Ancient Places (4 page)

Even though the basics of this story have been available for almost a century, the devil, as Roy Breckenridge continually points out, is in the details. He and his colleagues study the
events that took place around Lake Pend Oreille and the lower Clark Fork River at the end of the Pleistocene, trying to figure out how the Cordilleran Ice Sheet formed a wall that was tall enough and strong enough to back up an enormous Glacial Lake Missoula. They want to know how such a massive dam could have failed so catastrophically, and exactly what happened when three thousand square miles of deep water roared away. They are curious as to how the ice then re-formed to replay the same scene, with subtle variations, many times 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 Mesoproterozoic
era—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 contact
with 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 retaining a secluded site with quiet deep water and, over the years, it developed a research unit there that regularly performs acoustic experiments with small ships and submarines.

Breckenridge learned that naval technicians had constructed a sonic profile of the lake bathymetry by towing acoustic sources through the water, broadcast at different levels and captured by hydrophone receivers. Although the military project focused on the lake bottom’s shallowest sediments, Breckenridge guessed that their comprehensive data might also have something to say about the bedrock below them.

The Navy’s seismic data for the lake’s southern arm showed gradual slopes dropping off the east and west shores, but the east side alone was marked by a stark subsurface bench running directly down the lake from Green Monarch Ridge. This underwater bench, as well as a similar one still visible above lake level today, can be interpreted as the result of the tunneling jets of water that disintegrated the ice dam.

The naval data also revealed distinct stratigraphic units of debris on the lake bottom. Breckenridge believes that the deepest and thickest of these layers correspond to sediments left behind after the most recent ice-dam failure. Beneath that debris, the sonar outlined a classic U-shaped bedrock basin with a nearly flat center—the shape that defines glacier-carved valleys all over the world. Furthermore, the actual bottom of the lake was much deeper than he had anticipated. Along its south arm, the lake level of 2,050 feet, combined with a water depth of 1,150 feet, means that the top sediment layer lies 900 feet above sea level. Naval bathymetry showed that the depth of the bedrock basin approaches an astonishing 700 feet
below
sea level. This means that 1,600 feet of sediments rest below the water. In other words, the sediments themselves are much deeper than the deepest water in the lake.

While the hydrologic forces of high-pressure tunneling beneath an ice mass can reach impressive speeds, no model or study has shown that they can generate enough power to carve bedrock to any great extent below sea level. Breckenridge determined that the bottom of Lake Pend Oreille is actually an overdeepened glaciated basin that has been refilled with the debris of numerous Ice Age floods. It was created in much the same way as well-studied elongate lake valleys in British Columbia, glacier-carved valleys in the Alps, and coastal fjords in Norway. “So,” concluded Breckinridge, “those ice caves, which Largé thought were created by the glaciers, were in fact formed by the big floods. And the bottom of Lake Pend Oreille, which many geologists believed must have been dredged by the floods, was actually carved by ice.”

After the last of the Lake Missoula floods passed down the Columbia to the sea, vegetation sprouted on the massive gravel dumps and sand bars left in its wake. Mammoths and camels returned to browse their former haunts, and anadromous fish swam back upstream to spawn. People were there with them, spreading back across the landscape as conditions allowed.

The first written descriptions and accurate maps of this reclaimed world were forged by fur agent David Thompson. When Thompson surveyed his way from the Rockies to the Pacific between 1807 and 1812, he carefully plotted each new drainage that he entered, taking countless sextant shots and compass bearings. One of his maps depicts the interlaced rivulets formed by the Clark Fork River’s delta at the northeast corner of Lake Pend Oreille—the site of the ice dam that impounded Glacial Lake Missoula. Mountain ridges crawl across the landscape, marking the drainage divides for all the surrounding rivers and outlining the limits of Lake Missoula’s expanse upstream along the Clark Fork to Missoula and south through the Bitterroot Valley, as well as east and north up the Flathead River through the Mission Valley to Flathead Lake.

