Located on the east side of Highway 38 near the Portalnd Conservation Area, is an example of a Paleokarst, a fossil cave that is hundreds of millions of years old.  This fossil filled cave formed before southern Ontario was flooded by the Paleozoic seas

Some useful definitions

Karst -   landscape underlain by limestone which has been eroded by dissolution, producing ridges, towers, fissures, sinkholes and other characteristic landforms including caves.

Paleokarst is defined as karst that has been buried by younger rocks.   These younger rocks include sediments like sand and smaller rocks, stones and even boulders.

Clastic sediments are predominantly clay minerals and quartz particles, with minor amounts of Feldspars, micas, and heavy minerals.

• Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts.
• Breccia is a sedimentary rock composed of broken fragments of minerals or rocks cemented together by a find grained matrix
• Sandstone is a clastic sedimentary rock formed through transportation, deposition, compaction and cementation of different mineral composition of sand grains
• Shale is a find grained, laminated sedimentary rock consisting of tiny sized particles
• Mudstone is an extremely fine grained sedimentary rock consisting of a mixture of clay and silt sized particles

Marble is a metamorphic rock made of recrystallized calcite or dolomite

Examples of Paleokarst

Example 1 -  Located in the Liuhuang Valley is a paleocave dominated by sedimentary fill with a height of 6.1 m (Figure e). The lower part of this cave consists of well-sorted sand, the middle part consists of breccia, and the upper part consists of sand. Due to the complete filling of the cave, the cave fill affected the formation pressure, and a 1-m-thick fracture zone developed in the roof and sides of the channel (Figure f).

Example 2 - A completely filled paleocave, which is 3.6 m wide and 1.2 m high, located in the Yijianfang outcrop (Figure g). This cave is completely filled with pure calcite ( Figure h).

Portland Marble Cave

In the exposed Karst on Highway 38, you are looking at a billion-year-old marble cave, made up of metamorphic sedimentary limestone that is common to the Canadian shield rocks of the Grenville Province.  The metamorphic marble was dissolved approximately 500 million years ago to form the cave which subsequently filled up to the roof by sands and stones of the Nepean formation.    The sandstone that filled the cave shows cross-bedding which extends as horizontal “tongues” into the marble.  

Example of Conglomerate and clastic rocks embedded in the roof of the filled cave

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.

Questions

1. How many years ago did the marble dissolve to form the cave?
2. Look at the section identified by the arrow on the second photo as the former cave.    Looking at the stones embedded in the sediments, would you classify these stones as Breccia or conglomerate clasts?
3. What is the approximate size (height/length) of the largest stone you see.   
4. Next to the cave is a large marble outcrop, distinctive by its white colour when compared to the sedimentary rocks that lay over the top of the entire outcrop.   What is the height and length of this marble outcrop.  Look closely at the marble.  Do you see any evidence of sediments embedded in this stone does it appear to be sediment free?
5. Mandatory:  Take a photo from the west side of the road with the Portland Marble Cave in the background and include it in your found log.  Your face does not need to be in the photo.

References:

Road Rocks Ontario, by Nick Eyles

https://maps.niagararegion.ca/Metadata/md/DocumentUpload/2007-08-09%2014-17-49.pdf