Ian Madin has worked as a geologist with the Oregon Department of Geology and Mineral Industries for nearly 30 years. In his spare time he enjoys riding with Cycle Oregon and sharing stories about the incredibly cool geology of Oregon. We are delighted to continue his popular series with two reports from the 2021 Tygh Valley Rally. Up first, Weekend #1 (September 10-12, 2021) – Gravel Forward.
About Tygh Valley
The geologic landscape for the Tygh Valley Rally tells a story of two very different kinds of volcanic eruptions. The eastern ends of the two road rides traverse lava flows of the Columbia River Basalt, one of the earth’s three great flood basalt regions. The rest of the rides will cross a broad slope made of the Dalles Formation; lava flows and volcanic sand and gravel that were shed off ancient Cascade range volcanoes. You will also see a lot of loess, windblown silt deposited during the last great ice age. You won’t see much in the way of rock when you are riding up on the flats, but there is some great stuff down in the canyons that slice across the broad plateau.
The Cascade Range, which stretches from Mt. Lassen in northern California to Mt. Baker in northern Washington, is what geologists call a volcanic arc. Volcanic arcs are chains of volcanoes that are found around the world wherever there is a subduction zone. Subduction zones are part of the global system of moving tectonic plates, and are places where one plate is forced to slide beneath the other and sinks into the body of the earth. The sinking plates trigger melting of the surrounding rock as they descend, and the molten magma rises to form the volcanoes that make up the arc.
The volcanic arc in Oregon has been active for millions of years, and today’s High Cascade Range was built by the eruption of a dozen generations of volcanoes like Mt. Hood, Mt. Jefferson and the Three Sisters. Mt Hood is about a million years old, and has been built up over time by repeated eruptions, alternating between lava flows and more explosive eruptions like the 1980 eruption of Mt St. Helens. The most recent eruptions were relatively small events in the late 1700’s, and 1200’s, with other much larger eruptions about 1500 years and 20,000 years ago.
Over the last 6-8 million years, the ancestors of today’s Cascade volcanoes grew and died, each of them adding more eruptive material to broad aprons forming along the sides of the range. Some of this material was in the familiar form of lava flows, but the majority of it came in the form of volcanic rubble and debris. Many eruptions are explosive, and produce large masses of ash and broken lava. Lava can also erupt onto the snow and glaciers that cover the high volcanoes, producing a lahar, a muddy mix of ash, melted ice and broken lava which floods down the streams leading from the mountains. Rivers pick up the broken material and round and sort the fragments, leaving layers of sand and gravel intermixed with the lahars and lava flows. In this area, this mix is called the Dalles Formation.
The Dalles Formation is the result of the same types of volcanic eruptions we see happening all around the world today. In contrast, the Columbia River flood basalt was formed by an extremely rare volcanic process, which is a good thing, because flood basalt eruptions are global catastrophes on the scale of asteroid impacts. The Columbia River Basalt is a series of gigantic lava flows that erupted 16 million years ago from fissures extending from Enterprise, Oregon nearly to Spokane, Washington. Dozens of massive flows spread across Eastern Oregon and Washington, and then found their way west along an ancient course of the Columbia, passing through Portland and Salem, and in some cases reaching as far as Newport, Oregon, having travelled over 400 miles. Over the course of about 1 million years, lava thousands of feet thick covered an area roughly the size of Oregon. The impact on local life would have been absolutely catastrophic, and the gasses released during the eruptions would have dramatically altered the earth’s climate. Columbia River Basalt flows appear black, brown or reddish, and often show a pronounced columnar structure, the result of cracks that formed as the lava cooled.
Gravel Forward – Day 1
Leaving Tygh Valley, you will be riding towards the High Cascades up a canyon cut into the Dalles Formation, eventually climbing out onto the top of the plateau. Don’t expect to see much more than bits of gravel or old weathered lava in the road cuts. The road dips into the Columbia River Basalt that is under the Dalles Formation when you cross some canyons after the water stop, then you are back on the broad flat Dalles surface till Friend, where you pass onto a surface made of Columbia River Basalt covered in loess. The geologic interest picks up when you turn onto Highway 197 and descend through Butler Canyon. You will probably be going too fast to pay much attention to the basalt exposed in the roadcut, but you might keep an eye out for some of the well-formed basalt columns along the way. Basalt typically breaks into crude hexagonal columns, which form as the lava cools. The liquid lava solidifies at temperatures a bit below 2000 F, and then the hot, solid lava shrinks as it cools to room temperature, cracking into these hexagonal columns to accommodate the shrinkage. In some places like the Giant’s Causeway in Britain, or Devil’s Postpile in California, the columns are so regular and perfect that it is hard to believe that they are natural.