Thompson often traveled with tribal guides, who showed him the most efficient way to get from one place to the next. As he trekked across the region multiple times, he observed how local people circled through their known world to gather essential resources. Because of Thompson’s close attention, it is possible to trace the ways in which the Plateau peoples of the early nineteenth century fit into the landscape shaped by the Ice Age floods.

When paddling on Lake Pend Oreille near modern Sandpoint, Idaho, Thompson set his course—undoubtedly with the help of his Kalispel guide—by a distinctive rock that marks the narrowing of the river as it leaves the lake. This knoll, now known as Tank Hill, was stripped bare by the Ice Age floods, creating a landmark that served as a beacon for travelers on the lake and on land. The roiling waters dropped a pendant bar of gravel on Tank Hill’s downstream side; this lode of handy aggregate has served as a commercial pit for many years, providing the road gravel that transformed many of the tribal trails that Thompson traveled into modern highways.

When the fur agent journeyed southwest from the Pend Oreille to the Spokane drainage, his guides directed him to a trail that followed the path of the deluge as it burst from Lake Pend Oreille’s southern basin and overran a low divide, where it dropped sediments on a plain known today as Rathdrum Prairie. Bars of flood gravel there created several small lakes that appear on
Thompson’s maps. He recorded how water flowing from one of these pocket lakes, instead of following the expected stream course to the Spokane River, “disappears” into hundreds of feet of porous flood deposits.

In the spring of 1812, Thompson led a horse brigade along the north bank of the Spokane River. Where the river bent south to crash through a series of formidable falls that today mark the city’s center, he and his voyageurs followed a tribal trail west across grasslands spotted with well-spaced ponderosa pines. The brigade was heading for an ancient fishing village nine miles downstream from the falls, where a handful of Thompson’s men had built the Spokane House fur trading post two seasons before. In a typically terse daybook entry along the way, he marked his course by
“a range of Knowls to our Right.”

A knoll to Thompson meant a distinct feature, often rocky, and the one he sighted as he cruised through what is now north Spokane was a distinct circular mesa, fairly flat on top, that had been carved and recarved by successive Ice Age floods. Thompson took advantage of the open parkland around the southern edge of the knoll to trot his horses straight through to Spokane House. From that outpost, more dotted lines on his large map trace tribal trails through flood coulees of the Cheney-Palouse scablands all the way south to the Snake River.

The coarse gravels that form the beds of the Spokane and Little Spokane Rivers, dropped there by subtle flow changes in the last of the Lake Missoula floods, cover a range of sizes. For untold generations, several species of trout, as well as steelhead and salmon, thrashed their tails in these gravels to form redds for laying eggs. The fish nourished local people and visitors from far away—including David Thompson, who upon arriving at Spokane House in spring 1812 found
“all well, they have these 2 days caught many Trout.”

For the past thirteen thousand years, features carved by the Ice Age floods have shaped the way people live in and move across a large swath of the Inland Northwest landscape. The deluge may read like a signature origin myth to someone from the outside, but for anyone who travels along the many paths gouged by ice or swept clear by rushing water, each detail of the story points to a practical reality: from Mesoproterozoic time to the Pleistocene, from the sweep of open landscape to the range of a single plant species, from the frenzy of spawning fish to the continuous trickle of new people who have filtered into the region since the floods receded, from the breadth of Grand Coulee to a cool cavern just wide enough to slide into, and just right for storing food.

This is a story about a rock that flew. One thread of the tale begins in Oregon with a pioneer farmer named
Ellis Hughes, who worked a small parcel of pleasantly rolling land southeast of Portland between the hamlets of West Linn and Willamette, near the border of present-day Clackamas County. On a November day in 1902, Hughes was walking home from cutting firewood for the Willamette grade school when he spotted part of a rusty crosscut saw blade about fifty feet off his path. No one wasted valuable steel in those days, so he ventured into the woods for a closer look. There he found the saw piece resting against a large “metallic-looking rock protruding above the ground.” Nestled within a grove of recently cut stumps, the greater portion of the
mass was buried in the earth. A thicket of hazel bushes helped to mask its presence from the trail.