Welcome to Ilkley Moor! This cache is one of a series of caches that will take you on a tour of some of the many geological features that you can find at this scenic location.
Ilkley Moor
Ilkley Moor is an area of upland moorland, covering 676 hectares. Internationally, this type of moorland is rare, despite being relatively common in the North of England, and as a result has significant nature conservation interest. Mosaics of heather moorland, blanket bog, acid grassland and flowing streams make up the character of Ilkley Moor. Mixed with these are small stands of pine woodland, scattered trees, tarns and exposed rock faces.
Ilkley Moor is underlain by a succession of shale and course grained sandstones including the Addingham Edge Grit that forms the Cow and Calf rocks. The sand that forms these rocks was originally laid down on shifting sandbanks in a great river about 320 million years ago during the Carboniferous Period.
Deep burial caused the sand to harden into rock and subsequent earth movements and erosion brought the rock back to the surface again. By the end of the Ice Age, about 13,000 years ago, ice had scoured the moor and deposited the long low ridge of Lanshaw Delves, a glacial moraine. Since then peat has accumulated in sphagnum bogs.
Stream Erosion
Erosion is the action of surface processes (such as water flow or wind) that remove soil, rock, or dissolved material from one location, and then transport it to another location. Streams are one of the most effective methods that erode rock and sediment. In addition to eroding the bedrock and previously deposited sediments along its route, a stream constantly abrades (the wearing away at a surface over time by transported material) the individual rock and soil particles carried by its water.
How are they carried?
There are three types of ways that water transports material: dissolved matter, suspended load, and bed load, which I will explain below.
Dissolved matter is invisible, and is transported in the form of small molecules dissolved in the water. All streams carry some type of dissolved load. Materials comprising the dissolved load have the smallest particle size of the three load types.
Suspended load is composed of fine sediment particles suspended and transported through the stream. These materials are too large to be dissolved, but too small to lie on the bed of the stream. Stream flow keeps these suspended materials, such as clay and silt, from settling on the stream bed. Suspended load accounts for the largest majority of stream load.
Bed load rolls slowly along the floor of the stream. These include the largest and heaviest materials in the stream, ranging from sand and gravel to cobbles and boulders. There are two main ways to transport bed load: traction and saltation. Traction describes the “scooting and rolling” of particles along the bed. Saltation is a bounce-like movement, occurring when large particles are suspended in the stream for a short distance after which they fall to the bottom. The below image shows the three types of transportation:
Hydraulic Action
The ability of flowing water to dislodge and transport rock particles or sediment is called hydraulic action. In general, the greater the velocity of the water and the steeper the grade, the greater the hydraulic action capabilities of the stream, because there are more forces acting upon the particles, for example gravity.
Rocks are also broken up by friction and repeated impact with other rock fragments during transportation. For example, a rock fragment carried along in a river's current continuously bounces against other fragments and the river bottom and eventually is broken into smaller pieces. This is called mechanical weathering.
Logging your Find
In order to log your find, please send a message to my profile answering the below questions. The answers can be found by visiting the location, reading the above description, and taking observations in the field.
Look at the stream.
- What shape are the rocks the stream flows around?
- Why you think the rocks are this shape, and how do you think their shape will change over time as the actions detailed in the description above impact the rocks?
- What size are the loose rocks in the stream bed?
- Why do you think there aren't smaller rocks?
As Ilkley Moor is a stunning part of the world, you may optionally want to upload photos of your visit, of course I would love to see them! You can log your find immediately, but please send the answers within 24 hours. I will respond to your message as soon as I can. Logs where incomplete answers have been given, or where an email has not been received, may be deleted.