Geology of the Porcupine Mountains Overlying the Porcupine Volcanics is a produced major thrust faults and folding sequence of fluvial (river deposited) of Keweenawan strata. As a result of this The bedrock of the Porcupine sedimentary rock known as the Copper deformation, the Porcupine Mountains Mountains records part of the evolution Harbor Conglomerate. The deposition of anticline, Presque Isle syncline and Iron of the North American continent during these rocks marks the end of rift River synclineÐall features that can be the late Precambrian Era. Rock strata extension and the beginning of observed in the parkÐwere formed. within the mountains belong to the sedimentation in the post-rift basin. The Keweenawan rocks are notable for Keweenawan Supergroup, a thick Conglomerates of the Copper Harbor significant copper deposits. Secondary sequence of volcanic and sedimentary formation contain clasts derived primarily mineralization by copper, epidote, quartz, rocks (exceeding 25 km) deposited about from Keweenawan Supergroup volcanic prehnite, and pumpellyite fill voids, 1.1 billion years ago, during and shortly rocks. fissures and vesicles in basalt flows. after an episode of continental rifting. A series of thin basalt flows marks a Utilization of the region's copper deposits This Midcontinental rift system brief return to volcanism during Copper was begun by prehistoric peoples active represents the early stages of continental Harbor deposition. These flows, called in the region long before European breakup and is a major geologic feature the Lakeshore Traps, form one of the arrival. Historic extraction of the copper of the North Ameerican Continent. park’s most prominent geologic features; resource began in 1845, when the region Outside of the Lake Superior region, the extended basalt-capped escarpment witnessed the nation's first mining rush. most of the Midcontinental rift is overlooking the Lake of the Clouds. The Pleistocene glaciation has left concealed by younger rock strata. In the Lying atop the Copper Harbor abundant evidence in the Porcupine Porcupine Mountains, the rocks that filled sediment is the Nonesuch Shale. This Mountains. Lake Superior, relic lake the rift and post-rift basin are well body of black to gray shales and shorelines, erosion of the bedrock exposed along the park’s Lake Superior siltstones was deposited in a relatively comprising the mountains, and extensive shoreline, rocky river beds, and shallow body of water occupying the ground moraine deposits are reminders numerous ridges and escarpments. post-rift basin or valley. of the region's glacial heritage. The oldest rocks outcropping in the The youngest exposed rock in the The last advance of the Wisconsin Porcupine Mountains are the massive mountains are the red sandstones and GlacialÐthe GreatlakeanÐdeployed ice in andesite and rhyolite flows visible siltstones of the Freda Sandstone. the Porcupine Mountains region about throughout much of the park's interior Exceeding 3 km thick beneath Lake 12,000 years ago, retreating about 9,000 uplands and easily visible at the Summit Superior, these rocks mark a return to years ago. As the ice sheet melted, water Peak Scenic Area. These rocks, known fluvial sedimentation in the post-rift impounded between the ice front and as the Porcupine Volcanics, lie atop basin. The Presque Isle river flows adjacent uplands produced a sequence older continental flood basalts of the across the tilted edges of the Nonesuch of proglacial lakes. Although these lakes Portage Lake Lava Series. The Porcupine and Freda formations at the Presque Isle were relatively short lived, they left Volcanics were deposited by a large Scenic Area, providing excellent signature shorelines along the north flank stratovolcano just south and east of the exposures of both rocks. of the Mountains. Carved step-like into mountains. This form of volcanism is the side of the mountains, these old unique in the rift system, which was Following rift extension and post-rift beaches are among the park's most dominated by flood basalts issuing from sedimentation, continental compression, prominent glacial features. long fissures along the rift axis. attributed to the Grenville Orogeny, Geologic Map Porcupine Mountains Wilderness State Park

Key to Geologic Strata Freda Sandstone r Nonesuch Shale perio oungest u y e S Copper Harbor Lak Lakeshore Traps Union Bay Campground Porcupine Volcanics -Andesite

oldest Porcupine Volcanics ior pment Overlook -Rhyolite Lone Rock Escar e Super Lak Lake of the Clouds pring Geologic Fault n S Unio Anticline - fold dips er k away from axis e ea

k P p Riv t a n Syncline- fold dips L e m toward axis r n Big Car o r r ve W r o i G h i M te . Los Correction Line N t P in L e r Mirror Lake a A

e k n v e i ti cl R in e p r a

C

e l t t Cross i Presque Isle L e lin ton Lily P c Campground L ond yn er i S ttl er and Scenic Area e C iv arp R r R Pink ive Summit on Peak Ir Road ior e Super Lak Little Carp River Road

P re sq u NORTH e MILES Is le P R re iv sq er Michigan 0 1 2 4 ue Is le DNR Sy nc line A Field Guide to the Bedrock of Porcupine Mountains Wilderness State Park, Michigan

Freda Sandstone- Copper Harbor, Copper Harbor, Sandstone- Porcupine Volcanics, The youngest of the Conglomerate- This reddish-brown Rhyolite- Keweenawan rocks found in Conglomerate is a sedimentary rock often This pinkish or gray volcanic the Porcupine Mountains. sedimentary rock, like contains wave ripple marks rock is the oldest rock found This medium to fine-grained sandstone or shale, that and mud cracks, intriguing within the Porcupine sedimentary rock is usually contains many rounded evidence of the waters that Mountains. Summit Peak brown, red or tan. You can pebbles cemented together flowed in the Porcupine and most of the park’s see Freda Sandstone along by finer materials. The Mountains region one billion interior uplands are the Presque Isle River near pebbles within the years ago. This sandstone composed of rhyolite. Look Nawadaha Falls at the conglomerate are usually can be seen along the Lake for this rock along the trail park’s Presque Isle River durable igneous rocks like Superior shore, in the Union to the Summit Peak Scenic Area. rhyolite or granite. Bay campground. Observation Tower.

Nonesuch Shale- Lake Shore Traps, Basalt- Porcupine Volcanics, Native Copper- The Nonesuch Shale Basalt is the dark, fine- Andesite- Keweenawan rocks are contains layers of gray to grained volcanic rock that Named for the Andes known for their deposits of black shale, siltstone and caps the prominent Mountains in South native or metallic copper, sandstone, all sedimentary “escarpment” ridge of the America, this fine-grained, which is rare elsewhere in rocks. See the Nonesuch Porcupine Mountains. Basalt reddish-brown volcanic rock the world. Within the formation along the Presque often contains numerous can be seen in the Porcupine Mountains, Isle River from Manabezho small voids created by gas Porcupine Mountains along copper is found in the Falls downstream to Lake bubbles trapped in the the Upper Carp River near Copper Harbor Sandstone Superior. The Nonesuch molten rock when it cooled. Trap Falls. (Andesite and and Nonesuch Shale. contains widespread copper Can be seen at the Lake of rhyolite are often grouped deposits. the Clouds Scenic Area. together and called felsite).