Pictured Rocks

National Lakeshore

Michigan

photo of the lakeshore at Pictured Rocks

Pictured Rocks National Lakeshore, Michigan

The geological history recorded in the sedimentary rocks and surfical deposits of Pictured Rocks National Lakeshore (PRNL) is limited to two widely separated intervals of geologic time; the Cambrian and Early Ordovician Periods (more than 440 million years before present), and the late Quaternary Period (two million years before present) to the present. During the Cambrian and early Ordovician periods, sediments were deposited in the shallow seas and near-shore deltas that covered what is now northern Michigan. These deposits became the sandstone units that are exposed within PRNL. Except for their exposure near Lake Superior, a veneer of Quaternary glacial drift presently covers these units.

Bedrock geology

Bedrock is best observed in the western one-third of PRNL where cliffs rise up to 180 feet from Lake Superior. These extend along the lake about 17 miles from Munising to Beaver Basin. For a short distance inland from the escarpment, bedrock is occasionally exposed.

The sandstones of the park are derived from fluvial and shallow marine deposits of quartz sand and gravel that were shed northward off a regional range of mountains, the "Northern Michigan Highland" and southwestward from highlands in adjacent parts of "Canada" (Hamblin, 1958). The "Highland" was probably similar in lithology to present-day outcroppings of pre-Cambrian rocks west of PRNL.

The Jacobsville Formation, of late Precambrian (Keweenawan) age is the oldest formation exposed in the park. It is a fluvial, feldspar-rich quartz sandstone, deep red in color with white mottling. The red coloration is thought to be primary and the white, a result of subsequent leaching along joints and bedding planes (Hamblin, 1958). The Jacobsville, regionally quite variable in thickness, is 1,100 feet thick in the Grand Marais area, however usually only the top few feet rise above lake level within PRNL (e.g. vicinity of Au Sable Point). This formation was quarried for building stone in the late nineteenth century. Several buildings in Munising and Marquette were constructed using Jacobsville block or facing. The western side of Grand Island, just west of PRNL within the Hiawatha National Forest, features spectacular Jacobsville cliffs.

The Late Cambrian (500-520 million years old), light grey and pinkish grey to white Munising Formation lies unconformably above the Jacobsville. The unconformity represents a time lapse of several million years; Jacobsville rock was deformed and eroded prior to the deposition of the Munising Formation. The Munising Formation probably represents a complex shoreline/shallow water environment that was influenced by fluvial, wave, tidal and eolian processes (Haddox and Dott, 1990). The Munising is divided into three members: the Basal Conglomerate, the Chapel Rock sandstone and Miners Castle sandstone.

The Basal Conglomerate unit is 2 to 15 feet thick. Its fluvially deposited pebbles and cobbles are comprised of vein quartz, quartzite, and chert with lesser amounts of slate, basalt, granite, iron formation, and sandstone. Some of the clasts are derived from the underlying Jacobsville.

Above the Basal Conglomerate lies the 40 to 60 foot thickness of the Chapel Rock Member. This member is pinkish grey or light buff to brown, medium grained, quartz sandstone. The large-scale crossbedding of this unit can be observed at its type locality, Chapel Rock. East of the mouth of the Mosquito River, it comprises virtually the entire section exposed in the Pictured Rocks (Hamblin, 1958). A few thin, dark grey, clayey mudstones are scattered throughout the Chapel Rock Member. This member displays several other striking sedimentary structures including mud cracks, ripple marks, channels, animal trackways, clay pellets and clastic dykes. Some of these features can be seen in exposures near the mouth of the Mosquito River (Haddox and Dott, 1990).

The contact between the Chapel Rock Member and the overlying Miners Castle Member is easily identified by changes in color (related to presence of clay), in type of cross-bedding (large scale vs. small scale), and in geomorphic expression of the rock units. The resistant, light pinkish-gray Chapel Rock Member has large dimension cross beds and forms steep cliffs and rock benches; the crumbly, multi-hued (red, yellow, green, blue) and gray Miners Castle Member has small dimension cross beds and forms slopes. The differences between the two members suggests differences in sediment source areas and environment of deposition.

The Miners Castle Member is a soft, crumbly, quartz sandstone (with abundant garnet content) about 140 feet thick; its complete section is exposed in the Pictured Rocks Cliffs between Sand Point and Miners Castle. Sediments of this member are generally poorly sorted.

Capping the easily eroded Miners Castle Member of the Munising Formation in the western half of Pictured Rocks, is the resistant Early Ordovician (480-500 million years old) Au Train formation. The Au Train Formation is a light brown to white dolomitic sandstone that forms the resistant cap to the underlying softer sandstones. The numerous falls in Pictured Rocks National Lakeshore are the result of this caprock.

