Park in the parking lot for the conservation area. Walk west along the footpath, which leads to a boardwalk over a marshy area. The path then swings north among cedar trees to an open expanse of flat-lying white bedrock.
This large exposure offers an extensive bedding plane view of the quartz-rich sandstone typical of the Nepean Fm (formation). The sandstone beds are still in their original, horizontal position. The sandstone is composed almost entirely of well-sorted sand-size quartz grains that are cemented by quartz cement. There are abundant sets of trough cross bedding, from which the paleocurrent direction can be determined. Occasional sets of ripple marks show trends essentially perpendicular to the paleocurrent flow direction. The sand was likely deposited in a braided river environment, approximately 500 million years ago. However, the presence of a few elongate crystal pseudomorphs in the sandstone that are interpreted to have been gypsum (hydrated calcium-sulphate), suggests that the environment may have had a marine influence, or that the depositional waters may have occasionally become alkaline. This large outcrop displays crossbedding and ripple marks on a single bedding surface. Loose blocky slabs provide a view of composition, texture and parallel laminations (bedding) in the third dimension.
This locality illustrates preservation of:
- original horizontal bedding: same attitude as when deposited as loose sand
- primary structures: bedding, lamination, parting, crossbedding, ripple marks
- prominent secondary structure: joints, which here display consistent patterns (they occur in "sets")
- numerous parallel linear groups of glacially formed chatter marks, near the north side of the outcrop
The rock is an extremely mature siliciclastic sandstone, composed almost entirely of quartz grains held together by intergranular quartz cement — in most sections it breaks through the framework grains (i.e. it is well-indurated).
Directional data
- symmetric ripple marks seen on bedding may be shore-parallel
- asymmetric ripple marks and the arcuate traces of crossbedding indicate that paleocurrents were moving the unconsolidated sand southward, probably in a shallow braided stream system at the time of deposition (about 500 million years ago)
- several sets of chatter marks record the movement of ice, also southward, but much later during the Pleistocene Ice age, which ended as recently as 8000 years ago.
The outcrop surface displays abundant evidence of Pleistocene glaciation. Many top surfaces have been polished smooth by glaciation. Nested acurate chatter marks (opening towards the down ice direction) and glacial striations parallel to the axes of the chattermarks both indicate that the ice was moving in this area.
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Question to answer:
Send me the answer to the question below via eMail.
1. What is the texture of the rock?
2. Find the striations. Indicate which direction they are oriented, and from that, tell me what direction was the ice moving?
ALL INFORMATION COMPILED BY QUENTIN GALL, November 2010.