Take a moment to look back to the north as you cross Tygh Valley on Fairgrounds Road. Your descent through Butler Canyon cut across Tygh Ridge, which is a giant fold in the layers of the Columbia River Basalt. Between the Dalles and the northern side of Tygh Ridge, the lava layers slope gently down towards the Columbia River, then at Tygh Ridge they bend in a sharp wrinkle and tilt steeply to the south. In the Google Earth image below you can see where the upturned edges of a steeply tilted lava layer makes a series of upside-down “V” shapes where it intersects ridges.
Gravel Forward – Day 2
Day 2 starts out with a quick climb out of the valley and on to a broad plateau of Dalles Formation. On the way up the hill, you will have time to appreciate the roadcut on your left, which shows colorful layers of Dalles Formation sandstone. As you start up the plateau, you aren’t going to see much in the way of rocks, but you will pass by some mysterious features on the landscape that still baffle geologists. The Dalles Formation here is covered with a thin layer of wind-blown silt called loess. The loess originated as rock ground to fine flour beneath the glacier that covered the northern hemisphere during the last ice age. The fine rock dust was flushed down the Columbia River and then picked up by the wind and blown across the landscape. Here it is only a few feet thick, but supports the drylands wheat farming that is an economic engine for the area. Where the natural surface is preserved between the fields you may see Mima mounds if you look carefully. Mima mounds are regularly spaced, circular mounds a foot or two high and 10-20 feet across. They stud the eastern Oregon landscape in many areas where the natural ground surface has not been disturbed. Geologists still argue about the cause, but one currently popular theory suggests that long-lived colonies of burrowing rodents slowly build the mounds.
You may be able to see mounds from the road ( There are some on the left just as you first come up onto the plateau) but they are much easier to see from the air, and even easier to see using a laser topographic scanning system called lidar.
Let’s talk a little bit about gravel.
You have wobbled and skittered on it, gotten it stuck in your cleats, probably cursed it a few times, but what do you really know about gravel?
With the exception of water, gravel is the most widely used natural resource in modern society. Gravel literally underpins everything we build, and huge amounts are mined, transported and used every year. In 2018, Oregon mines produced nearly 40 million tons of gravel, which works out to 10 tons per Oregonian. That is enough to fill 2.4 million dump trucks. What did you do with your 10 tons?
From a geologist’s perspective, gravel is an aggregation of rock fragments whose size is greater than .078 inches. Fragments between .078 and .157 inches are called (if these numbers seem odd, its because they are actually defined in metric units) granules, between .157 and 2.5 inches pebbles, between 2.5 inches and 10 inches cobbles and greater than 10 inches boulders. So if the fragments are in the pebble size range, it would be called a pebble gravel, though in most natural deposits of gravel, there are usually a mixture of sizes. The range and variability of sizes is called sorting, and a well-sorted gravel would have fragments in a narrow size range, while a poorly sorted gravel would have a mixture of all sizes. In addition to size and sorting, the other defining characteristic of a body of gravel is rounding. If the fragments all have sharp points and edges, they are called angular, and if they are smooth , without points and edges they are called rounded. These three characteristics, size, sorting and rounding define how a gravel will behave when used in some sort of construction or engineering application, like roads.
Gravel is either mined by excavating a natural deposit, like the gravel from old river beds or it is produced by mining and crushing hard rock like basalt, granite or limestone. In both cases the gravel is run through a series of different sized screens to sort it into different sizes which are then blended to produce the desired product. Crushed rock is always very angular, but natural gravel has typically been rounded by centuries spent rolling along the bottom of a river. Different applications call for different gravel characteristics. Poorly sorted angular crushed rock can be compacted into a hard layers, as the angular fragments interlock and smaller pieces fill in the spaces between fragments. A well-sorted well-rounded gravel will not compact and remains loose with lots of open space between the grains, which is terrible for a road surface, but perfect for drainage.
Many types of rock are mined for the production of gravel for roads, but in Oregon the most common is basalt. Basalt is a hard, dense black rock that forms from the cooling of molten magma either on the surface as a lava flow, or underground, as a dike or sill. Basalt is very common throughout most of Oregon, which is important because the majority of the cost of producing gravel for building roads is transportation. Gravel is heavy, so road builders like to find sources close to their construction projects. There are currently over 1000 rock quarries permitted in Oregon, and at least as many small pits used for construction of forest roads.
There are several important factors in designing and constructing a gravel road. The road must be able to support the weight of vehicles, and the gravel surface must be drained, smooth and cohesive. A well-constructed road starts with removal of surface soil and compaction of the subsoil. A base layer of large gravel is laid down and compacted to provide drainage and to support loads. The surface gravel layer has to be a mix of angular pebble gravel with the right mix of finer fragments to compact into a smooth layer that will hold together. The shape of the road is very important, with a pronounced crown, meaning that the center is high and slopes smoothly down towards the edges.
When everything is done right, and gravel road can provide a smooth ride at high speeds for both vehicles and bicycles. Over time, the passage of vehicles at higher speeds causes washboards to develop so frequent regrading and application of gravel may be needed.