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<br />17: <br />-- <br />......: <br />~ <br />- <br />c <br />~' <br /> <br />. <br /> <br />. <br /> <br />15 <br /> <br />.. It if, in the opinion of this investigator, probably the <br />bt~t tXBmp1e in Colorado (and perhaps all the West) of a canyon caused <br />by ~uperpo~ition of a river (See page 4). To be sure, there are many <br />other eXMmple~ of this phenomenon, including nearby Lodore Canyon; but <br />thi~ one is such a good example because it is so compact and so obvious. <br />The structural relief of the anticline through which the river has cut <br />is relatively great, but the canyon is short (less than three miles long), <br />with broad floodplains underlain by young rocks at either end of the canyon. <br />b. The canyon contains one of the two eastern-most exposures <br />of Precambrian sedimentary rocks in Colorado. (The other is along the <br />IOYtheast side of Juniper Mountain.) This is significant because these <br /> <br />rocks contain a rare record of events which happened in this area approxi- <br /> <br />matoly 1200 million years ago. It is also significant that many of these <br />.l.yers of Precambrian rock in Cross Mountain Canyon are among the oldest <br />.known terrestrial red beds - sediments with a red color due to the presence <br />of hematite (iron oxide). The formation of hematite on land depends on the <br /> <br />prosence of excess oxygen in the earth's atmosphere to oxidize the iron. <br />.. Sfnce terresti'hl red beds older than 1200 million years are not known, it <br />fs assumed that excess oxygen was not present in the atmosphere until about <br />that t1me. Thus, rocks l1ke those of the U1nta Mountain Group are important <br />'clues to the evolut10n of the earth's atmosphere. <br /> <br />c. 'At the eastern end of the canyon, near river level, are ex- <br /> <br />.~..eell Int exposures of the "Great Unconformi ty." , This is the surface of <br /> <br />contact between 1200 million-year-old Precambrian strata and the 550 mil- <br /> <br />11on-year-old Cambr1an Lodore Formation. Thus, this unconformity repre- <br /> <br />sents a gap of approximately 650 million years in the geologic history of <br /> <br />this area - a time when there may have been extensive uplift and erosion. <br /> <br />... <br /> <br />~-~ <br />