Modern T abular Lobed Coral 50 VISIT OR GUIDE Oil Shale The fne laminations in this oil shale from the Eocene Green Ri ver F orma tion probably represent annual sets of seasonal layering of fne and fner grained sediment deposited at the bot tom of a large inland lak e Oil shales do not produce oil directly Instead they contain k ero gen a solid material that produces oil when the shales are distilled under extreme heat Moder n Br anc hed Cor al and Moder n T abular Lobed Cor al Coral is commonly found in w arm seas and has been abundant in the fossil record since the Ordo vician P eriod about 50 0 million year s ago Corals produce external sk eletons of calcium carbonate They grow as solitary indi viduals or in colonies Calcit eCement ed Sand Crystals This group of crystals from S outh Dakota contains more sand grains than calcite The crystals tak e their shape from crystalline cal cite that cements the sand grains together Gift from the Geolog y Museum Colorado School of Mines Sandst one The shape of this sandstone specimen from the Upper Cretaceous F rontier F orma tion Shirley Basin W yoming formed when it brok e along fractures flled with white calcite cement The sandstone is about 90 million year s old It w as deposited in a shallow marine set ting in southcentral W yoming After you have viewed the specimens in the case you will see two exhibits about three quarters of the way down the hallway in front of you on the right and lefthand walls The frst on your right Ripple Marks was formed in Late Cambrian Potsdam Sandstone found in Keeseville New Y ork Ripple Mar ks Ripples form in sand that is mo ved by wind w aves or w ater cur rents Ripples that are steeper on one side than the other are typically formed by directional cur rents lik e those found in streams Ripples that slope evenly from eac h side of their crests are usually formed by w ave action where w ater mo ves bac k and for th across the sand The broad rusty bands that cross these ripples are iron o xide stains that precipitated from moisture between the layer s of Late Cam brian P otsdam S andstone The roc k layer with the mir ror image of these ripples is on display at the National Museum of Natural History in W ashington DC Collected 4 December 1 886 b y Charles Doolittle W alcott Gift from the National Museum of Natural History Smithsonian Institution through George Switzer and Harold H Banks Jr The exhibit on the lefthand wall Intertidal Sand Bodies is directly opposite