A view of the Krukowski Quarry.
Digging Irma Hill and the Krukowski Quarry
Sites in north central Wisconsin hold a treasure of rare Precambrian fossils.
Abigail M. Bostwick and Anna N. Hess, Photos by Abigail M. Bostwick
More than 500 million years ago, shallow seas licked at the shores of the landmass including what is now known as Wisconsin. The continent teemed with life.
Evidence of these shallow marine soft–bodied forms from this time is few and far between. And yet, fossils have been preserved in a very unlikely location of the Badger State — Irma Hill, the third highest point of the state and an isolated sandstone outlier in a heavily glaciated region south of Tomahawk.
The former Irma beach today is a hill owned by the state and private property owners. The fossils found here have been compared to similar, though better–preserved, fossils in the Krukowski Quarry, located about 50 miles to the south in Mosinee.
In Wisconsin, the sea waters of the past left behind layer–upon–layer of sandy, sedimentary rocks such as sandstone, limestone and shale, which were home to a variety of soft–bodied organisms.
During the last 300 million years, the landmass containing Wisconsin shifted its tectonic position to the northern hemisphere, above sea level. The existence of patches of sandstone, such as Irma Hill, means that the Cambrian sea covered most, if not all, of northern Wisconsin at one time.
Irma Hill was near the center of the Wisconsin dome, which was higher in the landscape during Paleozoic time. Located at least 50 miles north of any other Cambrian rock outcrop, it's believed the sandstone was deposited on top of the former Penokean Mountain Range. Irma Hill stands today at 1,650 feet.
Irma Hill sandstone is comprised of medium– to fine–grained silica sedimentary rock and is one of the most isolated patches of Cambrian sandstone in the state. Most other sandstone patches have been wiped out due to erosion and glaciation.
Irma Hill is along the end moraine of the most recent Wisconsin glaciation. Glaciers moved across the Lincoln County area several times after the Ice Age began more than 1 million years ago. The glaciers transported vast loads of rocks and pulverized material as they scoured their way across the landscape. When the ice sheets melted or stagnated, glacial drift was deposited in the form of till, outwash and lacustrine deposits. Today, the drift is several hundred feet thick in many areas of Lincoln County where the moraine marked its furthest extent. Around 20 to 30 feet of glacial till covers the top of Irma Hill.
But glaciation did not completely cover, nor take away, all of Irma Hill's trademark sandstone. Rock outcrops show ripple marks, cross–bedding and grading, in addition to the trace fossils – all evidence of a past marine environment. The exposed layers along the road show at least six layers.
In comparison, Krukowski Quarry is comprised of upper Cambrian Mt. Simon–Wonewoc quartz arenite sandstone. In the late Cambrian period, this area was a beach and shoreline, and the quarry is well known for abundant fossil impressions. The area also was glaciated, though less till was deposited here than at Irma Hill.
Preservation of the soft tissue organisms that lived so long ago is exceptionally rare. Their lack of hard body parts, likelihood to quickly decay and reworking of sediment all mean these fossils were often obliterated. Erosion and glaciations also removed much of what survived.
At Irma Hill and Krukowski Quarry evidence of these organisms has survived and they are home to some of the world's best known soft–bodied fossils and trace fossils.
No shell animals have been recorded, but earlier soft–tissue life ranging from jellyfish, arthropods, Climactichnites and other unknown ichnofossils, possibly worm trails, can be found.
While evidence of jellyfish has not been found at Irma Hill, the Krukowski Quarry shows several stunning sandstone horizons of mass jellyfish stranding events.
Jellyfish were mobile, carnivorous animals at the time seas covered Wisconsin. Jellyfish were not often fossilized, though, because of their soft body parts. They also decay quickly. But at the site of Krukowski Quarry jellyfish likely became trapped when they settled on the surface in shallow water. As they moved their sub–umbrella down and pumped their bells to escape, their stomach and external cavities filled with sand. Stranded jellyfish have only been found in two locations worldwide — Krukowski Quarry and New Brunswick, Canada.
Perhaps one of the most spectacular trace fossils of Cambrian time is the Climactichnite, a tire–like trail found thus far exclusively in sandstone. It has never been seen in other rock types or periods. The trail is considered to be exceptionally unique and one of the oldest fossils ever to have been discovered.
Climactichnite have been observed in Australia; Ontario and Quebec in Canada; and New York, Missouri and Wisconsin.
In Wisconsin, they've been seen not only at Irma Hill and Krukowski Quarry, but also Chippewa Falls, Eau Claire, Black River Falls, New Lisbon and Mauston.
This track–maker lacked a skeleton and did not leave a fossil. What is left behind is the classic form of paired, lateral ridges that may be smooth or rough. Between these ridges are a series of alternating bars and furrows. Based on this appearance, the organism was named in 1860 after the Latin "climacis," for their "ladder–like" appearance.
It's believed the animal crawled at night or during times of low light rather than during sunny days because temperature and ultraviolet radiation from the sun likely would have hurt the Climactichnite. It likely crawled on the sand when it was damp but not full of water.
In every instance where Climactichnite traces are found, trails are overlain by another bed of sandstone without any film or layer of clay being deposited in between. This observation is true for Irma Hill Climactichnite fossils.
It's uncertain how old the Climactichnite is. It's often dated via trilobites, the species that most often occurs in the next overlying stratigraphic rock. One of the classic features of Climactichnites is the constant width. Trails display a small turning radius, indicating the animal didn't stretch out much when moving.
Climactichnites on Irma Hill range in width from 6 to 10 millimeters wide. Climactichnites seen at the Krukowski Quarry have been far more numerous, with around 30 trails in a 200–square–foot area.
Interpreting these trails is difficult and debate between scientists continues: Is this indeed a trace fossil or is it an impression from a very long, earthworm–type organism?
Ichnofossils and ripple marks
The final trace fossils observed in Irma Hill sandstone remain unidentified. Similar trace fossils are present at the Krukowski Quarry and are referred to as undescribed track way ichnofossils, or possibly worm trails.
Ripple marks also have been recorded at both sites but are not fossils, rather a record in the rock of the coastal environment.
Researching fossils found at Irma Hill and the Krukowski Quarry provides critical evidence of the history of life and shows some of the earliest prints left by the first life on earth. While collecting fossils from state lands is prohibited, they provide a rich understanding of Wisconsin's natural history.