The Emerald Horizon: The History of Nature in Iowa

1. Setting the Stage

1 SettingtheStage Iowa’s broad acres, “fair as a garden of the lord,” spread from river to river before a human foot had pressed the surface of the planet. . . . Seasons rolled by the same as now. . . . The struggle for life . . . was here yearly carried to the final issue. Man at last joined the struggle, and long before there were any historians, events of historical importance were enacted within the limits of Iowa. —Samuel Calvin, 1893 We think that we walk on solid ground, that the earth under our feet is inviolable. But this rectangle of land we today call Iowa has always been restless turf. Only the rate of its ceaseless transformation has changed with time. While in past millennia the landscape was modified at glacial speeds, now the character of our planet is transformed at ever-faster tempos, speeded onward by technological innovations and the cravings of growing numbers of people. Today the speed and magnitude of Earth’s ongoing transformation have reduced the ability of nature’s systems to function with integrity. And we search for methods of regaining the environmental stability that once was afforded as a matterofcourse.Toplotfuturedirectionswithwisdomandcoherence,wemust try to understand our landscape’s genesis and historic capabilities—commencing with the creation of the land we now inhabit. A Brief History of Land and Life Iowa’s origins might be traced to the birth of our planet around 4.5 billion years ago, when Earth coalesced from dust and gasses swirling in the solar nebula. Geologists believe that the major continental crusts emerged about 600 to 700 million years later, when lighter components of the molten mass cooled and solidified . Soon thereafter, these continental shields began their ceaseless wander of the planet’s surface, migrating as huge discrete slabs of rigid rocks (tectonic plates) that slid over deeper, more pliable layers. Time and again the shields mergedtoformasinglelandmass,onlytobreakintolargepiecesthatsometimes Setting the Stage | 3 reconfigured the borders of the tectonic plates. During its wanderings, our embryonic state spent considerable time near or south of the equator. By the Cenozoic Era, or Age of Mammals, which began about 65 million years ago, Iowa had reached its modern location on the planet and nestled into theheartoftheNorthAmericantectonicplate.Periodicallyadvancingseasthat hadflushedthemidcontinentforabouthalfabillionyears,leavingbehindlayers of limestone and other sedimentary rock thousands of feet thick, were giving way to dry ground that was reshaped by eons of weathering and erosion. Simple life-forms that first appeared perhaps 3.5 billion years ago had evolved into diverse mammals and flowering plants. North America’s climate was changing, in part because the rising Rocky Mountains were drying midcontinental air by blocking the flow of moist Pacific air. The stage was set for the appearance of North America’s drought-adapted midcontinental grasslands. Grasses developed and spread across the dry interiors of most continents during the Miocene, becoming firmly established over vast tracts of mid North America by five to seven million years ago or even earlier. In northeastern Nebraska , the discovery of large concentrations of animal and grass seed fossils reveal that savanna-like grasslands covered the region about twelve million years ago. Communities were likely very similar to those of today, although species differed. The expansion of the world’s grasslands was accompanied by the development of fast-running grazers and small burrowing animals. In North America, three-toed and one-toed horses, llamas, camels, mastodons, and rhinoceroses were among the common mammals, with fossils of all of these found in Iowa (Ashfall Fossil Beds State Historical Park 2006; Risser et al. 1981: 26, 31; Holmes Semken, personal communication). About 2.5 million years ago, a dramatic change in course was signaled: the climate became cooler and wetter, and the amount of snow falling surpassed the amountmeltingeachyear.Thisshifttriggeredperiodicmassiveglacialadvances. Continental ice sheets thousands of feet thick usurped enormous amounts of Earth’s water, lowering coastline sea levels by 300 to 400 feet. Glaciers crept southward across Iowa and into northern Missouri many times between 2.5 million and 500,000 years ago. Later glacial advances were less extensive in Iowa. Around 150,000 years ago, the leading edge of a continental ice sheet nudged its way from the east into southeastern Iowa. More recently, a broad tongue of advancing ice (the Des Moines Lobe) lapped southward into north-central Iowa, advancing to today’s city of Des Moines. This occurred several times, the most 4 | Setting the Stage recent such advance being around 15,000...