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August 2010 - archives

GSOC President's Field Trip to the Oldest Rock in Oregon

by Janet Rasmussen

This year, Larry Purchase led the Geological Society on its 75th President's Field Trip to the oldest rocks in Oregon. We camped at Delintment Lake, where the first GSOC field trip was held in 1935.

The focus was on fossil collecting. The oldest rocks (Devonian) outcrop on a private ranch, where we also found Jurassic-age fossils. We were fortunate that Larry was acquainted with the owners and we all had permission to collect. The trip began Friday, July 16, in the Ochoco Mountains of central Oregon, and continued through Monday, July 19. Most were ready to head straight home with our treasures on July 20.

An ammonite found at the publicly accessible site a few miles from Delintment Lake, on FS Road 43. These were found in sediments dating to 169 to 176 million years ago.

An artist's conception of a living ammonite. Ammonites existed from the Jurassic to the late Cretaceous, but then became extinct.

Illustration of ammonite shell forms from the 1904 book by Haeckel, Artforms of Nature.

The campground was unexpectedly crowded, but fortunately a large campsite accommodated half a dozen tents. We all shared campsites, and the camp host was very welcoming.

Doug and I walked partway around the lake Friday evening, through the rest of the campground. We saw this unusual twisted Ponderosa pine.

As we were returning, we found Anne, camped by herself at the far end from the rest of the group. She had brought drinks to share! About then, Clay drove up and decided to pitch his tent nearby. We had a before-dinner cocktail. Anne had borrowed a JetBoil stove for heating water. I told her how on another PFT I tried out my new one, and set the foam covering on fire. I believe I was sitting with the Smoody family at the time. I haven't used it since. She, however, was able to light the stove with the foam covering in place as I anxiously watched from a safe distance. It did not catch fire. Hmmm! A little more Maker's Mark, please! While we were visiting Anne and Clay, others drove to the Four Corners site to dig ammonites. I was anxious to get my own!

We met most evenings at the group picnic area to hash over our plans for the following day, and get a little geological background for what we would be seeing. On Friday night, our smaller group gathered at the big campsite.

On Saturday, I woke up early and went for a walk around the lake. It was so pretty and quiet as the sun had just come up. We had an uncomfortable night, having neglected to inflate the air mattress fully. It seemed full when the air was very warm, but as the night cooled, the air shrank, and we collided and occasionally subducted grumpily in a deep trench. Also, the air in the mattress became colder overnight, and I hadn't brought the foam pad I usually put over it. I was afraid it would deflate altogether and leave us lying on the hard ground.

Morning sun made the grass glow, and wildflowers were blooming everywhere. I wish I'd brought the book.

I scared a family of ducks out into open water.

Jay Van Tassell and Larry Purchase in their fossil-hunting shirts.

Larry brought an arsenal of tools, though mostly all we needed was a pocket knife or trowel.

We drove through the ranch to the oldest rock in Oregon, a Devonian limestone outcrop (about 380 million years old) on top of a rounded hill. It was a beautiful, lonesome place with interesting outcrops of dissimilar rock. The road became a track through the tall grass. At one point it became too steep and rutted for Peter's small car, but Clay went back to drive him and Thalia up to our parking area. We started off by having lunch.

It was a very hot day and there was not much cover or shade. After lunch, everyone was eager to climb up to the rock. I spotted a large juniper near the top of the slope, so I charged ahead, hoping for a private place to take a personal break. Doug was instructed to hold off the others by taking a photo of the ascent. He had great difficulty in slowing them down, we were all so excited to look at this unique rock!

This outcrop is described as "...100 feet of highly folded, massive, cherty, stromatoporoidal limestone...[which] consists largely of skeletons of organisms in their original growth positions and surrounded by a very fine granular matrix. The fauna included stromatoporoids...corals...and brachiopods. (Kleweno et al 1961) The photo is by Dr. Jay Van Tassell.

Leo looking for specimens at the base of the outcrop. The limestone was almost too hard to break even with a rock hammer. There were many fragments on the ground, though. We forgot the heat as we searched for pieces with visible fossils. I found 3 rocks of interest, containing horn corals, rugose corals, and stromatoporoids.

Peter and Thalia on the top.

Geologist Bev Vogt examines rock fragments in the grass near the oldest rock in Oregon.

This close-up photo is of rugose corals (Thamnopora cervicornis) in a limestone fragment. Corals developed over 500 million years ago, and of course are still found today. Some kinds are now extinct, including the rugose corals. Corals and ammonites are index fossils, meaning that a sedimentary layer can be dated by the species of coral or ammonite found there. Photo is by Dr. Jay Van Tassell.

This is a photo of the rugose coral in a thin section of similar rock taken through a polarizing microscope. Note the scale bar. Photo by Dr. Jay Van Tassell

A close-up of stromatoporoids (Gerronostroma elegans). Photo by Dr. Jay Van Tassell

A photo of stromatoporoids (compartmented layers) in thin section via polarizing microscope, also by Dr. Jay Van Tassell. Stromatoporoids were aquatic colonial invertebrates, believed to be related to sponges, that were responsible for the creation of large reefs in the Devonian period.

As I said before, the road, or path, to the outcrop was challenging. Our "service engine" light came on as we were driving, and on our return we came across this large car part in the track. Joseph Cohen, Jay Van Tassell, and Tom Walsh looked under several cars and speculated about what exactly it was. Tom determined its function, but its origin remained a mystery.

Larry stopped the outgoing autos to inquire if any were missing an essential piece. Although everyone thought their cars were running just fine, we decided to take it home, because 1) our warning light was on and 2) Larry wanted it for a centerpiece at next year's banquet. See the postscript at the end...

