This is an EarthCache in an urban location with plenty of easy parking just along the road. The location takes you into a hidden part of Ilkley that many will not know about and yet is very close to the town centre.
The focus of the cache is the underneath of the footbridge. You do not need to climb the wall but to answer all questions you might like to walk round the ramp to the top of the bridge, Since this cache is wheelchair friendly the whole area is smooth but please note it’s not level.
Here is some information about limestone and how it can create rock formations in and on other stone.
At this location you will notice that the bridge is made up of large concrete blocks which have been carved and put together. However the focus of this EarthCache is the formation of stalactite type deposits that are dripping through the cracks in the bridge. You may even see water dripping off the stalactites if it has rained recently This is happening because the construction of the bridge must be rich in limestone and as the rainwater passes through the structure and permeates out through the cracks it leaves deposits behind, just like stalactites in some cave systems.
The words stalactite and stalagmite can be traced back to the Greek word "stalassein," which means "to drip." This is fitting because it describes how they are formed in nature. The slow drip drip drip of ground water leaving behind deposits .Although they look lifelike and a little creepy, stalactites and stalagmites grow simply because of water running over and through inorganic material leave behind trace deposits which build up over time.
Limestone caves, where most stalactites and stalagmites are found, are mainly composed of calcite, a common mineral found in sedimentary rocks. Calcite molecules are made of calcium and carbonate ions, and are referred to as CaCO3, or calcium carbonate. When rainwater falls over a cave and trickles through rocks, it picks up carbon dioxide and minerals from limestone. If we add water, carbon dioxide and calcium carbonate together, we get the constituent parts needed to leave deposits: This is exactly what’s happening here as the rain water passes through this bridge.
You will notice, if you look at the formations directly above your head that these stalactites are in fact not solid but are hollow like a straw. This is a well know a common formation.
A soda straw (or simply straw) is a speleothem stalactite in the form of a hollow mineral cylindrical tube hanging down. They can be also known as tubular stalactites. Soda straws grow in places where water leaches slowly through cracks in rock, such as on the roofs of caves, or in this case a bridge. Soda straws in caves rarely grow more than a few millimetres a year and the rate of growth is related to the concentration of the mineral and the amount of water that flows, but may only average just a tenth of a millimetre per year. At this location it is fair to say that there is a regular flow of water, it is Yorkshire after all and since this is a man-made bridge the concentration of lime must be quite high. A soda straw can turn into a solid stalactite if the hole at the bottom is blocked.
Thanks for reading the information.
Hopefully you have enough to answer the questions below, thank for having a go.
- Please describe the underneath of the bridge, above your head and over the stream.
- How are the stalactites forming here?
- How long are the longest stalactite and so about how old would you estimate they are? Please give reasons for your answer.
If you feel willing and able please include a photo but this don’t give answers away and this is optional
Thanks for visiting the location and I look forward to reading your logs.