Deep Time: Lake Ontario’s Lucky Stones & Fossils

When did homo sapiens arise? Maybe 2 million years ago?

Nobody really knows exactly when we became “human,” but most of the rocks on our Lake Ontario beaches are at least a hundred times older than our species.

This is what the geologists mean by “deep time.”

Rocks, I have learned, lead more exciting lives than you might imagine. They are born, grow, crystallize, dissolve and re-form, changing ultimately to become clay or sand only to again become rock. This has been going on for about 4 billion years.

The geologists call the process The Rock Cycle. Just as souls in some religious beliefs are re-born, stones and boulders too undergo reincarnation. Rocks buried miles deep, are melted and transformed into new rocks, only to be uplifted by tectonics or erosion. Then weathering begins to transform them yet again.

Ontario’s south shore beaches are unusual. Thanks to glacier transport, the rocks and gravel came from many places to gather here. They include pebbles from the ancient rock of the Canadian Shield and the Adirondacks as well as more “local” sandstone and limestone pebbles.

One distinctive rock type I began collecting as a child is the “lucky rock.” According to local lore, the lucky ones are black pebbles with white bands of a crystalline mineral called calcium carbonate that circle the stone. The distinctive bands are formed after the stone cracked apart deep underground. Water saturated with minerals then bathed it and over time, the mineral precipitated and bonded the two pieces of rock back together. So lucky rocks really are fortunate to have survived their journey through time.

The Lake Ontario beach is a part of a system that is more complex and dynamic than we imagine. Who knew the life of a pebble could be so filled with adventure?  You erode out of the drumlin, tumble down to join the rest of the crowd, and here’s the water again – after ten thousand years of waiting inside a hill. Tumbling, rolling, crashing, smashing (at least if you’re not too big), you move along the shore at a right good clip. Some days you might travel several hundred feet.

Perhaps a really big storm hits and you end up in an “over-wash” and now you’re hanging out at the bar. The barrier bar that is. How long will you stay? Seedlings of cottonwood, sprouts of willow, roots of false indigo creep through the gravel, gripping, holding, and slowing the move. You might stay several decades or depart with the next big storm and move on again. Until you hit the jetty at Little Sodus Bay or get around it to move into the bay – and then stagnation.

I have long been fascinated by fossils. We have several types of fossils on the shores of Ontario. Some are casts or imprints left by hard bodied critters. I see these most often in a nondescript tan or brown rock that another beach comber recently identified looking like a burned brownie.  Another type of fossil is the “trace” which represents some sort of activity by a creature a couple hundred million years ago.

Worm tunnels that appear as holes are fairly common. The most conspicuous fossils I have found are in gray limestone beach pebbles and are of various marine animals whose bodies became mineralized with calcium carbonate. The bigger fossil creatures sometimes show the crystals of this mineral quite clearly.

During the Ordovician Era, Lake Ontario’s watershed was covered by a warm sunlit shallow sea. At this time all of Earth’s larger, more complex creatures were marine life. Crinoids (sea lilies), corals, graptolites, snail like gastropods, clam like brachiopods, and a variety of cephalopods with shells – ancestors to today’s cuttlefish and nautoloids – along with other creatures, swarmed in the shallows. Evidence from the fossil record shows that an abrupt change in climate caused the second most severe mass extinction of this thriving community of life in Earth’s history.

About 440 million years ago, for some as yet unknown reason, the amount of CO2 in the atmosphere decreased. Earth’s climate cooled and a global ice age followed.

Cooling temperatures and falling sea levels caused by water being locked up as ice in vast glaciers then wiped out the vast majority of marine life. As we entered the Silurian age, marine life slowly began to recover. It took a few million years, but most of the fossils we see today in Lake Ontario beach pebbles are of extinct life forms.

Photos, from above: limestone pebble with marine cephalopod; assortment of metamorphic rocks transported by glacial action to south shore beach – these are gneiss and granitic pebbles from Canada and or the St.Lawrence thousand is area; ‘healed’ rock aka lucky rock of Lake Ontario; and typical gravel barrier bar on Lake Ontario’s south shore (Blind Sodus Bay).