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Geology Trip North by Gerry Bennett

Page history last edited by Gerald Bennett 11 years, 5 months ago

Self Guided Tours of some Highways and byways In algoma






A Self Guided Geological Tour of part of Highway 17 North from Sault Ste Marie  

This page is under construction - Revisions and additions will be made from time to time.


Please note that the name and existence of commercial properties referred to below may have changed since this document was first written.

The tour begins by the St. Mary's river at the southernmost end of Church Street Sault Ste Marie, Ontario (60 Church Street).

These few pages will attempt to briefly comment on almost 3 billion years of the Earth's geological history as revealed in the rocks exposed along only 100 kilometers (62 miles) of Highway 17 north of Sault Ste. Marie. The guide is intended for use by those with little or no prior knowledge of geological concepts.

IMPORTANT: Because of high speed traffic you must at all times exercise caution when examining rock exposures along the highway. Side roads and off-road areas can usually be found to allow parking clear of highway traffic. Wear safety glasses if using a rock hammer. Avoid areas with overhanging, loose rocks. For reasons of content and safety we do not recommend this tour for young children.


0 km (0 miles) 60 Church Street. The office the Ministry of Natural Resources. Proceed north on Church

Street and Great Northern Road.

0.8 km  (0.5 miles) The shoreline of the Nipissing Great lakes.

This short, steep hill on Pim Street (also Bruce Street, 2nd Line etc) marks the former wave-cut shore of the Nipissing Great Lakes. About 5000 years ago the land in this area remained depressed by the great weight of the recently melted glaciers. The water level was higher than today, so Lakes Huron, Superior, Michigan and Nipissing were joined to form the Nipissing Great Lakes that drained eastward through Lake Nipissing and the Ottawa River.

The clay, which underlies much of Sault Ste. Marie above this hill, was deposited in still older glacial lakes when the water levels were much higher than even that of the Nipissing Great Lakes. The bedrock below the glacial lake clay deposits is red sandstone of the Jacobsville Formation. (1.1 billion years old) This sedimentary rock was formed from sand grains eroded from older rocks, transported by rivers and cemented together by iron, silica and calcium minerals. The Jacobsville sandstone is also exposed near Highway 17, where the Root River has washed away the overlying deposits of glacial sand and clay.

11.3 km (7.0 miles) The Archean Rocks of the Canadian Shield:

The “Canadian Shield” is the core of the North American Continent. Rocks of the Archean Eon are more than 2.5 billion years old and are the very oldest part of the Canadian Shield. Note the more rugged, rocky terrain of the Superior Province Canadian Shield in comparison to the relatively flat terrain of the area underlain by glacial lake deposits and much younger sandstone.

15.7 km (9.8 miles) Trout Lake Road. Continue north on Highway 17.


16.7 km (10.1 miles) Archean Granite, Granitic Gneiss intruded by a gabbro dike of undetermined absolute age.

(You may be able to park on an old section of Highway 17 branching east from he present highway). 

Here Highway 17 passes through an outcrop of light-colored (felsic), Archean granite and granitic gneiss (pronounced "nice") that have been intruded (cut through) by a dark colored (mafic) dike about 18 meters (60 feet) wide. 



The above photo shows the north contact of the Hayden gabbro (diabase) dike with granitic gneiss.


The granite and granite gneiss were formed from even older sedimentary and volcanic rocks, which were buried so deeply in the Earth and subjected to such heat and pressure, that they underwent partial melting.  This silica and aluminum-rich liquid gathered to form a large body of hot, viscous liquid (magma) which slowly rose upward because of it was less dense than the surrounding rocks. The magma eventually cooled and solidified deep within the Earth to form a rock we known as granite. The part which did not melt completely, cooled to form "gneiss" i.e. a light colored rock with dark layers or streaks a centimeter or so thick. In the billions of years since they were formed the gneiss and granites were thrust many kilometers upward to form the core of an ancient mountain range. Eventually, even the highest mountains were worn away and the old granite and gneiss were exposed for us to see.

