Traveler
Geologists use different features to read the past history of the Earth. Seen on the beach at Selkirk Provincial Park are two types of boulders (Photo 1). Each tells a very different history. One type is a long distant traveler.
Photo 1: Two boulders on the beach, Selkirk Provincial Park. The boulder on the left, labeled “LS”, is 385 million year old limestone, which is likely derived from the local bedrock (Onondaga Formation). The boulder on the right is 1 billion year old Precambrian gneiss, labelled “GN”, which is likely derived from the Grenville geological province, located 320 kilometers (200 miles), or more, to the north. During the Wisconsin glaciation, an ice sheet transported the Precambrian gneiss to this general area during the last ice age. The Precambrian gneiss is a long-distance traveler. Photo source: Andy Fyon, Lake Erie beach, Selkirk Provincial Park, September 19, 2022.
On the right is a 1 billion-year old dark-coloured rock made of silicate minerals that formed deep in the Earth. Those minerals are arranged in layers that were created by intense pressure, heat, and strain (squeezing of the rock). The rock responded to the heat and pressure the way plasticine flows when you push on it. Geologists call this type of metamorphic rock a gneiss (pronounced "nice"). The closest bedrock of this type is located about 320 km to the north in the Canadian Shield.
On the left is a limestone rock that formed about 385 million years ago in a subtropical ocean. Geologists call this the middle Devonian epoch. The limestone rock is made of the mineral calcite and locally contains the fossilized remains of ancient marine animals and corals. This type of rock forms the bedrock along the north shore of Lake Erie, in Selkirk Provincial Park.
Although the two rocks sit beside each other, the dark gneiss was carried at least 320 kilometers (200 miles) by an ice sheet, or glacier, during the last ice age. This was called the Wisconsin glaciation. The Wisconsin glaciation lasted from about 75,000 to 11,000 years ago. It was the last time that a great ice sheet covered covered this part of Ontario. The gneiss boulder was dropped in this area when the Wisconsin ice sheet melted about 10,000 years ago. The limestone likely was plucked from the local bedrock, but it does not appear to have travelled very far.
So, two rocks, two very different histories, one travelled a great distance thanks to the flow of the glacier that covered this area during the last ice age, and one that is local.
Next time you stand on a beach, look down at the pebbles and try to group them into different types based on colour, or texture, or minerals. That is the first step in gaining a deeper understanding of the geological forces that shaped the Earth.
Nov 7/22; Facebook post: Sept 19/22