Fossils are completely absent from the Jacobsville Formation and very rare in the Munising Formation; trilobites tracks (Cruziana and Rusophycus) have been found in the Miners Castle Member. The Au Train contains Early Ordovician cephalopod, conodont, brachiopod, and gastropod fossils.

Structurally, PRNL lies along the northwest edge of the Michigan Basin, thus sedimentary bedding dips gently toward the southeast. This dip causes the Precambrian Jacobsville to be exposed extensively on Grand Island, at the waters edge at Grand Portal Point and at Au Sable Point where it forms shoals in the lake creating a need for the lighthouse. The Jacobsville can also be seen in the gorge at Sable Falls.

Late Pleistocene and Holocene Geology/Surfical Geology

During the Pleistocene epoch, ice sheets of all four North American glacial stages advanced and retreated through the area (Dorr and Eschman, 1972). The Greatlakean advance, one of the last substages of the Wisconsinan glacial period, wiped the surface clean and left only its record. This ice sheet completed its advance near the present location of Two Creeks, Wisconsin (just southeast of the city of Green Bay) about 11,850 years ago. Ice then began slowly melting, sometimes stabilizing and occasionally re-advancing short distances. A brief re-advance, the Marquette substage, occurred about 10,000 years ago in northern Upper Michigan (Hughes, 1978; Farrand and Drexler, 1985).

Melting of glacial ice within the Superior Basin produced huge rivers that deposited millions of tons of pulverized rock rubble in various configurations to the south of the Superior basin. A sheet of outwash, various in thickness, was deposited along the south edge of PRNL between Wetmore and Seney by southward-flowing glacial streams. The material underlying the present-day Kingston Plain was deposited in this way. Occasionally great ice blocks detached from the glacier terminus and were buried in the outwash. Eventual melting of these blocks caused surface depressions, which, in some cases, became water filled. Kingston, Nevins, and Grand Sable Lakes are examples of these "kettle hole lakes." The Grand Sable Banks near Grand Marais originated as a glaciofluvial crevasse filling or kame terrace.

The high band of hummocky topography adjacent to Lake Superior in central Alger County was initially interpreted as a terminal moraine (Leverett, 1929; Bergquist 1936). Blewett and Rieck (1987) suggest that the "Munising Moraine" (and the parallel "Newberry Moraine" farther south) are better portrayed as complex ice wastage features since they contain much stratified drift and lack characteristics associated with active ice.

As the Marquette ice terminus retreated northward, a series of lower drainage outlets were uncovered. The ice front confined a large, west-to-east draining, meltwater river against ice-free land to the south. Drainage shifted from the south (along the Au TrainWhitefish Channel), to the east along the Pictured Rocks area (Drexler, 1981). Meltwater carved several channels into Cambrian sandstone bedrock; the most prominent of these are now occupied by Chapel Creek and Mosquito River and by Beaver Basin. As ice retreated completely from the Superior Basin, shorelines in the basin receded rapidly northward leaving the Pictured Rocks area "high and dry" about 9500 years ago (Farrand and Drexler, 1985). This occurred as outlet channels to the east remained at low levels due to the recent loading on the earth's crust by glacier ice.

Between 6,000 and 4,000 years before present, rebound of the earth's crust from its "depressed" state began to accelerate as land was relieved of the huge weight of the ice sheets. The rise of the outlet of ancestral Lake Superior (at North Bay, Ontario) caused the lake level to rise relatively quickly to a level roughly 35 feet higher than present Lake Superior (Larsen, 1987). This high lake stand has been designated glacial Lake Nipissing. As ancestral Lake Superior rose during the Nipissing transgression (about 5,000 years before present), the Grand Sable Banks were destabilized and part of the glaciofluvial deposit was reworked by wind to form the Grand Sable Dunes (Marsh and Marsh, 1987; Farrell and Hughes, 1985). During the Nipissing "high stand," Chapel Rock and Miners Castle as well as many less prominent features (such as perched sea caves near Little Beaver Lake Campground) were carved into the Cambrian sandstone by wave action. Beaver, Trappers, Little Beaver, Chapel, Little Chapel, and Miners Lakes represent embayments on ancient Lake Nipissing.

