When the sea dried up.
Photo courtesy Metcalfe Geoheritage Park Committee.
The large raised lines on this remarkable display rock are ancient mud cracks that formed when the bottom of the Ordovician ocean was exposed to the atmosphere. This unique specimen was collected during the construction of the nearby hydro generating station.
Continent Under Water
During the Ordovician Period our part of the world was covered with warm, shallow seas. What would later become North America was part of the paleocontinent Laurentia, situated close to the equator.
Image from Chapter 7 of the book “Four Billion Years and Counting” published by the Canadian Federation of Earth Science and Nimbus (available at the Almonte branch of the Mississippi Mills Library. 557.1 FOU). Image downloaded from http://www.fbycbook.com/chapter-7. Original by Ron Blakey ( http://jan.ucc.nau.edu/rcb7/globaltext2.html), with additions by Asia Reid.
Much of the North American protocontinent was under water during the Ordovician Period. Land is shown in green and brown on this these paleographic maps. The lighter blue areas represent shallow water conditions, and the darker blue indicates deeper ocean waters. Some modern geography has been added for orientation.
How did this rock form?
When the sea floor was exposed to the atmosphere, the muddy sediment dried in the sun. Mud cracks, also known geologically as desiccation cracks, formed as the surface layer contracted. The cracks were later filled with sand that accumulated in thick layers across our region. The sand was eroded from highlands and deposited in near-shore coastal conditions as the ocean level gradually rose to cover much of the continent.
This display specimen was recovered from the excavation of the building site for the 4.6 MW hydro generating station at the other side of Metcalfe Geoheritage Park. You’re looking at the underside of the rock.
Photo by Asia Reid.
The red lines of this image were added to highlight the intersecting pattern of the mud cracks. The sand filled cracks, which are more resistant to weathering, form rounded ridges.
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