3 CGB 3 – Gravenhurst
Sheila Ballantyne and Phillip Ruscica
GPS Coordinates (44°55’11.42″N , 79°23’6.26″W)
Parking: There are ample parking spots beside the outcrop at the farmers’ market or across the street near the marina.
The Gravenhurst site has two outcrop scans on the eastern side of the town. These stops are along Highway 169, and across the street from the Steamboating Monument.
Click here to open the bedrock geology legend on the OGS website
The images in the slides below highlight the foliated gneiss, cross-cutting pegmatite dykes, and pods of deformed mafic dyke material.The foliation is variable here but often sub-horizontal.
Stop 1 - Gravenhurst Gneiss
Digital Outcrop
Click on the arrows in the image below to view various perspectives of this outcrop. A link at the bottom left will take you to a 3D model of the outcrop in Sketchfab.
Digital Hand Samples
The following rock sample (VLS-21-38) is of the finely layered gneiss observed at the Gravenhurst outcrop. Click on the image (or here) to view the 3D model of the sample in Sketchfab! Note the alternating dark and light layers. The lighter layers are feldspar and quartz, while the darker grains are dominantly hornblende and biotite.
Thin Sections of Hand Samples
Sample VLS-21-38 was made into two thin sections, A and B. VLS-21-38A is cut parallel to the foliation, and VLS-21-38B is cut perpendicular to the foliation. These can be viewed in the slides below. Please note that thin section VLS-21-39 is also in these slides (#5 and #6) and the rock sample for VLS-21-39 is discussed further down in this section.
The following two thin section videos are of slide VLS-21-38A. These show the view parallel to foliation of sample VLS-21-38.
Plain Polarized Light (PPL) view of sample VLS-21-38A.
Crossed Polarized Light (XPL) view of sample VLS-21-38A.
The following two thin section videos are of slide 38B, these show the view perpendicular to foliation of sample VLS-21-38.
Plain Polarized Light (PPL) view of sample VLS-21-38B.
Crossed Polarized Light (XPL) view of sample VLS-21-38B.
Stop 2 - Gravenhurst Pegmatite Dyke
There are several pegmatite dykes throughout the long outcrop in Gravenhurst. Their relative ages are complicated and remain not well understood. Note the black pods of deformed mafic dyke here too, presumably of younger age than the pink pegmatite dyke.
Digital Outcrop
Please note that there is no 3D model made of sample VLS-21-39. The thin section scans are in the slides above, following VLS-21-38 A & B.
Thin Sections of Hand Samples
The following two thin section videos are close up rotating views of sample VLS-21-39. Note the large feldspar and quartz crystals, with minor biotite, hornblende, and unidentified opaque minerals.
Plain Polarized Light (PPL) view of sample VLS-21-39.
Crossed Polarized Light (XPL) view of sample VLS-21-39.
Additional photographs, detailed descriptions, and high resolution images of thin sections are available to post-secondary instructors. Please email virtual.petrology.network@utoronto.ca
All comments, questions, and concerns are always welcome at this email address.
Go to the next chapter: CGB 4 – Bala Falls!