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<br />DDT and other high potency pesticides brought the codling moth and other pests under <br />control. Today, spring frosts are the primary factor limiting fruit production in the Grand <br />Valley. Record cold temperatures in the winter of 1962-63 killed more than. 100,000 peach <br />trees, and in the spring of 1989 a severe frost caused the most complete bud kill in the <br />valley's history. Wind machines that mix in warmer air from higher elevations now are <br />common in the peach orchards, replacing smudge pots used in the past. <br /> <br />IV. <br /> <br />Imagine an inland sea covering at times much of the continental land mass of what is <br />now western Colorado, a sea coming in from the north and, at one period. extending all the <br />way to what is now the Gulf of Mexico. Such was the state of the Earth during a period <br />geologists call the Cretaceous, approximately _ million years ago. The Mancos shale that is <br />the product of this period underlies the entire Grand Valley, outcropping in the Book Cliffs <br />that form a distinctive northeast boundary for the valley. The sandy shores of this sea are <br />now the Dakota Sandstone formation, and the Mancos shales are remnants of "the shells and <br />skeletons of ionumerable marine animals: coiled ammonites, giant oysters, clams, and <br />swimming reptiles. ,,21 This area is the easternmost extension of the Colorado Plateau, with <br />its uplifted sedimentary layers still remarkably horizontal though deeply carved by water. <br />Somehow this plateau escaped the mountain building processes that occurred in the Rockies to <br />the east and the Sierra Nevadas on the west. <br />As already discussed. the salinity of these shales created problems with growing crops <br />in parts of the Grand Valley around the turn of the century, problems that were largely <br />addressed by the construction of a substantial drainage system. In effect, however, the <br />problem was just transferred downstream. There are many sources of salinity feeding into lhe <br />Colorado River: nearly half of the salts found in the river at Hoover Dam are thought to come <br /> <br />%I Halka Chronic, Roadside Oeolo"" of Colorado (Missoula: Mountain Press Publishing Co. 1980) at 256. <br /> <br />10 <br />