The Story of Silver Falls State Park--Part 2

 THE COLUMBIA RIVER BASALTS
  
Continuing with our story, we pick up at 16.5 million years ago when 100-mile long fissures in the ground formed in Northeastern Oregon and Southeastern Washington. Lavas poured out of these cracks and flowed toward the West. These ‘flood basalts’ were the effect of the creation of the Yellowstone Hotspot, which now sits several states away on its march through the Rocky Mountains. Instead of explosive ash and rock like Mt. St. Helens and like the Discovery Channel movie “Supervolcano,” they were the same type of rock that is on Hawaii. But unlike the small flows on Hawaii, these flows were massive and contained between 100 and 500 cubic miles of rock per flow! Per flow, there is enough basalt to create a layer between 5.5 and 27.5 feet thick over the entire state of Oregon!

  
While there is uncertainty to how exactly they flowed, a person could use candle wax as an analogy. If a candle is let burning long enough to pour across a flat surface, it will fill the lowest areas first. Once the lowest areas are filled, the basalt, just like the wax, will stop due to the terrain being too steep to pass and will start to bulge from the inflow of lava. If the stresses from the inflow become too great, then the lava will burst out one side and continue flowing to the lowest area.

 

When lava slowly begins to cool down, it cracks much like the way mud cracks form after a period of time with no rain. Instead of cracking in random patterns like mud cracks, basalt tends to produce fractures called joints in a hexagonal arrangement. Places around the world the show perfect hexagonal pillars are the Giants’ Causeway in Ireland, the Devil’s Post pile in California, and Toketee Falls in Oregon. At Silver Falls, the perfect hexagons have been eroded so much that it is hard to see even one. 

All regions of a lava flow do not necessarily produce these perfect columns, known as columnar basalt. In some regions, such as the side and top of a flow, and even the whole flow occasionally, a jointing pattern known as entablature forms. This type of pattern can be thought as a group of columns that have been left stationary at the top and pulled along the bottom or vice versa. Another pattern that may be encountered in basalt is known as massive. Massive jointing is so irregular, there is really no pattern at all. Another phenomena that is seen in some areas of the park, in particular along the path next to the Lower South Falls, is that the lava seems to have folded in on itself as it flowed across the land in a similar way that bread dough is folded in while kneaded.  

This rock found near Lower South Falls looks like it folded over as gas bubbles were forming.

The bubbles in the rock, which are now cavities, were formed by gas that was trapped within the basalt, much like how air gets trapped in bread when it gets cooked. You will find more of these cavities near the top of the flow than the bottom. In some areas it might even be possible to see elongated cavities near the top of a flow. As the gas bubbles rise, they begin to get longer and skinnier until they reach the surface and escape.

The Columbia River Basalts is made up of several groups. Two of these groups, which are in the Yakima Basalt Subgroup, are found within Silver Falls State Park. These two are known as the Grand Ronde Basalt and the Wanapum Basalt. Within Western Oregon itself, 21 separate flows have been identified; within Silver Falls State Park, 13 flows rising more than 500’ have been found, with 5 from the Wanapum Flows and 8 from the Grand Ronde Flows.  The flows generally slope down toward the West, which means that the flows came through a low spot in the Ancestral Cascades near the Opal Creek Wilderness.

The Separate flows are identified according to jointing patterns, the presence of large plagioclase, bubbles marking the tops of flows, and even the waterfalls themselves help to separate flows out. For instance, Twin Falls, Drake Falls, and the Lower North Falls each go over one separate flow. The pool and stream below each fall within the park is a relatively good marker for the top of another flow. The stream just above each waterfall is a general indicator for the top of a flow as well.

Some lava flows, such as one in the upper portion of South Falls, followed an ancient river valley and is only 10 to 20 feet high maximum. Other flows in the park can be 30 or more feet high. It should be noted that Crown Point on the Columbia River Gorge sits on a single flow that is over 1000 feet in thickness! In Silver Falls State Park, there is roughly 1.4 cubic miles of basalt, enough material to make over 5 Great Walls of China, or enough to make a road from San Francisco to New York City that is over 2 feet thick and 1 mile wide!

Due to cracks in the basalt, water can flow somewhat easily though it. Sometimes flows can reach more than 1000 gallons per minute! Due to this fact, throughout the year, and especially during the winter and spring, a visitor will see various small falls and water dripping from the rocks that are a result of cracks in the basalt. When you see water dripping through the rock from above the path that goes behind South Falls, don’t be alarmed, this is just natural and doesn’t mean the rock will fall on you. You should, however, be careful when you walk down the stairs and steep slopes when the water is running across them since they can be quite slippery.

Some of the Columbia River Basalt flows continued on through, and make up the bedrock of the South Salem Hills and on toward Depoe Bay. The red soils of the Willamette Valley Vineyards is the soil from the eroded basalt, this soil is known as Jory soil and is perfect for growing wine grapes due to the type of minerals found in the soil.

