And the waters prevailed, and were increased greatly upon the earth...
Genesis 7:18
The Kaskaskia is the
younger of the two and lies on top of the Tippecanoe. Globally, these
layers are filled with shallow-water marine fossils, since at that point
the floodwaters had not yet begun to cover the dry land.
Some of these rocks are visible near the center of the island on the
highest point. Referred to as the Ancient Island, its peak is about 275
feet above today’s Lake Huron.
Here, near the War of
1812-era Fort Holmes, some of the lower 
Kaskaskia sediments are exposed,
known as the Devonian Bois Blanc Formation. Below this half-mile-long
hill and across most of the island reside carbonate rocks of the upper
Tippecanoe Megasequence (Silurian St. Ignace Dolomite).
These two
sedimentary layers make up the bulk of the island and were most likely
deposited within the first 40 days of the Flood. They were exposed as the floodwaters receded and were further sculpted by the post-Flood Ice Age.
Scattered around the island are several prominent “sea” stacks composed
of broken and angular pieces of rock. These resistant spires, stacks,
arches, and even a cave are composed of Mackinac 
Breccia.
Breccia is
made up of broken, angular chunks of rock melded together.
*These stacks formed in a manner similar to a sinkhole and were the
product of salt dissolution and collapse. In this case, the sediments
composing the broken pieces were deposited on top of the St. Ignace
Dolomite. Below this dolomite lie earlier Flood rocks that are rich in
salt.
During the Flood’s continued advance, water was forced into the ground
and dissolved some of the salt. With no support from below, the
overlying sediments collapsed, dropping fragments from above into the
vacant cavity. Swirling groundwater cemented these broken fragments
together.
As the receding Flood stripped away the surface layers, the breccia
stacks were left behind as “sea” spires. Glacial lake water further
carved some of these into caves such as Skull Cave and arches such Arch Rock.
Native Americans referred to the island as Michilimackinac, or “place
of the great turtle,” because in profile the island resembles a turtle’s
back.
This appearance is due to erosion during the two
primary Ice Age lake levels, the older Lake Algonquin and the younger
Lake Nipissing.
The Ancient Island was all that was exposed above the
water line when Lake Algonquin crested at over 230 feet above the
present lake level. Its waves carved a steep cliff marking the edge.
As the lake levels further dropped, over two dozen successive shoreline
ridges were formed prior to the formation of Lake Nipissing.
The two prominent wave-cut cliffs and the numerous shoreline ridges
preserve evidence of the many changing Ice Age lake levels. These
features always seem to be present wherever Ice Age lakes resided.....In fact, these landforms can occur in just a few weeks. Creation scientists calculate that the Ice Age lasted about 500 to 700 years after the Flood. The development of these landforms easily fits within the biblical timeline." ICR