Unusual Unconformity on California's Lost Coast

Will someone save the unconformity? It's going to gone soon!

Well, okay, there's not much to be done about it, being that the exposure seen here is on the shoreline of one of the most violent storm-ridden coasts in California, and just a half mile or so from the northernmost land exposure of the San Andreas fault. It's at Shelter Cove on California's Lost Coast, one of the longest undeveloped stretches of coastline in the nation. Except for the small town of Shelter Cove, there is wilderness for a distance of about fifty miles, from Fort Bragg to Ferndale.

The underlying rock is part of the Franciscan Complex, a mixture of graywacke sandstone and shale that was deposited in the trench that once existed off the coast of California in Mesozoic and early Cenozoic time. The gray rocks were uplifted and eroded, and after a stretch of time, were covered by the tan-colored breccia or conglomerate. It was part of the wave-cut cliffs, but was isolated by a fluke of erosion. It's in the active wave zone, so it won't be long before it disappears. The wave-cut bench on which it sits may become a future unconformity if it is ever covered by sediment.

Where are the Ten Most Incredible Places You've Ever Stood? My Number 2: The Grandest Canyon of All

A clearing storm in Carbon Canyon, a tributary to the Grand Canyon

Of course, the Grand Canyon was going to appear on a list like this. It is one of the great spectacles of geology on planet Earth, and it has been entwined with my life many times over. I can trace the first inklings of my curiosity about geology to a vacation at the North Rim when I was a child of nine or ten. Picking up fossils in a meadow along the highway north of the park, I wondered how they could have ended up at 8,000 feet above sea level. A decade later I was a gawky thin teenager in his first year of college looking for direction in his life. A week below the rim on the New Hance and Grandview trails with an inspirational professor provided the impetus for following a career in geology. And then there has been the nearly three decades of leading students to this stunning place, introducing them to this most incredible gorge and the geological history it reveals. Yes, the Grand Canyon is one of the ten most incredible places I've ever stood.

The Desert View Watchtower on the eastern edge of the South Rim of Grand Canyon.

But there is a problem with picking the Grand Canyon as one of my "spots". It's a really big place! There are 200-plus river miles, the canyon is a mile deep and ten or fifteen miles wide, and has countless side canyons and tributaries (so many that a lot are named after their mileage along the river, i.e. Two-hundred Mile Canyon). Many of the side canyons would be national parks of their own in any other setting; Havasu Canyon and National Canyon are tens of miles long, and just as deep as the main gorge.

Mather Point, possibly. I didn't label this one!

So do you pick the rim? This is where most people see the canyon for the first time. There are two rims of course, the North and the South. Probably 90% of the park's visitors come to the South Rim, and that is where most of the facilities are located. It has some grand viewpoints, including, um, Grandview Point. I love visiting there, but it isn't the most incredible part I've stood on.

The North Rim is distinctly different. A thousand feet higher than the South Rim, it is covered with an extensive cool forest of fir and ponderosa. It's lonelier, with a single resort, a small camper store and a campground (check out the excellent Geogypsy Traveler blog for the perspectives of a North Rim ranger). No matter where I am on the North Rim, it feels more wild. It's one of my most cherished places in the world. But I can't pick out a single spot that I've stood on that set it apart from other areas of the canyon.

There are the archaeological sites. People have lived on and in the Grand Canyon for more than 4,000 years, and have left behind intriguing clues about their lives and beliefs, including the split-twig figurines and multitudes of petroglyphs and pictographs. Had I not ended up a geologist, I would most certainly have followed archaeology as a career. The Grand Canyon has some great archaeology, but I couldn't pick a single spot that represents all the canyon means to me.

It took a lot of consideration, but in the end, I knew the "spot" would have to be Hance Rapids at Mile 77 on the Colorado River. So many things in my life converged at this spot. The New Hance Trail reaches the river at this point. I walked the trail in 1976 on my first geology field studies trip and stood on the shore of the Colorado River for the first time. I had walked through 1.7 billion years of earth history to reach this spot, seeing the rocks I had been studying in class for the previous two months. The history of the canyon came alive to me as I walked in wonderment, seeing the crossbedding in sandstone caused by wind blowing over dunes 300 million years ago, the footprints of pre-dinosaurian reptiles and amphibians, the ripplemarks of long-gone rivers and beaches, and fossils from times before multicelled life existed on the planet.

It is at this point that river-runners first encounter the Granite Gorge, the Inner Canyon of the Grand Canyon. The rocks are schist and gneiss 1.7 billion years old  that formed in the roots of a long-gone mountain range that until recently was hidden in the deep crust of the lithosphere. Only in the last five or six million years has the Colorado River exposed these rocks to view. It was near this spot that John Wesley Powell wrote his immortal words about the Grand Canyon: "We are now ready to start on our way down the Great Unknown...We are three quarters of a mile in the depths of the earth...We have an unknown distance yet to run, an unknown river yet to explore. What falls there are, we know not; what rocks beset the channel, we know not; what walls rise over the river, we know not."

The Great Unconformity separates these ancient rocks from the only somewhat younger rocks of the Grand Canyon Supergroup (the reddish sediments on the right in the picture below).  The Supergroup is a group of late Proterozoic sediments that are three times as thick as the main sequence of Paleozoic rocks that make the upper 4,000 feet of the Grand Canyon cliffs. They were faulted, tilted, eroded, and ultimately buried by the advancing Cambrian sea of 515 million years ago. It may be the most famous unconformity on the planet.

This was the spot where I became a geologist.

