A Legacy of Fire and Water EarthCache
A Legacy of Fire and Water
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When you drive through the Madras region, you see a gently rolling plain dotted with low hills and flat-topped buttes. Since the plain appears to extend uninterruptedly to the foothills of the Cascades, little would you imagine that between you and the mountains lurk three awesome canyons, totally invisible until you nearly reach the brink.
Three rivers--the Deschutes, the Crooked, and the Metolius--have cut these gashes into the plain and have laid open for observation a sequence of remarkable geologic events.
Beginning in the Pliocene Epoch, approximately 10-12 million years ago, numerous lava flows were forming the base for the High Cascades and westward flowing streams were forced to seek north or south courses to reach the sea, thus the ancestral Deschutes River and it's tributaries became established. While the Cascades were building, earth movements were causing a sinking of the land suface, forming the Deschutes basin. The first materials to fill this basin were from eroded sediments, airborne volcanic materials, and ash flows. Ash flows occur when gas charged magma erupts rapidly, up to 100 mph, from a volcanic vent. The mixture of ash, pumice fragments and gas tends to fuse from retained heat so completely that it resembles a lava flow. This layer is called the Dalles Formation, and is typically thick , brown to dark gray sandstone and conglomerate. The top or youngest layer is light gray pumiceous sandstone. Numerous lava flows are interbedded in the Dalles Formation from vents within the basin. These can recognized in the canyon walls as more resistant dark brown layers with prominent columnar jointing.
At the end of the Pliocene Epoch or beginning of the Pleistocene Epoch, approximately 2-3 million years ago, the local volcanic vents erupted basltic lava, capping over the Dalles sediments. These lavas are called the Rimrock Basalt, since they characteristically outline the edges of the canyon. This layer is thin to thick gray olivine basalt. At this time the land had started to rise. Uplift caused the rivers to run faster and to erode deep channels. The Deschutes, Crooked and Metolius Rivers cut rapidly into the soft layers of the Dalles Formation. The steep sided canyons were gouged nearly to their present depth by the late Pleistocene time.
About 50,000 years ago, surges of red-hot basaltic lava poured into the upper canyon of the Crooked River. By the time it reached the narrow canyon of the Deschutes, about 4 miles below the mouth of the Metolius, it was beginning to cool and form a dam. A great pool of lava rose and flooded the canyon to within 200' of the rims. The erosive force of water is not easily thwarted, and after the lava cooled, the rivers proceeded to wear it away, again reaching it's original depth. This layer is called the Intra-Canyon Basalt Layer. You can see areas where remnants of Intra-Canyon lava lie against the walls of the former canyon. The brownish-black basalt is in sharp contrast to the lighter colored Dalles beds. The island is also a remant of the Intra-Canyon lava, the sheer basalt cliffs rising 450' above the lake give an idea as to the tremendous volume of lava that once filled the valleys and the powerful forces of erosion that have cut steadily through it.
To log this EarthCache, please answer the following questions and send them to us in an email. Do not post them with your log.
For bonus points, send us a picture of you or your GPS in this beautiful place.
1) Why does one side of the canyon look different than the other?
2) How many different layers can you see across the river?
3) What color is the water in the lake?
4) What does the sign say at the coordinates?
Please do not post a picture showing the sign.
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