Ancient Places Read Online Free Page B

over.
    Sometimes scaling down to the finer points of the story, all the way to refrigerated basements and cold air wells, can provide insight. And sometimes you have to consider the much grander progression of deep time.
    As dinosaurs roamed the earth during the Cretaceous epoch, around a hundred million years ago, a massive island terrane docked against the western edge of North America and then slowly pulled away, creating a long valley that emerges from British Columbia’s Kootenay Lake, follows a brief run of river across the border into Idaho, then crosses a gentle divide into the Clark Fork–Pend Oreille drainage. This valley is known to geologists as the Purcell Trench. During the Miocene epoch, more than twenty million years ago, a river flowed through the trench, cutting a meandering course through less resistant rock exposures and fault zones.
    Much more recently, at the end of the Pleistocene, the Purcell Lobe of the Cordilleran Ice Sheet moved down the pathway of the Purcell Trench, filling the ancestral valley occupied today by Lake Pend Oreille. When this frozen river pushed through the basin, it collided with Green Monarch Ridge on the eastern rim. Forged from some of the oldest rocks on the continent—the ancient Belt bedrock of the Mesoproterozoicera—Green Monarch provided a solid terminal buttress for the ice. As the glacier continued to advance, more and more ice piled up behind the ridge, gradually thickening into the dam that created the first of a series of Glacial Lake Missoulas.
    Geologists who have studied the dams that developed at this site maintain that whenever the depth of Lake Missoula approached 2,000 feet, the water’s sheer weight began to compromise the ice cleaving to the base of Green Monarch Ridge. Small cracks began to appear, and streams of water flowed into the cracks, boring tunnels beneath the ice plug. The combined forces from the weight of the dam and the volume of Lake Missoula pressurized the water flowing into these tunnels so that the pathways enlarged very quickly. Jets of water churning along Green Monarch’s solid wall undermined the dam and caused its sudden failure. (YouTube videos monitoring the removal of modern concrete dams attest to the power of this process.)
    After the last glacier retreated, Lake Pend Oreille remained. Today it is recognized as the largest and deepest body of water in the Idaho Panhandle. Its natural surface level lies about 2,050 feet above sea level; the ice-carved mountains that surround it reach to 6,000 feet and more. Its waters plunge as much as 1,150 feet deep along its southern reach. During his investigations around the lake, one question that intrigued Roy Breckinridge was whether such great depth was the result of the grinding ice sheet or the repeated slashing floods. Although the carving power of glaciers is well documented,some geologists contended that the pressurized water shooting from the ice dams would have eaten away enough bedrock to significantly deepen Lake Pend Oreille.
    Breckinridge believed that the answer might lie on the bottom of the lake. He knew that the most dynamic part of a glacier is its forward toe, which makes both first and last contactwith raw ground. That is where the ice’s bulldozing power performs its most drastic razing of the landscape. Since the ice lobe that filled the Pend Oreille basin would have repeatedly gouged its southern edge, and since the main discharges of Glacial Lake Missoula would have been ripped through that same area, Breckenridge and his team were drawn to the lake’s southern arm. The sediments there, they reasoned, might well show the difference between what had been sculpted by ice and what had been eroded by floods.
    By chance, during World War II the US Navy had established a training base along the southwestern edge of the lake, exactly where the floodwaters had once poured out. Although this station was decommissioned in 1946, the Navy understood the advantages of