![]() Glacial lake water further carved some of these into caves such as Skull Cave and arches such as the previously mentioned Arch Rock. Swirling groundwater cemented these broken fragments together, and, rather oddly, they became more resistant than the surrounding bedrock.Īs the receding Flood stripped away the surface layers, the breccia stacks were left behind as “sea” spires. With no support from below, the overlying sediments collapsed, dropping fragments from above into the vacant cavity. ![]() ![]() Below this dolomite lie earlier Flood rocks that are rich in salt.ĭuring the Flood’s continued advance, water was forced into the ground and dissolved some of the salt. In this case, the sediments composing the broken pieces were deposited on top of the St. These stacks formed in a manner similar to a sinkhole and were the product of salt dissolution and collapse. Arch Rock is a rare formation of limestone breccia and stands on the Lake Huron shoreline 146 feet above the water. The largest stack of Mackinac Breccia is called Sugar Loaf and is about 75 feet tall. Breccia is made up of broken, angular chunks of rock melded together. These resistant spires, stacks, arches, and even a cave are composed of Mackinac Breccia. Scattered around the island are several prominent “sea” stacks composed of broken and angular pieces of rock. 2 They were exposed as the floodwaters receded and were further sculpted by the post-Flood Ice Age. These two sedimentary layers make up the bulk of the island and were most likely deposited within the first 40 days of the Flood. Below this half-mile-long hill and across most of the island reside carbonate rocks of the upper Tippecanoe Megasequence (Silurian St. 1 Here, near the War of 1812-era Fort Holmes, some of the lower Kaskaskia sediments are exposed, known as the Devonian Bois Blanc Formation. Referred to as the Ancient Island, its peak is about 275 feet above today’s Lake Huron. ![]() Some of these rocks are visible near the center of the island on the highest point. 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. The Kaskaskia is the younger of the two and lies on top of the Tippecanoe. Mackinac Island exposes rocks from two of the earliest megasequences of the Flood, the Tippecanoe and the Kaskaskia. Photo of the majestic Grand Hotel on Mackinac Island circa late 1920s, built in 1887 ![]()
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