After dinner that evening, Doug and I joined a small group digging for ammonites a few miles from the campsite. Chris, Larry, and Joseph were already there. It was a steep slope and uncomfortable but hard to stop once we began finding the lovely fragments and imprints of ammonites. Sometimes we found an ammonite on one surface and an imprint of a clam on the opposite side. Some called these "ammoclams" and some called them "clammonites".

On Day 3 of our trip, we returned to the ranch and parked near a small quarry. It was a very hot day. We hiked up to the crocodile site. Next to Larry's left foot is the darker brown rock where the fossils were found. Below is a copied online article from National Geographic News.

"March 22, 2007—It endured a rocky ride—literally—but this ancient "sea monster" from Asia has found a place in the United States to call home.

The fossil remains of a crocodile-like reptile called Thalattosuchia have been discovered in rocks in the Blue Mountains of eastern Oregon—about 5,000 miles (8,050 kilometers) from where it most likely died, researchers announced on Monday. So far about 50 percent of the animal, including the upper leg bone and rib fragments seen here (bottom), have been unearthed.

"This creature lived in Jurassic times, so it's 150 to 180 million years old," retired University of Oregon geologist William Orr said in a press release. Orr provided expert advice to the excavation team.

"It probably lived in an area from Japan to East Timor, somewhere in the western Pacific in a tropical estuarine environment."

The reptile, the oldest ever found in Oregon, is a rare discovery in North America. But similar fossils have been found throughout Southeast Asia, so experts believe that the remains were carried to the U.S. by plate tectonics. As the section of Earth's crust containing the fossils moved eastward, the Pacific plate collided with the North American plate, pushing the bones into the mountains.

The 6- to 8-foot-long (1.8- to 2.4-meter-long) creature, shown in an artist's conception (top), is part of a group that scientists think represents an evolutionary transition for this line of crocodilians. Features from related fossils suggest that the animals were evolving from being semiaquatic to entirely ocean dwelling.

The newfound fossils will go to the University of Iowa for further study before going on display at an Oregon museum.

—Victoria Jaggard"

Part of the group enjoyes the shade of a juniper during the hottest part of the afternoon. On the left stands Evelyn Pratt with her rock hammer, who has been a GSOC member for 42 years! Photo by Larry Purchase

It was too hot to look for the rarer large ammonites, some of which measure 16" across ("ammoSquatches" per Clay Kelleher). I wandered back to the cars with Doug, and then we angled up the hill looking for the rest of the group. He got sidetracked looking at rocks on the ground--several people found very nice fossils just lying on the surface. I continued over several ridges until at last I heard some voices in the distance. I found Chris and Leo digging up large numbers of small ammonites near the shade of a juniper, and joined them. There are no photos of this little dig because I got so excited by my finds that I couldn't stop to take pictures. At some point I heard Doug calling for me and brought him to the site as well. Eventually others came, and everyone had a great time. I did take this photo later of some fossils that I found at the shady spot less than a mile from where the Jurassic crocodile was found.

Doug found a portion of a very large ammonite on a hillslope. I put my little perfect ammonite on top for scale. No one found a whole large ammonite, but they have been taken from this area.

After another lovely day of digging in the dirt, we returned to camp for dinner and conversation. Doug and I inflated our mattress again to the very limit of its capacity. We had a much more comfortable night.

Joseph, Bart, and Peter confer over a map back at camp.

Most went to bed early, but a few of us stayed up for a little campfire and conversation. Photo by Larry Purchase

Chris brought ingredients for s'mores, which were shared around. Photo by Larry Purchase.

The nights were beautiful. We left the rain fly off our tent to enjoy the stars. But we couldn't stay awake long enough to really appreciate them.

On our last morning in camp, several of us walked partway around the lake. We decided that after touring some back roads to the north, looking at geology, we would go on to John Day for a motel before heading home.

Bev and Bart led the final day of the field trip. Bart addresses the group at one of the first stops along FS Road 6370.

Bart's method is to bring typical rock specimens of an area from his vast and well-catalogued collection in order to illustrate the geology of each stop. He also breaks off representative samples from the rocks in situ. This day's stops were based on the 2009 Geological Society of America field trip guide written by U of O's Todd LaMaskin, who has some new ideas about the processes shaping the very complicated geology of NE Oregon. Photo by Larry Purchase

While I wandered down the road for a nature break, I spotted this little rabbit hiding motionless in the shade of a juniper tree. The day was very warm.

These darling brachiopod fossils (Rhynchotrema sp.) were found near stops 6 and 7 on FS 6370, where we also found ammonites and clams. The age of the formation in which these were found was approximately 175-183 million years old. I had to chip the softer sandy sediments away from the little oyster cracker-like fossils, and wash them in lots of water while rubbing them to separate the individual shells. We raised a cloud of dust digging for "keepers" and also smaller fragments that could be used for the placecards on next year's annual banquet. Finally, well coated with sweat and dust, we headed for a paved road leading to a motel. Another fabulous President's Field Trip! Thanks, Larry!

For further reading on the fossils found in the Blue Mountains of NE Oregon, go to William and Elizabeth Orr's Oregon Fossils, 2009, pp 61-75.

A postscript: After we returned from the trip, we took our car in to the dealer and learned that the service light indicated only a minor temporary glitch which was resolved.

Then we had a call from Larry: "Do you still have that car part?" Evidently, it had come from his Honda CRV. Although it had run all right for awhile, it quit while his wife Wenonah was driving one day. They drove down in their rental car to retrieve the part from us one evening, in hopes of reducing the cost of a very expensive fix.

Better yet, I remembered that auto comprehensive insurance covers hitting rocks with cars (learned from personal experience)---they checked, and most of the cost of the repairs were covered. Something to remember next time you run over a rock, geologists!

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August 2010 - archives