STRATIGRAPHIC COLUMN OF IOWA 2017 Age Maximum Age Maximum (million years thickness (million years thickness before present) System Rocks Rock-unit names (ft) before present) System Rocks Rock-unit names (ft) 0 359 Holocene units “Maple Mill” (DeForest Fm.) 100 Holo- cene (Grassy Creek - 300 0.01 Saverton) Dows Peoria 60/200 alluvium 100 0.02 Upper Lime Creek Fm. 300 Glasford Loveland 95 383 Cedar Valley Group 400 0.6 Wolf Creek & DEVONIAN Pleistocene Alburnett fms. (“A & B” tills), 400 unnamed alluv. Middle Wapsipinicon Group 180 393 QUATERNARY 1.3 L unnamed 100 419 1.7 U “C-tills” & 80 433 Gower Fm. 150 unnamed alluv. 2.6 Scotch Grove Fm. 300 Plio- cene Unnamed 5 (“salt & pepper”) 200 Hopkinton Fm. 13 Lower 160 NEO- Mio- cene GENE SILURIAN Blanding - Mosalem 23 fms. 160 PALEO- not present in Iowa GENE 444 66 Maquoketa Fm. 350 74 Manson Impact 10,000 Upper 458 Galena Group 290 83 Niobrara Fm. 50 80 Upper Platteville Fm. 91 St. Peter - Glenwood Fort Benton Group 265 450 Middle fms. 95 470 Dakota and Windrow 500 ORDOVICIAN Fm. Prairie du Chien Group 650 Lower 102 CRETACEOUS 485 Jordan Sandstone 130 Lower 145 St. Lawrence Fm. 240 Lone Rock Fm. 180 160 Fort Dodge Fm. 75 g Upper Wonewoc Fm. 150 JUR- g Eau Claire Fm. 250 201 ASSIC 505 TRI- Mt. Simon Sandstone 1400 CAMBRIAN ASSIC L-M 252 not present in Iowa 541 PERM- unnamed 800 IAN 900 299 “red 25,000 Keweenawan clastics” Wabaunsee Group 260 1050 Supergroup volcanics 40,000 Shawnee Group 185 1100 1280 mafic intrusives Douglas & Upper Lansing groups 150 granites, Kansas City Group 150 1480 rhyolites, metamorphics Bronson Group 120 307 1640 500 PROTEROZOIC Sioux Quartzite Marmaton Group 200 1760 granites, 1800 PRECAMBRIAN gneisses, Middle Cherokee Group 700 PENNSYLVANIAN metavolcanics 315 2100 Caseyville Fm. 85 2500 Lower 323 gneisses, mafics, iron formation Pella Fm. 75 2910 ARCHEAN Upper 331 ages from GSA Geologic Time Scale v 4.0 “St. Louis” Fm. 160 KEY 340 sandstone, igneous- sand metamorphic Warsaw - Keokuk fms. 200 Middle siltstone, silt gypsum, anhydrite g Burling- 140 Gilmore ton Fm. shale, coal City Fm. mudstone 160 chert limestone MISSISSIPPIAN Maynes Creek Fm. 150 oolite Lower North Hill Gp. 120 dolomite g glauconite 359 diamicton breccia (glacial till) 300 Trowbridge Hall, Iowa City, Iowa 52242 319-335-1575 www.iowageologicalsurvey.org .