An Ancient Sea Floor
Over five hundred million years ago the spot on which you are standing was the bed of a shallow
saltwater sea. Small two-shelled brachiopods lived their adult lives on the sandy bottom.
Insect-like trilobites scurried about the rippled sand, scavenging. There were also hyoliths,
cone-shaped mollusks resembling snails that lay flat on the bottom.
Trilobites, worms and other animals left tracks and burrows sometimes preserved as trace
fossils.
How do we know these facts? Careful study of the rock cliffs along the Red Cedar Trail and
throughout the valley reveals a series of layers representing changing environmental conditions.
Within the rocks are preserved fossils -- the remains of creatures or their trails and borings that
lived in those distant times.
Geologists, using knowledge of the rate of breakdown of radioactive elements in the rocks and
comparisons with rock strata and fossils elsewhere, have dated the rocks along the trail at
between about 520 and 525 million years old, within the upper (late) Cambrian Period. Study of
living sea animals and deposition of sediments on modern sea floors help scientists understand
the ancient environments and their inhabitants.
It is thought that the sea encroached on the land and retreated five times during the late Cambrian
and first half of the Ordovician Periods, represented by the rocks of western and southern
Wisconsin. Along the trail such rocks are classified as the Eau Claire and Wonewoc geological
formations. West of the trail, on higher ground, can be seen the Lone Rock, St. Lawrence,
Jordan, and Oneota Formations. Underlying the Ear Claire is the Mt. Simon Fm.
Oneota Fm.
Jordan Fm.
St. Lawrence Fm.
Lone Rock Fm.
Wonewoc Fm.
Eau Claire Fm.
Mount Simon Fm.
You won't find dinosaur fossils here -- the rocks are too old. Dinosaurs arrived much later,
during the Mesozoic Era, which lasted from 245 to 65 million years ago.
Sandstone, Shale, Claystone, and Limestone
In some areas the sea crashed on rocky shores during its advance over the land, slowly grinding
rocks into sand, which was then suspended in the seawater. Elsewhere stream or wind erosion
brought sediment from the land to the sea. In the turbulent near-shore environment only the
larger, heavier sand particles settled out of suspension, later to be cemented together as a coarse
sandstone. Further offshore, smaller particles settled out and became medium and fine-grained
sandstones. Even further offshore (or on quiet tidal flats), very fine-grained clay and silt or mud
particles settled and became claystone and shales.
Limestone, the third main type of sedimentary rock found in this area, formed from calcium
carbonate precipitated from the water and from the remains of animal shells. Sandy dolomitic
limestones may be seen west of Menomonie in the St. Lawrence formation and in several of the
Ordovician formations.
Sedimentary layers may build up slowly over exceedingly long periods of time or all at once in
layers up to several feet thick as the result of hurricane-like storms. Naturally, lower layers will
be older than those above (the law of superposition). Presence of fossils indicates a marine
(ocean) environment since there were no land or freshwater creatures at the time.
The Ice Ages
The Red Cedar Valley occupies a unique position relative to Ice Age remnants. Beyond the
Chippewa River to the Southeast is the famous Driftless Area. It is though this area was never
glaciated. North of the Red Cedar Valley, the Ice Age Trail snakes across the landscape marking
the southernmost extent of the last great glaciation during the Wisconsin Stage (70,000 to 10,000
years ago).
Earlier glaciations, from two million to 70,000 years ago, did affect the Red Cedar Trail area,
scouring the landscape and filling up low areas with outwash, but in general the area is hillier
and more rugged than more recently glaciated regions to the North. Rounded granitic pebbles
brought to the area by glaciers may be found throughout the Red Cedar Valley.