The Parnassia Shorelines

Mississippi River at Playfairville looking upstream (photo by Cathy Keddy)
Mississippi River at Playfairville looking upstream (photo by Cathy Keddy)

Here, amidst a stretch of rapids on the Mississippi River, one can find an unusual array of plants, species that are more typical of the shorelines of the Great Lakes. This location is unique in Lanark County. One of the more striking species is the white flowered Grass of Parnassis (Parnassia glauca), which is not a grass, but a white-flowered herb with glossy leaves.

How did these plants arrive in Lanark County? Perhaps some of them arrived near the end of the last ice age, when a large glacial lake covered much of eastern Ontario. You can see a map of this lake on page 8 of my guide to the natural environment of Lanark County. This lake covered roughly the half southwestern side of the what is now Lanark County. The shorelines of this lake would have provided direct connection to the Great Lakes. At that time, the surrounding uplands were still covered with tundra.

As well there is apparently something special about this location that allows these plants to survive only here in the county. I suspect it has something to do with the calcareous rock, this being one of the few sections of the Mississippi River that flows through marble bedrock. As well, it is likely that the natural fluctuations in water level are important. Each spring, high water pulses may remove woody plants that would otherwise cover the shoreline.

Although the land here is private, you can get a fine view of the shoreline by looking upstream from the bridge at Playfairville. You can also see rapids and an old mill.

According to The Plants of Lanark County:

Rare plants in this area include: Eleocharis flavescens var. olivacea (Olive-brown Spike-rush), Ophioglossum pusillum (Adder’s-tongue Fern), Panicum tuckermanii (Tuckerman’s Panic Grass), Parnassia glauca (Grass-of-parnassus), Pilea fontana (Clearweed), Platanthera flava var. herbiola (Tubercled Orchid) (also provincially rare), Prenanthes alba (White-lettuce), Spiranthes lucida (Shining Ladies’-tresses), Torreyochloa pallida var. fernaldii (Fernald’s Manna Grass), Toxicodendron radicans ssp. negundo (Climbing Poison Ivy), Triglochin maritimum (Arrow-grass), and Trisetum melicoides (Melica-like Trisetum).”