The Grand Ronde Flows

The Grand Ronde Flows in Silver falls State Park compose every flow on the North Fork below the cavern under North Falls, and every flow below the cavern under South Falls. Most of these flows are more resistant to erosion than the flows above the caverns. At a combine total of at least eight flows, the Grand Ronde Flows make up the largest percentage of lava flows in the park. These flows are between 16.5 and 15.6 million years old and came from 100-mile long cracks known as the Chief Joseph Dike Swarm. Only five of these flows can be seen on the South Fork of Silver Creek because three of the flows “pinch out,” meaning that the flow of lava either changed direction, or the flows stopped completely. For example, the top lava flow in this unit is about 140 feet thick on the North Fork, but it becomes as small as 70 feet thick on the South Fork.

Two lava flows in this unit that create a good marker are the two just above the trail on the Lower South Falls. These two flows are the same two that are just above the trail on the Middle North Falls, and the same two that you can see about halfway up Double Falls. 

A Time of Peace

Between the Grand Ronde Flows and the Wanapum Flows, life in Silver Falls returned to a gentle pace for about 400,000 years or more. As the basalt from the Grand Ronde flows began to be eroded by ancient creeks, soils began to develop with trees similar to those seen today at the park. A thick bed of ash, sand, clay, and biological material slowly accumulated over time to a thickness of more than 25 feet in some areas and is known as the Vantage Interbed. But the peace would not last as some new flows from the Columbia River Basalts would soon overwhelm the region.

Underneath the trail at both the South Falls and the North Falls are the Grand Ronde Lava Flows. Sand, mud, and ash make up the layer that you can see at the back of the cavern under North Falls and the eroded out section next to the trail behind South Falls. That sedimentary unit is the Vantage Interbed. Looking up underneath South Falls, it is hard to determine the top of the Vantage Interbed due to the ceiling of the cavern being so eroded.

The holes you see in the ceiling were originally tree casts like those you can see in the North Falls cavern if you shine a flashlight upward. Over time, minerals of different colors have been deposited as the tree casts eroded away, leaving small caves with various colors. When both South Falls and North Falls were shorter, the pools below carved into the Vantage Interbed and created the caverns. Later on, as water trickled through the cracks in the basalt at South Falls, the tree casts were eroded outward to become the caves that you see today

The Wanapum Flows

As 15.2 million years ago approached, another sequence of basalt flows would soon come to this area. This new sequence is known as the Wanapum Flows, but only the Frenchman Springs Member of the Wanapum Flows is displayed in this park. In Oregon, this member contains 21 individual flows, but in Silver Falls State Park, only 5 of them are displayed.

The first lava flow that came in after the Vantage Interbed was a flow from the Sand Hollow Submember. This caused mass destruction as it plowed across the land swallowing trees on its way toward the Willamette Valley.  As the lava cooled around the trees, the trees burned up from the outside inward. While the forest has been gone for a long

 time, a person could look up into the ceiling of the cavern at North Falls and see the casts of trees, like those you would find at Lava Cast Forest in Central Oregon. If you happen to have a flashlight with you, you could point it up into the opening and see the charred remains of the tree.

The trees that were of large enough across ended up having their cast preserved in the rock. Those that had smaller diameters ended up completely burning up before the lava could cool around them. It is most likely that these tree casts would be found anywhere under the basalt and you can even see them at Winter Falls and North Falls. Even South Falls has tree casts, but at this location, water has damaged them by eroding the walls and depositing various minerals. The best one can be seen with green mineral deposits on the side of its walls but you may have to climb a little on the rocks to get a closer look. This flow also contains some fairly large plagioclase crystals within the rock. Most animals would have escaped the 30 mph flow long before it came through, but some were likely caught beneath the burning weight of the lava flow.

The next three members, where two can be seen at South Falls, were from the Silver Falls Submember, named after the area it is found the most. The first one is easily distinguishable from the top, more resistant flow and the bottom, less resistant flow with the tree casts. The second may be hard to see for those that don’t know where to look. Here at the South Falls you see something unique—a “pinch out” of one of the two flows. One could see this to the left of the South Falls. It is thick in the middle and thins out on each side like pinching the skin on top of your hand. This flow filled in a low area which might have been the ancestral South Fork of Silver Creek.

In this photo of South Falls, one can see the pinch-out flow as the second lava flow down.

At North Falls, it may be a little difficult to distinguish the lower basalt of Sand Hollow that is immediately above the cavern, and the first flow of the Silver Falls Submember that occupies the very top of the cliff. The second flow of Silver Falls Submember, which is the only flow of the three that is not included at the South Falls, is the bottom flow at Upper North Falls. The middle flow at this waterfall is the lava flow that pinches out at South Falls.

The final flow was from the Ginkgo Submember and is the top flow at both South Falls and Upper North Falls. At South Falls, it is the most resistant to weathering and has allowed the waterfall to plunge into the pool below instead of onto rocks like at North Falls. By 14.5 million years ago, the Columbia River Basalts ended their deadly rampage through the region on their way toward the Oregon Coast. 

 The third part of the series is here: http://beavercountryramblings.blogspot.com/2012/12/the-story-of-silver-falls-state-park_1769.html