In Washington state there are relatively few windows into the deepest reaches of geologic time and the oldest are in the easternmost quadrant of the state, including the Spokane-area. One of the oldest is the so-called Willow Lake Aureole near Four Lakes, just west of Spokane. About 50 million years ago a balloon of magma from deep in the crust intruded upon a 1.45 billion year old layer of the ancient Belt Basin (the Wallace Formation). The relatively recent (15,000 to 17,000 years ago) Ice Age floods blasted away a covering layer of young basalt, helping to expose the much older rock. What’s revealed is a dynamic piece of natural art–frozen in time–with the magma (now granite) swirling amongst greenish chunks of the ancient rock as though the Wallace rocks are green raisins in a pudding of granite and sparkling black amphibolite.The intricately weathered “Granite Dells” in Yavapai County, Arizona, are among the oldest rocks (~1.4 billion years in age) in the American Southwest.In dramatic display in nearly every direction in Sedona, AZ, the Schnebly Hill sandstone is at least as red as the Chugwater sandstone to the north. At 279 million years old, the Schnebly dates back to the late Paleozoic Epoch, when fishes began to emerge. The Schnebly is pinstriped with narrow layers of Fort Apache limestone. This photo was taken in southeast Sedona, looking south.Consisting of volcanic rocks that pre-date the sprawling Columbia River Basalt Group, Cathedral Rock is the most striking feature of the Sheep Rock unit of the John Day Fossil Beds. The prominent aqua-blue stripe near the top, and bluish hue below, is created by celadonite, a mineral that forms in alkaline, volcanic ash beds when the beds are intruded by pressurized hot water.The bedrock for much of Washington and Oregon, east of the Cascades, is basalt–the remains of lava that spewed into vast sheets covering more than 80,000 square miles. The vast bulk of the flows occurred between 14 and 17 million years ago. When liquid basalt comes into contact with water it often changes shape and color. Sometimes it cools into a sharp, glass-like material known as palagonite that occasionally includes volcanic glass. The Deep Creek gorge west of Spokane has some of the most colorful palagonite in the region, in Grande Ronde basalt that overwhelmed much of eastern Washington more than 16 million years ago.Near the Medicine Wheel on the west shoulder of the Bighorns are stunning outcrops of Bighorn Dolomite, like this one, formed from the carbonate skeletons of marine life in ancient seas–a half billion years ago. The outcrop is nearly 10,000 above sea level. It’s worth pondering, even if it makes you light-headed.My first glimpse at the fortress-like spires near Antelope, Oregon, came late in the day on September 23, 2021. It looked and felt like I’d just landed on an alien planet. This is the Clarno unit of the John Day Fossil beds. The Clarno is home to the oldest rocks in the fossil beds–the solidified remains of volcanic mudflows–known as lahars–that inundated what were then tropical forests, between 40 and 54 million years ago. With a few very rare exceptions the oldest rock within an hour of Spokane is the Prichard Formation. It is the basement of the so-called Belt Supergroup, which includes the aforementioned Wallace Formation. The Prichard is typically dull silt and mudstone (argillite) but it really lights up under stress. What you see in this photo is a closeup of Hauser Lake Gneiss. Gneiss (pronounced “nice”) is a metamorphic rock created when sedimentary or igneous rock is subjected to stone-bending heat and pressure. And that is what happened to a large segment of the Prichard when it was heavily folded during a period of intense faulting south of Mt. Spokane. This was during the Eocene epoch, some 50 million years ago. In other words, you need the ~1.5 billion year old Prichard rock to metamorphose and create the ostensibly younger gneiss. Hauser Lake Gneiss, often streaked with amphibolite, is a dazzling rock, and here’s a close-up of it, all stressed out, in a road cut along the east shore of Hauser Lake in the Idaho panhandle. There are also beautiful outcrops of the gneiss at the Saltese uplands, a popular hiking, biking and jogging venue just west of Liberty Lake, WA, near the Idaho border. Just look down, it’s impossible to miss.A wider view of that signature ridge in the Painted Hills unit of the John Day Fossil beds.Outcrop of Cambrian quartzite (~500 million years old) near the peak of Steptoe Butte in Whitman County, with the Palouse wind farm in the distance.Closeup of Hat Rock, a landmark along the Columbia River northeast of Hermiston, OR. Hat Rock shows up in the Journals of Lewis & Clark. What I like about it (apart from its fez-like shape) are the thin and fluted columns about the thickness of a baseball bat. This is a characteristic of the Pomona basalt flow, one of the youngest (11 million years) and rarest of the Columbia River Basalt Group flows. Why? I have no idea.This a breath-taking view of the core of Montana’s Garnet Range, east of Missoula. The rocks in the face are some of the youngest in the Belt Supergroup sequence, although that still places them at more than a billion years old. The outcrop is visible from I-90 in the Bearmouth area along the Clark Fork River.Blades of Grande Ronde basalt at the northern tip of Rock Lake, one of the great gouges from the ice age floods.Not far from Cathedral Rock at the John Day Fossil beds is the sprawling Turtle Cove complex of aqua-blue, volcanic sediments–infused with celadonite–dating back nearly 30 million years. Inter-bedded with basalt flows in the Spokane area is the clay-rich Latah Formation, portions of which are more than 20 million years old. The formation yields plant and insect fossils. In the Spokane area sections of the Latah can be found in Deep Creek, and along the Fish Lake Trail within a couple miles of the west Spokane trailhead.
Sunset on the Granite Dells near Prescott, AZ, with the Watson Lake reservoir in the foreground.Rugged cliff of the ancient, pink granite high in the Bighorns.Closeup of the 30+ million year old yellow and red fossilized soils (paleosols) at the Painted Hills unit of the John Day Fossil beds.
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