The very dark colored (mafic) "dike" forms much of the central part of the outcrop. It can also be seen in the road cut on the west side of Highway 17. A dike is a steeply dipping, tabular body of rock representing where magma was forced upward through continuous fractures in the granitic rocks. Dikes range in thickness from less than a cm up to hundreds of meters across and may be from a few meters to hundred of kilometers in length. Dikes of basalt composition (gabbro/diabase) are the most common type in the Algoma area.

If you look closely at the gabbro within a few cm of the contact with the granitic rocks you will notice that the minerals in the gabbro are so small that they difficult to distinguish with the naked eye.  This is in marked contrast to the much larger mineral grains in the central portion of the dike.  The distinctly small grain size at the border of most intrusive igneous rocks is an indication of the rapid cooling of the magma where it makes contact with the older, cool granite. Geologists often use this “chilled edge” feature to determine the age relationship between two adjacent igneous rocks.

This dike was formed from an iron and magnesium-rich magma; some of which may have flowed out on the surface as lava to cool as a dark volcanic rock known as basalt (since eroded away).

17.3 km (12.2 miles)  Highway 556 to Searchmont and Ranger Lake.

Continue on Highway 17 north.

22.0 km (13.6 miles) View of the Goulais River Valley. The low, flat area along the Goulais River is underlain by sandstone of Jacobsville Formation.

28.2 km /(17.5 /miles) Goulais River Bridge: Continue north on Highway 17



39.1Km. (24.7 miles) Archean metavolcanic rocks (Flows): On the east side of Highway 17.


This outcrop dark-green and pale-green rock (with much graffiti when I last viewed it) is made up of the oldest solidified lava flows in this area (over 2.7 billion years old). These lava flows were subjected to intense heat and pressure during deep burial.  The heat and pressure conditions were sufficient to from a new suite of minerals but the overall chemical composition is only slightly modified.  A rock with such a changed mineral assemblage is known as a “metavolcanic rock”.

 In spite of the changes in these rocks, a geologist can see evidence of poorly preserved  “pillow structures” (as apple-green streaks) which tell him not only that these rocks were originally basaltic lava flows, but also, that the lava cooled under water.  



The above is a view of the outcrop of Atchean metavolcanic on the East side of Highway 17 N.



41.1   km /(25.5 /Miles) Jacobsville Formation (about 1 billion years old):

Outcrops of mottled reddish and buff-colored sandstone of the Jacobsville Formation are on the east side of the highway. This is the same formation, which underlies the cities of Sault Ste Marie Ontario and Michigan. The differences in color are due to the presence of two forms of iron oxides as cement holding the quartz and feldspar grains. The red/maroon color reflects the presence of the more oxidized (ferric) form of iron while the pale- buff colored areas are due to the presence less oxidized (ferrous iron). 

Sedimentary rocks are generally laid down in almost horizontal layers called "beds". However at this location the indistinct beds are tilted steeply as a result of nearby fault activity. A "fault" is a break in the rocks due to an earthquake. The movement along the adjacent fault blocks drags the normally horizontal bedding to an orientation more parallel to the plane of the fault.

43 km (26.7 miles) Harmony River. Continue north on Highway 17



55.2   km (34.3 miles) Chippewa Falls Unconformity:


--Picnic tables and washrooms are available in season.


Please be careful when walking on rock surfaces during periods of high water.

Take the trail for about 100 meters (yards) east from the parking lot to the rocky area overlooking the pool and the bridge. You should see an area of almost black rocks lying over pinkish granite. The dark rock is basalt, a solidified lava flow of Keweenawan age (about 1.1 billion years old). The basalt flow lies on pinkish granite 2.6 –2.8 billion years old. Pillow structures are present in the basalt near the contact with the granite. Such structures only form when basalt flows into water.

The contact between the basalt and the granite marks the surface of the Earth a billion years ago.


Such an old erosion surface is what geologists call an "unconformity" and indicates a long period of erosion following a period of tectonic uplift when formerly deep-seated rocks were thrust upward and eventually eroded. We know that there was a long period of erosion because the granite once formed the core of an Archean mountain chain.  We also know that most of this erosion took place before 2.45 billion years ago.




