Andrew’s Outcrops

Andrew is one of the many polite teenagers that Lanark County produces by the bushel load.   He also has a keen interest in geology.  This past weekend I took the opportunity to visit outcrops that Andrew had been telling me about.  

Most of the outcrops that Andrew showed me consist of flat lying sandstone exhibiting paired vertical burrows on the top surface.   I expect that everyone would identify this sandstone as a shallow marine facies of the Nepean (upper Keeseville) Formation of the Potsdam Group.   Below are photographs of the top surfaces of  two such sandstone outcrops, each showing paired vertical burrowing.

One outcrop stood out as being particularly impressive and is likely worth a second visit.  Below are photographs of this outcrop.This outcrop can be divided into three parts, with at least two visible unconformities (separating layers 1 and 2; and separating layers 2 from 3).  Starting from the base you notice:
1.)  at least 30 vertical feet of a silicious quartz cobble conglomerate; 
2.)  followed by a foot of sandstone with layers of quartz pebbles and a pitted layer (that may represent eroded evaporite minerals?);
3.)  with a top layer showing bedded  sandstone cut by vertical burrows, some of which extend down into the middle layer (perhaps evidencing the transgression of Cambrian/Ordovician seas and the reworking of fluvial sandstone?).

The two lower layers are of continental/fluvial origin, while the upper layer is of shallow marine origin.  

The first three photographs show the basal quartz cobble conglomerate.   The fourth photograph shows two unconformities, with the basal quartz cobble conglomerate at the bottom of the photo.   The fifth photo shows vertical burrowing and an uncomformity.  The sixth photo shows vertical burrowing in the top layer (shallow marine sandstone), overlying what appears to be two fluvial events: a pebble layer, over  sandstone, over a pebble layer, over sandstone (with the pebble layers possibly being lag deposits formed by aeolian deflation?).

           
I expect that everyone would assign the top burrowed bed to a shallow marine facies of  the Nepean (upper Keeseville) Formation.    Most people would likely  assign the quartz cobble conglomerate bed  to the Covey Hill (Ausable) Formation of the Cambrian/Ordovician Potsdam Group, with some assigning it a Precambrian age.   The middle bed is likely a fluvial facies of the Nepean (lower Keeseville) formation.

Andrew also showed me a mica mine in the Precambrian rocks and a lichen covered, deeply fractured, sandstone ridge that lies on an adjoining property

The photographed outcrops fall within Lot 7 of Concession VIII in North Burgess Township (now Burgess Ward of Tay Valley Township), about four miles (6.5 kilometers) south of Perth, and west of the intersection of Stanley Road with the road to McLaren Lake.   The lichen covered sandstone ridge can be found east of the road to McLaren Lake, on Lot 8 of Concession VIII.

If you were one of the original settlers who had been granted the land upon which Andrew’s parents have their house, or granted one of the adjoining lots, I suspect you’d have complained about the lack of soil.  If you have an interest in geology, which Andrew has, you’d be pleased to be living there.

Christopher Brett
Perth, Ontario   

Suggested Reading:
David G. Lowe, R.W.C. Arnott, Godfrey S. Nowlan, A.D. McCracken, 2017
Lithostratigraphic and allostratigraphic framework of the Cambrian–Ordovician Potsdam Group and correlations across Early Paleozoic southern Laurentia; Canadian Journal of Earth Sciences, Published on the web 6 February 2017,    doi: 10.1139/cjes-2016-0151