Copper Harbor Conglomerate
Mary Macdonald Preserve at Horseshoe Harbor
Located at the tip of the Keweenaw Peninsula on the shores of Lake Superior, Horseshoe Harbor has a beautiful outcrop of the unique rock, Copper Harbor Conglomerate. Like all sedimentary rocks, the Copper Harbor Conglomerate formed in a four stage process- erosion & transport, deposition and cementation. EarthCache visitors will “read the rocks”, examining the features of the Copper Harbor Conglomerate to understand the conditions present 1.1 billion years ago at the time of its formation.
Starting Point: Copper Harbor traveling east on highway 41 until you reach (latitude N 47o 27.757’ longitude W87o49.359’), turning North onto the two track road. Navigate to the trailhead at (N 47o 28.153’ W 87o48.224’). Proceed on foot to Stop 1. Remember to stay on established trails and practice “Leave No Trace” ethics.
Materials Needed for Visit
Background Information: Earth History of the Keweenaw Region
Lake Superior’s deep basin began forming 1100 million years ago when a ‘hot spot’ in the mantel below caused the crust here to stretch, thin and began pulling apart forming a great split scientists call the Midcontinent Rift. The rift, trying to break apart the eastern half of the North American continent from its western half, stretched more than 1500 miles from Eastern Kansas, arching through the Lake Superior region and extending southeasterly to Detroit (figure 2). Today we see this same process of continental rifting happening at the East Africa rift.
For millions of years during rifting, lava poured out of the cracked and fractured region covering the land with one lava flow after another, each layer cooling to form hard basaltic rock. As the years past these volcanic rock layers piled on top of one another became thicker and thicker, especially in the center of the rift where the lava emerged. The incredible weight of these rocks caused the thinned crust to slowly sag downward for hundreds of feet forming a large bowl shaped depression (figure 3). After 25 million years the rift eruptions ended rather abruptly and a long period of weathering & erosion of the exposed rocks began. Over the next several hundred million years the eroded sediments were slowly deposited and began to fill in the basin creating layers of conglomerate, shale, and sandstone rock.
During glaciation two million years ago these rocks became exposed to the surface again. As the glaciers advanced they scraped the overlying rocks away, leaving behind the more resistant basalt and conglomerate layers and creating the Superior Basin. As the last glaciers disappeared around 10,000 years ago their melt waters filled the basin creating Lake Superior.
Tour of Horseshoe Harbor Copper Harbor Conglomerate
Stop 1: N 47o 28.383’ W87o48.074’
Weathering – Breaking Rocks Apart
The first stage in the formation of sedimentary rocks like the Copper Harbor Conglomerate is the weathering of exposed rock. The fossil record tells us that there were no land plants one billion years ago to protect the land from the harsh weathering processes of sun, rain, ice and wind. Over time these elements broke apart the rocky surface into smaller bits and pieces of rock. By closely examining the sediment types that compose the Copper Harbor Conglomerate we can interpret what the older rocks were that made up the edge of the basin.
Erosion and Transportation- Sediments on the Move
The energy of wind and water can transport sediments to a new location. The dip created by the midcontinent rift left a downward slope on either side of the Superior basin. As rain fell on the land, these slopes allowed water to collect into streams and run downhill towards the center of the basin. These rushing waters provided the energy to erode or move the broken sediments from their parent rocks toward the center of the basin. Today we can see evidence of this slope in the tilted rocks all around us.
Stop 2: N 47o 28.398’ W87o48.257’
Deposition – Sediments settling down
When the energy of a river’s current is not strong enough to carry the sediments, they drop out, settle and deposit on the bottom. Bigger pieces, like the cobbles, settle out first because they require larger amounts of energy to move them. Smaller pieces such as sand and silt travel further downstream and usually only settle out in calm waters such as in puddles or ponds.
Layer after layer of the eroded rock and minerals were deposited on top of each other. Each layer of sediments represents the conditions present on Earth during the time of their formation. Digging through the layers of Copper Harbor Conglomerate throughout the Superior Rift Basin earth scientists have been able to determine that the sediment layers are similar to present day alluvial fans found in semi-arid regions.
Alluvial fans are fan shaped deposits of sediments that form when a fast flowing stream exits a canyon onto a flatter, open basin. The stream flattens, slows, and spreads out over the basin depositing the sediments it transported. The fan builds as time passes and layer after layer of sediments are laid down. Heavier sediments (gravels) are deposited first near the edge of the basin at the steeper, upper section of the fan. The lighter, fine-grained materials like sand and silt are carried to the fan’s edge where the slope is much more gradual. Typically the middle of an alluvial fan can have both layers of gravels deposited during years of heavy rain and flooding, as well as sand or mud layers deposited during drier years.
Cementation – Sediments turning into Solid Rock
Overtime these alluvial fans deposited sediments which were eventually covered by many other layers of sediments- silt and then sand- until the entire rift was filled. The layers were pressed down more and more through time, squeezing the sediments closer together. Groundwater passed through these compacted layers, precipitating minerals, which act like cement to bond the clasts together causing them to form the hardened rock unit we see in front of us today.
Access to Horseshoe Harbor
This cache is located within The Nature Conservancy’s Mary MacDonald Preserve in Michigan. For general information, please call 517-316-0300, or visit our website http://nature.org/michigan. All preserve rules and regulations apply. Park in parking lots only. Check the Conservancy website for preserve hours.
The preserve is remote and can only be reached by traveling down seasonal roads, some of which can be difficult for low clearance vehicles. No motorized vehicles, camping or fires are allowed on the beach. Please read all signs and follow posted guidelines while visiting the area. Be sure to travel on pre-established roads and trails, and be sure to practice ‘Leave No Trace’ guidelines.
Connection to the Earth Science Curriculum
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