Potholes, in geology, are cylindrical pits formed in the rocky bed of a turbulent stream. Potholes are formed and enlarged by the abrading action of pebbles and cobbles that are carried by eddies, or circular water currents that move against the main current of a stream. Potholes are most commonly found at the bottoms of eddies in rivers.
Sand, gravel, and rocks become trapped in the spinning whirlpools and spiralling eddies, and over time, the grinding action of these abrasive materials drill holes known as Potholes into the underlying bedrock. The interiors of potholes tend to be smooth and regular. By definition the hardness of the grinding media must be the same or higher than the rock at the bottom of stream where the potholes are forming.
The bed of the Onkaparinga River at this Earthcache location has at least 50 of these rare and unique geolocial features within a stretch of 100m. Check the shape, diameter and depth of the potholes you see along the river bed.
To claim this Earthcache you need to have the following earthscience tasks validated by prompt email or message. Do not post the answers in your logs.
- Are the potholes round or elliptical? Which shape is most common?
- What is the diameter and depth of the largest pothole you can see?
- Can you see the stones that have formed the potholes? What size these stones?
To visit the Earthcache location you should use the unmarked Punchball waterhole trail. Reference child waypoints have been added to the cache notes. The trail is steep and will be slippery when wet. The river flows quickly in winter and you are advised to take care near the riverbank. Given that potholes are a river flow feature you may think you will not see the features if the river is flowing. Interestingly the largest potholes are not in the middle of the current riverbed. What does this mean? Well my opinion is that river flows were significantly higher before Mt Bold Reservoir was built. Perhaps you have another opinion?