Located on Airport Road in North Bay is a rare outcrop of unique stones called Corestones (also called Woolsack).
Corestones, are rounded boulders that can be found individually, or in piles, sometimes laying on top of bedrock layers or in exposed outcrops.
Corestones or Woolsack weathering in sandstone at Externsteine rocks, Teutoburg Forest, Germany
Corestones are an example of jointed bedrock that have undergone Spheroidal weathering, a form of chemical weathering that decays the Saprolite or “rotton rock” that is easily weathered rock surrounding the corestone, massive rocks that include granite, dolerite, basalt and sedimentary rocks including sandstone.
When Saprolite is exposed by physical erosion, concentric layers peel, or spall off the rock, similar to layers of a peeled onion. This peeling leaves behind a rounded, un-weathered stone ( corestone). More intense weathering results in a continuous transition from saprolite to laterite.
Laterite is both a soil and a rock type, rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and prolonged weathering of the underlying parent rock, usually when there are conditions of high temperatures and heavy rainfall with alternate wet and dry periods.
Both Laterite and Saprolite are made up of eroded rock called Regolith, a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestrial planets and moons.
How did these stones get here?
Ontario’s Geology only records a partial record of the past, with environments left unrecorded by the rocks left for geologists to study. For example, there is a huge gap from the Devonian era 350 million years ago to just a few million years ago. This gap in the geologic timeline is called an Unconformity. This likely results from global, geological changes, which may have been caused by sea level regressions, which in turn stopped or ceased sedimentation.
In southern Ontario, the Precambrian-Paleozoic unconformity indicates that the underlying rocks were exposed in a subaerial environment and subjected to periods of weathering and erosion. Geologists believe, like elsewhere in North America, the interval between the Devonian and Pelistocene ice ages was subjected to more a more modern, warmer, wetter climate, and this subjected these rocks to chemical weathering. Minerals including feldspar were dissolved forming cracks around the harder core stone, causing the softer minerals in the rocks to “rot” and dissolve leaving a soft deposit of regolith or grus.
This deep weathering happened underground and the once hard gneiss rock weathered, while the softer minerals dissolved, leaving a highly resistant quartz.
This outcrop likely survived further erosion from advancing glaciers thanks to its location in the lee of the Escarpment.
Logging Requirements
To Log this Earthcache
Please send me your answers within 4 days of posting your found log. If there is more than one cacher in your party, include the names in your group. Only one person needs to send me the group answers. No spoiler photos, please. Found logs posted without proof you visited the site will be deleted
1. Look at the rouned corestones sticking out from the embankment on Airport Rd. Find the largest stone closest to the light post, the one on the right of the three large core stones. What is the height and length of this stone?
2. Based on the description, what type of weathering is attributed to the rounded corestones?
3. Look closely at the regolith layers that surround the corestones. Based on the description found on the cache page, would you say that this layer is the finer grained laterite, or the coarse grained saprolite? Look closely at the grain size and colour of the material. Do you believe there is a high iron concent and if so, why?
4. From the information on the cache page, what do geologists call a gap in the geologic timeline?
5. From the information on the cache page, describe the environmental conditions that were present to cause the erosion of these corestones?
6. Mandatory: Take an original photo with one of the core stones in the background ( don't use the largest stone from question # 1) and include a personal item like your GPS, phone or a thumbs up. You do not need to show your face in the photo. Include this in your found log.
References:
Road Rocks Ontario by Nick Eyles
SE Ontario: Precambrian-Paleozoic unconformity and related mineralization (gov.on.ca)