Slowing of rebound, downcutting of channels through unconsolidated material, shifting of outlets to the south, and climatic change subsequently caused a lowering of Lake Superior to near its present level (Farrand and Drexler, 1985; Larsen, 1987)(1). As erosion lowered the Lake Nipissing outlet to the modern Lake Superior level during a 1,600-year period, lake currents deposited a succession of parallel beach ridges from the Nipissing level to the present beach. These closely spaced ridges which form a "corrugated plain" (Bergquist, 1936), are evident in the vicinity of Au Sable Point, along the trail from Little Beaver Lake Campground to Lake Superior, on Sand Point, and on the tombolo between Trout and Murray's Bay on Grand Island.

Since much of eastern Upper Michigan is characterized by low relief and a covering of glacial drift, bedrock only occasionally controls surface geomorphology. Where the veneer of drift is thin, as in most of PRNL, a gentle, east-west trending, southward dipping cuesta that formed on the resistant Au Train formation is evident (Dory and Eschman, 1972, p.98). Within PRNL, this cuesta comprises the Pictured Rocks themselves. All north-flowing streams in Alger County form waterfalls as they cross the cuesta (e.g. Miners Falls, Au Train Falls, Laughing Whitefish Falls).

(1) Influences of differential rebound and shifting lake outlets make the regional picture of events complex and appropriately referencing past lake levels to levels of the present is difficult. Statements made herein about lake levels are meant to convey a general picture of large scale patterns only.

References (asterisk denotes major source for text)

Agassiz, L. 1850. Lake Superior: Its Physical Character, Vegetation and Animals Compared to Those of Other and Similar Regions. Boston, Gould, Kendall and Lincoln. 428 p.

Bergquist, S.C. 1936. The Pleistocene history of the Tahquamenon and Manistique drainage region of the northern peninsula of Michigan. Michigan Geological Survey Publication 40, Geological Series 34, part 1 pp.7-148.

Blewett, W.L. and R.L. Rieck. 1987. Reinterpretation of a portion of the Munising Moraine in northern Michigan. Geological Society of America Bulletin 98:169-175

Dorr, J.A. and D.F. Eschman. 1972. Geology of Michigan. University of Michigan Press. 476p.

*Drexler, C.W. 1981. Outlet channels for the post-Duluth lakes of the Upper Peninsula of Michigan. PhD dissertation, University of Michigan, Ann Arbor. 295 p.

Drexler, C.W., W.R. Farrand and J.D. Hughes. 1983. Correlation of glacial lakes in the Superior Basin with eastward discharge events from Lake Agassiz. in Teller, J.T. and L. Clayton (eds.) Glacial Lake Agassiz. Geological Association of Canada Special Paper 26, pp. 309-329.

Farrand, W.R. and C.W. Drexler. 1985. Late Wisconsinan and Holocene history of the Lake Superior Basin. in Karrow, P.F. and P.E. Calkin (eds.) Quaternary Evolution of the Great Lakes. Geological Association of Canada Special Paper 30, pp. 18-32.

Farrell, J.P. and J.D. Hughes. 1985. Long term implications, from a geomorphological standpoint, of maintaining H-58 in its present location at Grand Sable Lake. Contract report to the National Park Service, Munising, MI. 78 p.

Foster, J.W. and J.D. Whitney. 1851. Report on the geology of the Lake Superior Land District, Part II, Washington, D.C., U.S. 32nd Congress, Special Session, Senate Executive Document #4. 419 p.

*Haddox, C.A. and R.H. Dott. 1990. Cambrian shoreline deposits in northern Michigan. Journal of Sedimentary Petrology 60:697-716.

*Hamblin, W.K. 1958. Cambrian sandstones of Northern Michigan. Michigan Department of Conservation, Geological Survey Division, Publication 51. 146 p.

Hough, J.L. 1958. Geology of the Great Lakes. University of Illinois Press, Urbana. 313 p.

Hughes, J.D. 1978. Marquette buried forest 9850 years old: Abstract for the American Association for the Advancement of Science Annual Meeting, February 12-17.

Larsen, C.E. 1987. Geological history of Glacial Lake Algonquin and the Upper Great Lakes. U.S. Geological Survey Bulletin 1801. 36 p.

Leverett, F. 1929. Moraines and shorelines of the Lake Superior region. U.S. Geological Survey Prof. Paper 154-A. 72 p.

Marsh, W.M. and B.D. Marsh. 1987. Wind erosion and sand dune formation on high Lake Superior bluffs. Geographiska Annaler 69:379-391.

Taylor, F.B. 1895. The Nipissing beach on the north Superior shore. American Geologist 15:304-314.

TOP OF PAGE

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

TOP OF PAGE

A general photo album for this park can be found here.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

TOP OF PAGE

Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.

TOP OF PAGE

Information about the park's research program is available on the park's research webpage.

For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.

TOP OF PAGE