This rock is known as the Rock of the Tattoo Goddess, due to the number of lines on it. The lines however are actually fossils. They are what is known as a log jam.
Look around and you will see linear structures, in a miss mach of patterns. This is known as a log jam. During the carboniferous period, this was a river channel, and during periods of flood, debris was washed downstream. Included in this debris , was logs and branches of trees further upstream. Eventually one branch or log would get caught, and then others would also be caught, eventually the channel becoming blocked by a jam of branches. It still happens today in river channels, such as in the River Aire. Behind this log jam, sediments then built up. Now, there were a variety of plants during this period, and as we have today there were different species. Hence, why not all of the log jam looks the same.
Paleocurrents
A palaeocurrent is the direction of flow at the time the sediment was deposited, and may also be referred to as the palaeoflow.
Palaeocurrent indicators
There are different types of palaeocurrent indicators in sedimentary rocks:
(1) Unidirectional indicators are features that give the direction of flow:
Cross lamination. Cross-lamination is produced by ripples migrating in the direction of the flow of the current.
Cross bedding. In Cross-bedding the inclination of the cross-beds indicates the transport direction and the current flow. The style and size of cross bedding can be used to estimate current velocity, and orientation of cross-beds allows determination direction of paleoflow.
Clast imbrication. This is when areas of gravel become orientated in strong flows, with their longer axis dipping upstream when viewed from the side.
Flute casts, are scours created by vortices in a flow, the steep edge marking the upstream edge.
(2) Flow axis indicators are structures that provide information about the axis of the current but do not differentiate between upstream and downstream directions.
Primary current lineations on bedding planes are measured by determining the orientation of the lines of grains.
Groove casts are elongate scours caused by the indentation of a particle carried within a flow that give the flow axis.
Elongate clast orientation may provide information if needle-like minerals, elongate fossils such as belemnites, or pieces of wood show a parallel alignment in the flow.
Channel and scour margins can be used as indicators because the cut bank of a channel lies parallel to the direction of flow.
In the case of this Log Jam, looking at its allignment can help give us an indication of the paleocurrent direction of the watercourse, in a log jam, the logs can be arranged perpendicular to the overall sediment transportation direction in the watercourse.
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) Please look at the allignment of the fossils, in which direction do they generally point towards?
(2) Using the information you have gained from task 1, please tell me the direction of the paleocurrent.
(3) Please measure the longest and thickest of the fossils, and tell me the diemenstions in cm.
(4) Are the fossils all over the rock, or in one specific bit, if so, which bit?
What we have here is a fossil. Looking at the alignment of a log jam it can help give us an indication of the paleocurrent direction of the watercourse, in a log jam, the logs can be arranged perpendicular to the overall sediment transportation direction in the watercourse.