The above figure shows unconformity at Cheppawa falls


82.8 kin (51.5 miles) The Black Forest Motel  - 


Continue North on the highway.

The very dark colored rocks along the highway are mainly basaltic volcanic rocks of Keweenawan age (about 1.1 billion years old). Basalt is the most common type of mafic volcanic rock.

About 0.5 km (0.3 Miles) north of the Black Forest Motel rock-cuts expose a bright-pink felsic intrusion in the mafic (basalt) flows. The intrusion is best exposed on the east side of the highway. The intrusion is made up of pink-weathering felsite about 40 meters wide (?) and is easily identified among the dark basalts.  “Felsite” is a general name for a fine-grained intrusive rock of granitic composition.

The intrusive nature of the body can be shown by the numerous inclusions of altered basalt within the felsite near both contacts (be sure to examine the north contact zone as exposed on the west side of the highway).  The banding seen within the intrusion is known as flow-banding. Felsic magma is a very viscous liquid behaving more like a weak solid than a fluid.

Patches and seams of grey-weathering domeykite (Cu3As) are found within the intrusion.

(Please note that the above description is in part from memory without access to notes and subject to the usual caveats).


88.8 km /(55.2 /Miles) Keweenawan conglomerate and basalt flow:

Find a safe place to park north of the outcrop and walk back.

This conglomerate is loosely cemented gravel laid down about 1.1 billion years by fast flowing streams. Although about a billion years old, this conglomerate was never deeply buried and hence is much like it was when it was laid down. The clasts are mainly those of Archean granitic rocks and Keweenawan basalt. If you look closely at the contact between the conglomerate and the overlying basalt lava flow you may be able to see the "fingers" of once molten lava, which trickled down among the boulders of the gravel that was later cemented by white calcite (calcium carbonate) deposited by percolating ground water.



92.6 km (57.5 miles) Quebec Mining Co./Montreal Mining Co. boundary sign. Continue north on Highway 17.


99.6 km (61.9 miles) Ropy lava flow top on Keweenawan basalt lava flow:

On the east side of the highway a large flat rock face dips steeply to the south. Find a safe place to park.

 If you stand back and look up at the southward sloping flat rock surface you can see and area of "ropy structure" a few meters (yards) across. It resembles the surface of a viscous liquid such as corn syrup as is poured out. These curving, ropy structures mark the upper liquid surface of a lava flow that cooled about a billion years ago. Please do not take samples. You can find identical features on the surfaces of lava, which recently flowed from Hawaiian volcanoes. The convex side of the ropy folds point up the outcrop indicating that the lava flowed in that direction. This shows that at this point the land surface once tilted in an easterly or northeasterly direction. Note the many light colored "amygdules" in the basaltic flows. "Amygdules" are gas cavities (bubbles), which were trapped in solidifying lava. Later the cavities were filled by minerals deposited from the groundwater percolating along fractures in the volcanic flows.

If you walk north from the ropy lava outcrop and observe closely the rocks of the east side of the highway you will see the smooth flow tops of several mafic flows dipping steeply to the south.  The area below the reddish, smooth, curving, upper surface of the flow is marked by abundant amygdules while the base of the flow commonly contains few but larger amygdules.


100.4 km  62.4 (Miles) The Daisy Stone:

Park on a side road north of the outcrop and walk back. The "Daisy Stone" is a local name for an unusual variety of basalt lava flow. Basalt is made up mainly of the minerals feldspar and pyroxene. The daisy stone exhibits and unusual radial development of feldspar crystals giving the appearance of daisies. The most likely cause of this remarkable igneous texture is the result of rapid crystal growth in a super-cooled magma with few nuclei of crystallization.  Nuclei of crystallization is a framework of atoms so arranged that they from the beginning of a mineral molecule.


 Gerald Bennett, 1998/2009

658 Norman Crescent


Midland, Ontario











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