This EarthCache is at Ghaistrill's Strid which is on the River North up stream from Grassington. Here the Wharfe flows through a narrow cataract.
There is no need to enter the water, the Strid is deep, very deep.
The bedrock here is limestone, though there are enough EarthCaches to discuss limestone, we are here to look for evidence of erosion.
Erosion can be caused by a number of factors:
Abrasion - this is where the rock surface is scraped by other rocks and particles carried in the water (or wind). Imagine a washing machine full of rocks, as it spins the rocks come into contact with the side of the drum and bang and scrape against it. In the case of a washing machine, this would eventually damage the drum, in the case of rock, it causes a circular feature, which gradually over time gets bigger. Stones which have angular features, gradually become round pebbles due to the abrasion of them coming into contact with other stones.
Corrosion - where rocks are broken down by chemicals and/or processes acting on them such as acid contact, heating, or rusting. This type of erosion can be seen in buildings, where sharp features become blunted and curved, as the corrosive action affects the stone. Flaking of rock can also take place.
Hydraulic Action - this is where the action of the water itself erodes the rock over time. In the case of a coastline which consists of sand or clay this causes the coastline to gradually retreat.
Freeze-thaw - this is where water freezes in the cracks of the rock, expands and contracts with changing temperatures and eventually the rock erodes.
This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. How wide is the Strid on the day that you visited?
2. What erosive action do you think has taken place here, please explain your rationale?
3. What physical evidence of erosion can you see, and feel here?