Guide to Rocks and Minerals of Illinois
A variety of rock types occur within the glacial deposits in Illinois. Some of the most distinctive include the igneous and metamorphic rocks known as erratics that originate only from specific regions to the north. For example, Lake Superior agates from the Lake Superior region of northern Michigan (see quartz) and, rarely, native copper from the upper Keweenaw Peninsula of Michigan have been found in Illinois.
Within the glacial deposits of Illinois are several unique and distinctive glacial erratics that indicate the specific areas where they originated. Knowing the source areas for specific types of rocks found within Illinois glacial deposits provides evidence for the flow paths (or directions) that the various continental glacial lobes traveled on their journey to Illinois, during the Pleistocene Ice Age.
Unique Glacial Erratics
A distinctive type of glacial erratic called an omar has been collected from glacial deposits in Illinois. Omars refer specifically to a dark gray to black, metamorphosed, tough, fine-grained sandstone (a graywacke). Omars are characterized by the occurrence of white to cream-colored, usually spherical, calcareous concretions that commonly weather out, leaving rounded depressions (pits) in the rock. The source rock for omars is the Precambrian age, Omarolluk Formation of the Belcher Group, which is exposed in the Belcher Islands on the southeastern side of Hudson Bay in the Northwest Territories of Canada. The occurrence of omars in Illinois indicates a flow path from Hudson Bay, south across Ontario, into the Lake Michigan basin, and south into Illinois.
A second, more distinctive and unusual type of glacial erratic found in Illinois is the jasper conglomerate, also called puddingstone. These conglomerates were transported from the Precambrian aged, Canadian Shield rocks of the Lorrain Formation. They crop out north and northwest of Bruce Mines village, which is located along the far northwestern shore of Lake Huron in Ontario. In the early 1800s, English settlers there gave the stone its unusual name "puddingstone" because it looked like boiled suet pudding with currants and red cherries. Jasper is a fine-grained, red to brown, iron-bearing chert, and the jasper conglomerate is considered to be a transitional rock—from sedimentary to metamorphic. The jasper conglomerates were originally sandstone conglomerates containing white quartz sand and clasts of red jasper. The original sediments were eroded from older rocks and deposited by flowing water in streams and rivers. The material was buried and, with time, formed a sedimentary rock. Heat and pressure (metamorphism) transformed them into a low-grade metamorphic rock. The light creamy-white, fine-grained matrix that surrounds the jasper pebbles and other types of pebbles and stones is primarily a white quartzite. The jasper pebbles vary in color from red to brown and pink to purple. Some jasper conglomerates contain minerals such as chromite, corundum, platinum, diamonds, gold, sapphire, and zircon. Although somewhat rare in Illinois, jasper conglomerate erratics are found in northeastern-source glacial deposits. These tightly cemented conglomerates make a good ornamental stone when cut and polished.
Tillite is defined as a sedimentary conglomerate formed by the consolidation or lithification (cementation) of glacial tills from an ancient glacial deposit.
Fairly common in the glacial deposits of Illinois, these tillites were derived from the Precambrian age, Canadian Shield rocks of the Bruce, Gowganda, and Lorrain Formations of Ontario. Tillite (also called diamictite and mixtite) is made up of sediment that was carried or deposited by glaciers and later cemented to form rock. In general, tillites consist of a fairly fine-grained matix that contains pebbles and larger size pieces of distinctive rock types. Most tillite erratics have a dark green, gray, or grayish brown matrix that contains lighter-colored, fairly angular igneous or metamorphic pebbles. The Ontario source area for tillites is larger than that for the jasper conglomerate, which probably accounts for the greater abundance of Precambrian age tillite erratics in Illinois glacial deposits of Pleistocene age.
Rocks and Minerals of Illinois Table of Contents
The printed version of Guide to Rocks and Minerals in Illinois can be purchased from the Shop ISGS Web site.
Updated 11/11/11 SLD