Road Log for Highway 17 North
For maps and diagrams relating to this road log click here .
0 km (0 Miles) 60 Church Street. At the St. Mary's River. Proceed north on Church Street and Great Northern Road.
0.8 km = 0.5 Miles. The former 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 7,500 years ago the land in this area remained depressed by the weight of the recently melted glaciers causing the water level of the Great Lakes to be higher than today. The high water levels meant that Lakes Huron, Superior, Michigan and Nipissing were joined to form the "Nipissing Great Lakes". During this period the Great Lakes drained to the Atlantic through Lake Huron via the French and Ottawa Rivers. The clay deposit which underlies much of Sault Ste. Marie above this hill was deposited during even older stages of the glacial lakes when the water levels were much higher than even that of the Nipissing Great Lakes.
The bedrock, now buried under the clay deposits in the Sault Ste Marie area, is red sandstone of the 1.1 billion year old, Jacobsville Formation (a.k.a. Jacobsville Sandstone). This sedimentary rock was formed from sediment eroded from the older, mainly granitic rocks, transported by rivers into a basin and cemented together by iron, silica and calcium-bearing minerals.
For a legend relating to this road log click here
The sandstone of the Jacobsville Formation is exposed near where Highway 17 crosses the Root River. If water conditions of the Root River allow, you may be able to access the Jacobsville Sandstone at the Root River via a short road leading from the east side of Highway 17 near this point.
11.3 km (7.0 miles) The Archean Rocks of the Canadian Shield:
A legend for the formations in the area is shown here
The Canadian Shield is the ancient core of the North American Continent. At this point the outcrops of mainly granitic rocks are more than 2.6 billion years. Note the more rugged, rocky terrain of the Superior Province of the Canadian Shield in comparison to the relatively flat terrain of the area underlain by glacial lake deposits and much younger sandstone of the .
15.7 km (9.8 miles) Trout Lake Road. Continue north on Highway 17.
STOP 17-1 - N46deg. 38.279min : W84deg.18.272min
16.7 km (10.1 miles) Granite, granitic gneiss and diabase of the Canadian Shield.
You may be able to park on an earlier section of Highway 17 branching east from the present highway just south of the rock-cut.
Here Highway 17 passes through a rock-cut of light-coloured, Archean granite and granite gneiss (pronounced "nice") which has been intruded by a dark-coloured (mafic) dike about 18 meters (60 feet) wide, known locally as the "Hayden Diabase".
The very dark-coloured (mafic) rock forms much of the central part of the outcrop. The dark-coloured rock is a gabbro dike. A dike is a steeply dipping, tabular body of rock which represents a place where magma was forced upward through continuous fractures within the much older granitic rocks. The continuation of this dike can be seen in the outcrop on the west side of Highway 17. This dike was once an iron and magnesium-rich (basaltic) magma; some of which may have flowed out on the surface as lava to cool as a dark, volcanic "basalt"; most, or all of which has eroded away long ago. That part of the magma which cooled to solidify within the fracture below the present surface is now visible as a "dike". Dikes commonly form parallel "dike swarms" over wide areas of the Earth's crust where tensional forces produced widening fractures.
Sharp eyes may notice another steeply-dipping, dark-coloured dike within the "Hayden Diabase" near it's southern boundary. The younger dike (we know it is younger because it cuts across (intrudes) the surrounding gabbro. The younger dike is less than a meter wide and may be a lamprophyre dike.
17.3 km (12.2 miles) Highway 556 leading 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.
STOP 17-2
39.1 km (24.7 miles)
Archean metavolcanic rocks (lava flows).
On the east side of Highway 17 about 200 meters (yards) south of McCauley's Hotel. These dark-green to pale-green metavolcanic rocks are about 2.7 billion years old, and are among the oldest rocks in the area. These ancient lava flows were subjected to great heat and pressure during deep burial. As a result new minerals were formed to make a "metamorphic rock". (Hence the term "meta"volcanic rock.) In spite of the changes in these rocks, a trained geologist can see evidence of ancient pillow structures which show that the lava flowed and cooled under water. (See Stop ELT-13 of the "Roadside Geology of Elliot Lake Roads" for more on pillow structures.)
STOP 17-3 N46deg 49.831min : W084deg 23.593min
41km (25.5 Miles) Jacobsville sandstone (about 1.1 billion years old):
Outcrops of mottled, red and buff-coloured sandstone of the Jacobsville Formation exposed are on the east side of the highway. This is the sedimentary rock which also underlies the cities of Sault Ste. Marie, Ontario and Michigan, and much of the surrounding area (see map 1). The differences in colour is due to the presence of two forms of iron oxide. The red colour is due to the oxidized form of iron (ferric iron) while the paler colours are the result of a gain of electrons to make ferrous iron. The color variations probably occured after deposition
Sedimentary rocks are first, deposited as sediment in horizontal, or almost so, layers called "beds". However here the indistinct beds seem to be tilted steeply as a result of nearby fault activity following deposition. A "fault" is a regional-scale fracture in the rocks due to an earthquake that occurred many millions of years ago.
