http://www.museum.state.il.us/gallery2/main.php?g2_itemId=843 en-us Gallery 2 RSS Module, version 1.1.0 Wed, 19 Jun 2013 08:29:15 -0500 120 http://www.museum.state.il.us/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=847 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=843 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=880 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=880
varved clay sedimentary LaSalle County, Illinois Varves are alternating layers of fine silt and clay that form in lakes along the edges of glaciers. The dark layers are clay and organic matter that settle out beneath ice-covered lakes during winter. The lighter, thicker layers are silt, deposited during spring and summer when the lakes are ice-free. Geologists correlate these annual layers from one lake to another to measure the pace of retreating glaciers.]]> photo Fri, 16 Sep 2005 14:54:59 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=877 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=877
tillite sedimentary central Illinois This rock formed in Canada 595 million years ago. Thick deposits of Precambrian glacial till (a mixture of clay, silt, sand, gravel, and boulders dumped on the landscape by retreating glaciers) were fused into a hard rock called tillite. Between 10,000 and 70,000 years ago, chunks of tillite were brought to Illinois by glaciers.]]> photo Fri, 16 Sep 2005 14:54:59 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=874 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=874
stromatolite sedimentary Cambrian Saratoga Springs, New York These layered formations are called stromatolites. In the ocean, sticky mats of bacteria and photosynthetic cyanobacteria trapped minerals and marine sediments. Bacteria continued to grow through the sediment, adding new layers. Sometimes fossils of the bacteria are preserved in the layers. Stromatolites are still forming today in water too salty for snails and other grazing animals that would eat the bacteria.]]> photo Fri, 16 Sep 2005 14:54:59 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=871 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=871
shale sedimentary with trace fossils Milan, Illinois Upland plants of the Pennsylvanian Period were preserved in the mud of a stream bottom that, over time, turned into shale.]]> photo Fri, 16 Sep 2005 14:55:46 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=868 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=868
petrified wood fossil southeastern California This fossil was a tree buried by volcanic ash. Groundwater flowed through the silica-rich ash and slowly replaced the original wood with a variety of silica called jasper. Although no actual wood remains, the silica has faithfully preserved the details of its structure, including its annual tree rings.]]> photo Fri, 16 Sep 2005 14:55:46 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=864 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=864
opalized wood fossil Utah Sometimes wood is replaced with a form of silica called opal. The preservation is so perfect that the cellular structure of the original wood is clearly visible under a microscope.]]> photo Fri, 16 Sep 2005 14:55:46 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=861 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=861
nautiloid cephalopod fossil Orietites brooki]]> photo Fri, 16 Sep 2005 14:55:46 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=858 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=858
marble metamorphic Campan, France This rock started as limestone formed from the remains of animals that died and fell to the sea floor. Later, magma heated and squeezed the limestone, causing it to recrystallize and form marble. This specimen was cut and polished.]]> photo Fri, 16 Sep 2005 14:55:46 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=854 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=854
Orbicular grantie igneous Sweden Orbicules (spherical crystal structures) sometimes occur in granite. Concentric bands of black hornblende and white feldspar show that the structures were undergoing crystallization while the rock was still molten.]]> photo Fri, 16 Sep 2005 14:55:46 -0500 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=851 http://www.museum.state.il.us/gallery2/main.php?g2_itemId=851
devonian corals fossils Thedford, Ontario These are the fossilized skeletons of tiny animals (Heliophyllum halli) that lived over 360 million years ago. As corals grow, they secrete thin layers of calcium carbonate on their skeleton. This leaves ridges that mark annual growth and finer lines that record monthly and daily growth. Studying these lines, we discover that the Devonian year was 400 days long and divided into 13 lunar months.]]> photo Fri, 16 Sep 2005 14:54:25 -0500