CUESTAS
Rarely are sedimentary rocks lying perfectly flat. Usually they have some dip, or
inclination, to the horizontal. When they do, the resultant landforms are often cuestas,
which are shown below. The stronger rocks layers form the caprocks while the weaker rock
layers (shown in white) are rapidly eroded and form escarpments, or steep slopes.
Source: Unknown
Michigan's cuestas, formed along the margins of the Michigan Basin, are mostly buried
by thick deposits of glacial drift. In the Paleozoic area (the Michigan Basin) the edges
of the rock layers ("bowls") were leveled off and their rocks exposed. The thin
edges the hard resistant formations were worn away so that the outer edges are now high
cliffs or escarpments. Such a structure --- a short steep slope over the exposed edges of
rock layers, and a long gentle slope in the opposite direction down the top of the strata,
is called a cuesta. The summit of a cuesta may be miles in length. The steep slope of the
Cambrian cuesta borders Lake Superior. In the red and white sandstone of the escarpment
the famous Pictured Rocks, Miner’s Castle, Chapel Rock have been carved. Over the
edge of the Cuesta escarpment streams fall in misty beauty to the lake and the Tahquamenon
drops in majestic grandeur over its ledges from quiet river reaches to rapids below. On
the back of the escarpment, the Cambrian rocks slope gently under the Southern Peninsula.
The Ordovician Trenton limestone stands as an escarpment farther south,
floored on the Cambrian sandstones, but is not so pronounced as another escarpment eight
to twelve miles still farther south --- the edge of the massive Niagara (Silurian)
limestone. The softer rocks between the Trenton limestone and the Niagara dolomite were
eroded into a deep moat between the two resistant formations so that these two rock
formations stand like ramparts across the peninsula guarding the lands to the south.
Other, soft rock rims were also eroded to troughs, therein forming the curving channels
where we now find Green Bay and Georgian Bays, Lakes Michigan, Huron, Erie, and Ontario.
In these channels, curving around the Michigan Basin, a river system was developed which
carved the basins for four of our Great Lakes. The Devonian limestones were carved into
low cuesta ramparts around the northern edge of the Southern Peninsula from Thunder Bay to
Traverse Bay. But the Devonian strata of the southeastern part of the State are flatter
and were therefore not carved to pronounced escarpments like the Niagara was.
Finally, when the older underlying shales were eroded, the resistant edges of the Mississippian formations were left as cuestas that almost encircle the Southern Peninsula.
As shown in the diagrams below, cuestas developed in a basin have
escarpments that face outward. In Wisconsin, cuestas developed along the margins of the
Wisconsin Dome have escarpments that face inward, toward the center of the dome. In the
Great Lakes region, the major "cuesta formers" are dolomites and occasionally a
resistant sandstone bed. Weaker rocks include the shales and some sandstones.
The thick sequence of Silurian (Niagara) dolomite that surrounds, and dips gently
under, the Michigan Basin is very resistant to erosion. Therefore, it tends to form a
prominent topographic feature (the Niagara cuesta) and it outcrops in many places as high,
rocky cliffs. The eroded edge of these rocks forms an escarpment that can be traced almost
continuously along the eastern part of Wisconsin, the Upper Peninsula of Michigan, and on
east to New York State were Niagara Falls is formed by waters flowing off the Niagara
dolomite onto the softer underlying shales. This escarpment is generally referred to as
the Niagara Escarpment.
In the UP and along the western and eastern margins of the Michigan basin the Niagara
Cuesta, formed on the resistant Niagara Dolomite, is prominent. The Niagara Cuesta forms
impressive ridges along the northern and western edge of the Door Peninsula, along the
southern UP near US-2, and on Manitoulin Island in Lake Huron. Niagara Falls originally
developed on (fell over) the escarpment of this cuesta.
Source: Unknown
This material has been compiled for educational use only, and may not be reproduced without permission. One copy may be printed for personal use. Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.