Rolfe, D: Rock River Country of Northern Illinois (Classic R

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Rolfe, D: Rock River Country of Northern Illinois (Classic R

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O knihe

Excerpt from The Rock River Country of Northern Illinois

These materials were deposited very largely under water - in widespread seas which during much Of Paleo zoic time (see table on page 29) submerged all of northern Illinois. Sometimes this sea was fairly deep; at other times its waters were so Shallow that they barely covered the surface or parts of it.

The clay and sand came from other, older rocks in the highlands of Canada - rocks which through the ages had been worn down and decomposed or broken into tiny bits, being carried into the sea by streams or blown in by winds, and Often sorted by waves and currents.

Lime came from older rocks, too; but because it was so easily dissolved by percolating waters, it entered the sea in solution. At periods when the waters Of the sea were clear, and other conditions were favorable, there was much plant and animal life in the sea. There were times when its waters fairly teemed with little animals which secreted shells or skeletons from the lime and other mineral matter dissolved in the sea water. As these animals died, their hard parts were deposited as sediment on the bottom Of the sea. Although most of the Shells and skeletons were broken to bits or ground to powder by the action Of the waves and currents, great numbers of them escaped destruction, and still remain imbedded as fossils in the rock, recording the character of the life in those ancient seas.

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