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MailTribune.com
  • Table Rocks: 7 million years in the making

  • Editor's Note: Oregon Outdoors kicks off its newest feature today, "The Rocks Speak," a column by local geologist Jad D'Allura aimed at shedding light on the ground beneath your feet.
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  • Editor's Note: Oregon Outdoors kicks off its newest feature today, "The Rocks Speak," a column by local geologist Jad D'Allura aimed at shedding light on the ground beneath your feet.
    Two of the most notable features of our valley are the flat-topped mesas of the Table Rocks. To understand their formation, we have to return to a time 7 million years ago and a place, Olson Mountain, south of Lost Creek Reservoir.
    Fluid lava, andesite (named after analogous rocks in the Andes Mountains), erupted from a shield volcano (similar but steeper than Mauna Loa, Hawaii) and flowed 44 miles down the ancestral Rogue River. Great clouds of steam — and resultant mega-fish fry — would have been seen for miles.
    The easternmost extent of that flow is Eagle Rock just west of Lower Table Rock. As the Rogue River cut to a deeper level, thin flat ledges of lava, like marooned sailors, remain along the canyon above river level.
    Solidified lava, being hard, forced the Rogue River to erode the softer rock on either side as it cut new channels. In the northern part of the Rogue Valley where the Rogue River encountered even weaker rocks, it cut a wide floodplain while carrying rock and soil to the Pacific Ocean.
    In the process, the Table Rocks were isolated about 600 feet above the valley floor to produce "inverted topography." After 7 million years of erosion, what was once low (lava in the river bottom) is now high. There's a lesson in humility somewhere in there.
    The most diagnostic features of the flow can be seen from afar. They're vertical cracks called columnar joints. These polygonal cracks formed after the lava ponded, cooling and contracting from the top and bottom.
    Along the trail above the parking area to Upper Table Rock, discover small sandy cuts that expose the easily eroded, pale-orange sandstone of the ancient Payne Cliffs Formation. Keen eyes will detect a loose, cobbly deposit, small remnants of the 7-million-year-old Rogue River, on the trail below the lava.
    Once at the top, walk to the southwest edge to view huge blocks detaching from the edge of the Rock to either slip slowly downward or tumble into debris as softer rock.
    Take advantage of BLM's guided hikes during the spring that explain cultural and scientific wonders. Stay on the trail to avoid the Mother of All Poison Oak. Most of all, enjoy.
    Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net
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