A long time before the loess was blown skyward, a series of other landscapes existed in the Loess Hills region, landscapes very different from the ones that exist today. The varied histories lie recorded within the fossils and rock layers, that for the most part, are hidden today beneath the Loess Hills, a mute testimony to slowly but constantly changing climates, communities, and landscapes. At least six times, the Loess Hills region was smoothed by advancing glaciers, which as they receded left piles of loose debris to be carved by rain and rivers. A long time before that, warm, carbonate-rich seas cyclically covered the region of the Loess Hills, leaving behind deposits that were compressed into bedrock over still more ancient solid rock.

If you were able to cut through the Loess Hills, deep into the earth, the knife would easily slide through the unconsolidated earth nearly to the base of the hills and then would, with difficulty, saw through several horizontal layers of successively older solid rock. Sooner or later, the knife would dig deep enough to hit Precambrian bedrock one to two billion years old, a part of the same rock system that forms todayÕs Rocky Mountains. However, although the younger layered rocks were later uplifted and then washed away into the western mountain states, they have remained as a cover over much of the Midwest. Thus, the Precambrian rocks can be seen at ground surface in Iowa only in one place, to the far northwestern corner of the state at the Gitchee Manitou State Preserve. They are also evident above ground in central Minnesota, Canada, and Wisconsin.

Lying on top of the ancient rock are layers on top of layers of rocks deposited at successively younger times, each under somewhat different conditions. Fossils embedded within the layers portray life from the earliest beginnings through the evolving complexity of non-marine and marine life. If the lowest of the deeply buried rocks were exposed, it would tell of approximately 500 million years ago or the late Cambrian times, when a shallow inland sea periodically covered the region of the Loess Hills, leaving shoreline, offshore deposits, and inner shelf. Fossils in sandstone and some other Cambrian rocks would reveal that life back then was limited to primitive forms: trilobites, algae, brachiopods, and burrowing wormlike organisms.

During the Ordovician Period, 450 million or so years ago, a warm shallow inland sea once again filled the region. The seaÕs sediments formed a diversity of rock types, reflecting the emergence of an even greater diversity of life. Algae and all the major invertebrate groups including mollusks, corals, echinoderms, such as beautifully delicate crinoid Òsea liliesÓ, and annelid worms, were very abundantly present.

The loess of which the Loess Hills are composed is mostly quartz silt-small-grained particles that are somewhat bigger than clay but not as large as grains of sand, with quartz being the most abundant mineral. The accumulations of loose, unconsolidated, lightweight silt are remarkably homogeneous, commonly lacking rock, gravel, and the horizontal stratifications so typical of soil and rock layers. Even though the loess color below the upper soil layer is often a uniform brownish yellow, the color can range from brown to gray, and some deposits are strongly molted.

Although the loess deposits consist of uniform-sized grains, a microscopic examination of loess from the eastern and western Loess Hills would show some differences in the particle size. Coarse silt, difficult for the wind to hold aloft, first dropped and is the most concentrated in and near the most western bluffs. The concentration of smaller, lighter clay, and fine silt particles increases with the distance from the floodplain.

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