Horsetail Falls, a large seasonal waterfall on Moore Creek, with Mt. Owen as its backdrop, there is a large parking area on the side of the Lyell Highway. From the carpark, a boardwalk propped on the side of the hill with a few steps winds its way uphill around the mountain, providing great views. A far less developed track continues from the lookout and following it will lead you to the top of the falls and the track to Mt Owen, however it is narrow, wet, and not for inexperienced walkers or the light-hearted.
For Geomum walking out to Horsetail falls took her out of her comfort zone, an irrational fear of falling sometimes gets the better of her. She found holding GeoDads arm tightly and focusing on the ‘pretty rocks’ on the left side of the track while they ascended, the option that kept her rational, this is how this EC was born. (Do not worry the WP’s are on the easier sections of the track) There are some spectacular geological features in the area, the feature we chose to teach you about today is Puddingstone, the first thing you need to know about Puddingstone is it is not real! Well, it is, but Geologists dislike the term as it is non-scientific and essentially nonsensical. Geologists prefer the term Conglomerate.
Conglomerate is a clastic sedimentary rock made up of rounded clasts that are greater than two millimetres in diameter. A clast is a piece of rock or mineral, ranging in size from tiny, like a grain of sand, upto the size of a house. The spaces between the clasts are generally filled with sand and clay size particles. The rock is bound together by a cement that is usually composed of calcite or quartz.
Conglomerate can have a variety of compositions. As a clastic sedimentary rock, it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz or feldspar, or they can be sedimentary, metamorphic, or igneous rock fragments. Clasts of quartzite, sandstone, limestone, granite, basalt, and gneiss are especially common. The matrix that binds the clasts together can be a mixture of sand, mud, and chemical cement. Common chemical cements are calcite or quartz.
Conglomerate forms where sediments of rounded clasts accumulate. It takes a strong water current to transport and produce a rounded shape clast. The environment of deposition might be along a swiftly flowing stream or a beach with strong waves. These conditions might only be met during times of extreme flow or wave action.
To form a conglomerate, there must also be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveals that they were tumbled for some distance by running water or moving waves.
Conglomerates often begin when a sediment consisting mainly of pebble and cobble size clasts is being deposited. The finer size sand and clay, which fill the spaces between the larger clasts, is often deposited later on top of the large clasts and then sifts down between them to fill the interstitial spaces. After compaction, the precipitation of a chemical cement between the grains will bind the sediment into a rock.
We need to mention that Conglomerate and Breccia are similar rocks. They are both made of clasts. The difference is in the shape of the clasts. Conglomerate is made up mostly of subrounded to rounded clasts. However, breccia is made up mostly of subangular to angular clasts.
Sedimentary clasts can be angular or rounded. They both start out at an outcrop This outcrop is known as the "source area" for the clasts. In the source area, chemical and physical weathering act upon the rock, causing it to break or disintegrate into smaller pieces. These pieces are usually subangular to angular. If the clasts accumulate near the outcrop and form into a rock, that rock will have angular pieces and be a breccia.
If the pieces are transported by a stream or the action of waves, the clasts will be abraded against one another and against other clasts on the bottom of the stream. That abrasion will, over time, cause their angular shapes to become subrounded to rounded. If the rounded clasts are deposited and formed into a rock, that rock with rounded clasts will be a conglomerate.
Analysis of conglomerate can sometimes be used as a prospecting tool. For example, most diamond deposits are hosted in kimberlite. If a conglomerate contains clasts of kimberlite, then the source of that kimberlite must be upstream of the location where the kimberlite clast was deposited.
In rare instances, conglomerate can be a "fossil placer deposit" containing gold, diamonds, or other valuable minerals. These conglomerates are mined, crushed, and processed as ores.
To log this Earth Cache, we require you to visit GZ and the listed waypoints to make some observations, consider the information given, then message us with the following answers to the best of your ability.
1, At GZ, facing the mountainside on which the walkway is attached, firstly pay attention to the bigger rocks and then the little rocks, are they the same, would you describe them as Conglomerate, why do you think some are bigger than others? What type of sorting applies to clasts in these larger rocks?
2, Moving on to WP2, again looking at the closest rocks. Can the same type of rocks be seen here? Are there any other notable features here, describe them, don’t worry if you don’t know the correct geological name.
3, WP3, Comparing the closest rocks on the mountainside to those over at Horsetail falls (just look from a distance we are not asking you to climb up there) do you believe they are the same rocks, why? why not? Note the differences.
4, Bonus points to anyone who can correctly identify any other clastic type rocks along the walkway!
5, A photo of your team or GPS with the falls in the background.
You are welcome to log your find straight away to keep your TB's and Stats in order but please message us with your answers within a couple of days. Cachers who do not fulfil this Earth Cache requirement will have their logs deleted without notice.
Source: Geology.com, openeducationalberta, chegg.