I returned to Hance Rapids last summer for the first time in thirty years, only this time I came by raft rather than by foot. I am still processing that journey in my mind, but I already know that it was one of the most significant events in my life. I became aware of the fragile nature of life, first among the plants and animals in this challenging environment, but also of my own. Aside from the profound risk of driving a car every day, I came the closest I've ever been to realizing the possibility of death when I was dumped into the near freezing water and rode the toughest rapid for more than a quarter of a mile. We went over more than 150 rapids in seventeen days, and Hance was the first of the monster rapids, rated 8 or 9 on a scale of 10.

Some pictures surfaced a couple of years ago on my Facebook page of that first profound adventure that I had in 1976. It's remarkable how little the river and the rocks have changed, but I realized how much has changed in our understanding of how the rocks of the canyon accumulated, and how the canyon itself came into being. Plate tectonics had been accepted only a few years prior to my arrival in the canyon, and the tectonic history was only just then getting worked out. Parts of the story are still mysterious.

I also realize the massive changes in my own life since then. Back then, there was a dedicated geology professor discussing the history of the canyon, and a young man making life-changing decisions in the professor's class. The young man had not yet married, there were no children in his life, he had not earned a living on his own. He was just starting out.

Today, my kids are grown, I get awards for longevity at my job, and I'm closer to the end of my career than I am to its beginning. I am a teacher now, but I continue to be a student as well, seeking out new places, and revisiting the old ones from the past for continuing enlightenment. The canyon will continue to exist, changed only in a few ways during my tenure on the planet. It knows or cares little of the latest life form that scrabbles about its surface, and it will shrug off the gigantic reservoirs we've constructed to try and control the river.

The canyon is an incredible place. It gives us perspective in so many ways, and that's why it ended up as number two on the list of the most incredible places I've ever stood.

Just who is that thin person in the yellow jacket??

The Delightful Details: Why I love the Grand Canyon

As my readers may know, I made it to the Grand Canyon as my last trip of 2011 where I saw a nice sunset, and a glorious sunrise. It's kind of a yearly tradition to stop at the canyon, as several family members live near by (in the southwest, 200 miles is 'near'; if you get that close, it is worth the diversion!). There are many things to love about a place like the Grand Canyon. For me, on this trip, it was the details. I don't think I had walked out to Powell Point in past visits, but it looks to be one of my favorite South Rim stops now. The point juts out well into the canyon, and the short trail has steep drop-offs on both sides. The view extends a great distance both east and west. It's easy to get lost in the grandeur of the view. This time I was looking closer, at some of the details. In the picture above, look at the center-right, in the deepest part of the canyon. See if you can pick out where I zoomed in on the picture below.

I zoomed closer, and concentrated on the scene below. It was a familiar one for me, as it is repeated many times in the Grand Canyon, as well as several other spots around the southwest. There are two main elements here, a light brown layer of sandstone on the top, about 200 feet thick, and a sheer cliff below made up of metamorphic rock and granitic intrusions. The story told in these rocks is part of what led me into geology; I walked across a surface like this during my first geology field trip in 1976 and was duly impressed. I wanted to know more.

 

The contact between the two rock sequences is a nonconformity, an erosional boundary between two contrasting rock types. Many people who see the Grand Canyon for the first time might think that the rocks represent some sort of continuous record of deposition, a complete encyclopedia of earth history. This couldn't be further from the truth. The succession of rocks in the Grand Canyon has been interrupted many times by uplift and erosion, and huge gaps occur in the sequence. There are more rocks missing than there are present. The gap shown in the picture above spans something like a billion years!

The sedimentary rock is called the Tapeats Sandstone. It is typical of the kind of sand that might accumulate along a shoreline, although in this case, the beach existed 525 million years ago. It would have been a strange environment to those of us in the world today...there was no life on land, just barren rock and sediment. There was life in the sea, but few species that would be familiar to us. The fossils most commonly found are trilobites, arthropods that are vaguely similar to pillbugs and horseshoe crabs. There were sponges, and some brachiopods (lampshells), and many strange and wonderful soft-bodied creatures who left little in the way of a fossil record (google 'Burgess Shale' if you want to know what they looked like).

The rock beneath is exceedingly complex, and a complete understanding of its origin is still to be determined by further research. We have discovered enough evidence to tell a dramatic story that included deposition of sediments and lavas on a sea floor around two billion years ago, the subsequent crushing and deformation of those rocks, and their rise from the deep crust to form the core of a gigantic mountain range around 1.8 to 1.7 billion years ago. This was followed by intense erosion for hundreds of millions of years, and then renewed mountain-building activity around a billion years ago. These mountains were also worn away until a nearly flat surface remained, interrupted here and there by low hills and ridges of resistant rock.

Then the land slowly sank, and sea transgressed across the landscape. The beach sands of the Tapeats buried all but the highest of the hills, which projected as modest islands. They weren't completely inundated until several tens of millions of years later. That's what I found incredible about the Grand Canyon on my first trip all those years ago. When you know what to look for, you can see these islands exposed in the canyon walls.

Many years ago, the first geologists to explore the secrets of the Grand Canyon called this "The Great Unconformity". I agree...

I place my hand on a billion year gap in time at Frenchman Mountain near Las Vegas. This is the same unconformity as the one seen in the deepest part of Grand Canyon

The rock samples pictured above are to be found along the Trail of Time between Verkamps Visitor Center and Yavapai Point on the South Rim, a recently completed exhibit. The sponsors and innovators did a great job of bringing alive the concept of geologic time.