, and gneiss.

In Illinois, conglomerates commonly are found at the base of sandstone formations and as beds in the lower “Coal Measures.” They also are found in some gravel deposits.

SANDSTONE (8)

SANDSTONE is a clastic sedimentary rock consisting of sand-sized grains (one sixty-fourth to one-sixteenth inch in diameter) held together by a cementing material. As sandstones become finer grained they grade into siltstones; as they become coarser grained they grade into conglomerate. The shape of sand grains in sandstones ranges from rounded to angular.

Quartz is the dominant mineral in sandstone, but other rock grains and mineral grains (especially chalcedony, feldspar, muscovite, hornblende, magnetite, or garnet) generally are present.

Sandstones are commonly cemented by carbonates, silica, iron oxides, or clays. Most sandstones are a shade of gray or brown, but the color may vary from gray or white to yellow, brown, or red. The color probably depends on the type of cement, the amount of organic material present, and the amount and degree of oxidation of iron in the rock.