PIER COUNTY PARK AT ROCKBRIDGE
Lying along the west side of the Pine River in Richland County is a bold
north-trending ridge of sandstone, about a half mile long, standing 60
feet above the surrounding plain. The south end of this sandstone mass
lies within Pier County Park on WI 80 next to the village of Rockbridge.
The north end lies on private land. The ridge is fragmented into several
blocks by joints, or deep vertical cracks, and has been deeply eroded at
its base on both its east and west sides by streams that have flowed on
one or both sides at various times throughout its history. Today, the West
Branch of the Pine River flows southeast toward the ridge where it has
worked its way through one of the joints and under the sandstone mass
to join the Pine River on the east side of the ridge. During flooding, both
streams lap against the ridge, continuing their erosive work.
Where the West Branch cuts through the ridge, it has eroded stone
along the joint to make a triangular passage, known as the Rockbridge,
about 20 feet wide and 10 feet high. At the ridge’s south end, at some
point in the distant past, an overhanging slab of stone broke off and now
stands upright like the point of a giant sword driven into the land. Long
ago a road ran through the gap between the sandstone mass and this
broken piece. That road is now a dead-end parking area that allows for
easy viewing of the natural bridge.
The site has not been studied extensively, so there remains some uncertainty among geologists about the composition of the sandstone. The top
layers of the ridge could be composed at least partly of sandstone of the
Tunnel City Group, deposited around 500 million years ago in Cambrian time. The Ironton Sandstone, a strongly cemented member of the
Wonewoc Formation, which underlies the Tunnel City, might be another
component. Its iron oxide and silica cement resists erosion, so the Ironton
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The West Branch
of the Pine River
along a joint
between blocks of
sandstone. This is
the east side of the
A view of the west side of the sandstone ridge looking north. The West Branch of
the Pine River flows to the base of the sandstone mass then north along its base to
Rockbridge (out of sight at center).
Sandstone is responsible for many of the resistant ridges in the Driftless
Area. The reddish streaks in the sandstone at Pier County Park indicate the
presence of iron. The more erodible sandstone near the water level could
be that of the softer member of the Wonewoc, the Galesville Sandstone.
Cross-bedding, or sweeping layers in the sandstone, indicate that wind-formed sand dunes once lay here, probably near a Cambrian seashore.
Archeologists have found evidence in Pier County Park that Native
Americans used the overhangs of the sandstone for shelter during winter
months. The land was donated to Richland County in the early 1920s
in order to preserve its unique beauty. Workers excavated a tunnel that
allows visitors to walk through to the other side of the ridge. Stairways
on both sides of the ridge take visitors to it summit where a path runs the
length of it within the park.