Fossils on a Mountaintop
It is so easy to forget that the world as you know it is a tiny snapshot in time. It is has been changing for millions of years by raising mountains and wearing them back to dust and by carving out oceans and filling them in again. Many time, if you look closely, the places we know are filled with clues of a different time. You don’t expect to find fossils on a mountain top, but on Chichagof Island small, shelled creatures are frozen in rock among the lichens and deer trails. Their presence are a time capsule of life 360-375 million years ago just waiting to be discovered!
Hoonah, Alaska has an extensive road network – 300 miles of road wind through forests of Spruce and Hemlock. Making a road takes a lot of rock and that has resulted in dozens of rock pits scattered throughout the road system. I had heard of fossils on the mountaintops, but a discovery by a coworker a few years ago of fossils in a rockpit made them more attainable to see and document. I recently drove the miles of winding roads to that pit with a camera in hand to learn-about-and-observe life long ago.
Like cracking an egg the pieces of rock were jigsaw puzzle pieces that contained clues of the lives of Devonian creatures. In the pit, we found a variety of forms of animals frozen in the rock and some of them reminded me of animals in the oceans today. Shells, sponges, corals, and some trilobites! Seeing them caused me to reflect on “deep time” and the evolution of species – it also reminded me of how little I knew about the geologic history of the island of geology itself! What were the names of these creatures? What type of rock were they buried in? Why had they been lifted up from the ocean bottom to this spot?
I began to research the geologic history of Chichagof Island and discovered the region is well researched. For decades the USGS, Forest Service, and other research institutions had documented the bedrock and formations of Chichagof. Through a USGS Map viewer I learned that much of northern Chichagof Island was classified as the “Freshwater bay” formation. I read through the description of the formation on the website and couldn’t help but think of one thing : look at all that jargon! Scientific jargon is almost impossible to comprehend unless you are in the field, so I made it my mission to crack the jargon.
I copied the description below from the USGS website to describe the Freshwater Formation. Once you’ve read through it skip to the next section to crack the jargon!
“Freshwater Bay Formation on Chichagof Island is composed of green and red andesite and basalt flows, breccia, and tuff, pyroclastic rhyolite deposits, minor amounts of interbedded conglomeratic volcanic graywacke, grayish-black argillite, and dark-gray limestone (Loney and others, 1963). The correlative but more sedimentary-rock-rich Port Refugio Formation on Prince of Wales Island consists of km-thick sections of siltstone, shale, volcanogenic graywacke, conglomerate, and minor limestone that alternate with km-thick sections of pillow basalt intercalated with minor chert, shale, limestone and aquagene tuff (Eberlein and others, 1983). Unit also includes the Coronados Volcanics and the Saint Joseph Island Volcanics found on western Prince of Wales Island and adjacent islands (Eberlein and others, 1983). The Port Refugio Formation may be a distal facies of the Freshwater Bay Formation. Eberlein and Churkin (1970, p. 43) stated that “many of the graywackes are largely reworked basaltic lavas that contain euhedral crystals of plagioclase and pyroxene that resemble the phenocrysts in the basaltic flows of the formation,” and that many of the conglomerate clasts are andesitic or basaltic rocks. Volcanic flows are found throughout the unit and are up to a hundred meters thick (Eberlein and Churkin, 1970). Age control from the Freshwater Bay is derived from included brachiopods, including Cyrtospirifer, mollusks, and corals of Frasnian (Late Devonian) age (Loney and others, 1975) and conodonts of Famennian (Late Devonian) age (Karl, 1999). Eberlein and Churkin (1970) reported Late Devonian “beautifully preserved” brachiopods that Savage and others (1978) assigned a middle to late Famennian age and that are associated with vascular plant fossils”Alaska USGS https://alaska.usgs.gov/science/geology/state_map/interactive_map/AKgeologic_map.html
Breaking Down the Rock Jargon
Not sure about rocks of the Devonian Era? Come learn along with me!
- green and red andesite
- Andesite is a fine-grained igneous rock. That means it is typically found in lava flows.
- basalt flows,
- A fine-grain dark rock found in lava flows. It’s earth’s most abundant bedrock!
- A rock made of other rocks. Typically the rock particles are greater than 2mm in size and are glued together with rock debris as flows downhill to a collection spot
- an igneous rock formed from explosive volcanic eruptions. It is made from material that is ejected and then glued together
- pyroclastic rhyolite deposits,
- Ryhyolite is an igneous rock with a high silica content. Pyroclastic flows are ash and gas that form dense clouds and flow down a volcano.
- minor amounts of interbedded conglomeratic volcanic graywacke,
- graywacke (pronouced gray-wackah) is a type of sedimentary rock. It can be formed in deep oceans.
- grayish-black argillite,
- and dark-gray limestone
- A sedimentary rock made of calcium carbonate that most commonly forms in shallow seas
Breaking Down the Animal Jargon
As if life these days isn’t hard to enough to keep track of check out all the names of the creatures that lived long ago!
- A clam-like marine mammal although not closely related to clams. There are over 300 species of brachiopods currently living.
- A type of brachiopod that are now extinct. They have a shell shaped like a bat in flight.
- Soft bodied animals covered in a shell in the order mollusca such as snails or clams
- and corals of Frasnian (Late Devonian) age (Loney and others, 1975) and conodonts of Famennian (Late Devonian) age (Karl, 1999). Eberlein and Churkin (1970) reported Late Devonian “beautifully preserved” brachiopods that Savage and others (1978) assigned a middle to late Famennian age and that are associated with vascular plant fossils
- Deep time is a crazy, crazy thing. The chart above does an adequate job of showing off the “Devonian Era” in purple. It’s far before the time of the dinosaurs. Humans have been around for about 200,000 years and the T-Rex lived during the Cretacous Period. Life was really just getting going in the Devonian period!
I know this will not be the last time that I come across fossils on Chichagof Island. I look forward to when I do and to expanding my knowledge of a time long past.