43 km (26.7 miles) Harmony River. Continue north on Highway 17.
STOP 17-4 N46deg 55.759 min W084deg 25.540min
55.2 km (34.3 miles) Chippewa Falls unconformity
Follow the trail for about 100 meters (yards)east from the parking lot to the rocky area overlooking the river to the bridge on Highway 17. You should see an area of almost black-coloured rocks lying over pinkish granite. The dark rock is basalt, a solidified lava flow of Keweenawan age (about 1.1 billion years old (1.1 bybp). The basalt flow lies upon pinkish granite more than 2.6 billion years old. The contact between the basalt and the granite marks the surface of the Earth about a billion years ago. Such an old erosion surface is what geologists call an "unconformity". An unconformity indicates a period of tectonic uplift when formerly deeply buried rocks were thrust up during a period of mountain building and eventually eroded and covered by younger rocks.
82.8 km (51.5 miles) The location of the former Black Forest Motel. Continue north along Highway 17.
The very dark-colored rocks along the highway are mainly basalt volcanic rocks of Keweenawan age (about 1.1 billion years old). Basalt is the most common type of volcanic rock.
STOP 17-5 N46deg 58.712 min : W084deg 46.057 min
Rock-cuts expose an series of outcrops of bright-pink to orange-colored felsite (a general term for a fine-grained, light-coloured, igneous rock) and dark-coloured (mafic) basalt flows. The felsite is best exposed on the east side of the highway. The it is made up of pinkish-weathering felsite about 40 meters wide(?) and is easily identified among the dark basalts.
Some geologists have examined this outcrop and determined that the pink felsite is a rhyolite volcanic flow (solidified lava) that once flowed upon the basaltic surface and was later covered by basalic flows. Both the south and north contacts between felsite and basaltic rocks are exposed along Highway 17 in this area. If you closely examine the nature of the contacts between the felsite and the basalt you will see that fragments of the basalt are included witrhn the felsite. This shows conclusively that the felsite has intruded into the basalt flows.
/p>Patches and seams of the grey-weathering mineral "domeykite" (Cu3As) are found within the felsite intrusion.
This is a rare mineral in this area so it is requested that you refrain from taking specimens.STOP 17-6 N47deg 00.201min : W084deg 47.118 min
88.8 km (55.2 Miles) Keweenawan conglomerate and basalt flow.
Find a safe place to park north of the outcrop and walk back. A conglomerate is an ancient gravel deposit cemented by calcite (calcium carbonate). It is therefore a sedimentary rock made up of sand, pebbles, cobbles and boulders of older rocks deposited by fast flowing streams. Although more than a billion years old, this conglomerate was never deeply buried and is much as it was when it was laid down. The clasts are mainly cobbles and boulders of Archean granitic rocks loosely cemented by white calcite. If you look closely at the contact between the conglomerate and the overlying basalt lava flow on the east side of the highway you will see "fingers" of once molten lava that trickled down among the boulders of the conglomerate.
STOP 17-7
99.6 km (61.9 miles) N47deg 05.465 : W084 deg 43.130 min 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 sloping flat rock surface you can see and area of "ropy structure" a few meters (yards)across in the central portion of the outcrop. The ropy structure resembles the surface of a viscous liquid such as cold, corn syrup as is poured out.
These curving, ropy structures mark the upper 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 upwards along the outcrop indicating that the lava flowed in that direction. This shows that, at this location, the land surface once tilted in an easterly or north easterly direction (i.e away from Lake Superior) in contrast to the present southward dipping flow surface indicating a collapse of the area now underlain by Lake Superior
Note the many light-coloured "amygdules" up to a few centimetres across in the basaltic flows. "Amygdules" represent spherical, gas cavities which were trapped in solidifying lava, and eventually filled by minerals deposited from the hot water percolating along fractures in the volcanic flows.
If you walk about 50 meters (150 feet) north from the ropy lava outcrop you will see the smooth, amygdaloidal, ropy (pahoehoe) flow tops of several mafic flows dipping steeply to the south.
Can you estimate the true thickness of some of these basaltic volcanic flows?
STOP 17-8 N47deg 05.809 : W084deg 42.872 min
100.4 km 62.4 (Miles) The Daisy Stone:
The "Daisy Stone" is a local name for an unusual variety of basaltic lava flow. This basaltic rock is made up mainly of elongated tablets of plagioclase feldspar that form a rosette pattern somewhat resembling daisies. It is believed that these feldspar crystals grew rapidly in a supercooled basaltic liquid. If there are relatively few centers of crystalization in a basaltic magma crystallization may be delayed until the temperature of the magma falls below a temperature at which crystallization would normally occur. When crystallization eventually occurs under such "supercooled" conditions crystal growth takes place rapidly with several feldspar crystals growing outward from a single center of crystallization.
END OF TRIP
Gerald Bennett. g.bennett@geoben.ca