Sediment Primer 101 EarthCache

The Diagram is a rough drawing for orientation. It does not reflect ALL the rocks, nor is it to scale. Remember - No Duplications!

Descriptions:

1. Hornfel

This metamorphic rock was formed under intense heat. The properties of the rock are fused and blended with the various other rocks making up the resultant Basal. This sample carries two notable features: 1. The even smooth rounding indicates it was exposed to flowing water for an extended time. The abrasive run off of water has resulted in a smooth polished surface devoid of any angular edges. 2. With the dull black colouring, this rock would have an essential Iron (Fe) chemical constituent. The fine particles that it is made of imply it was of Mafic origin, probably Basalt and the source sediment would have been mud or clay.

2. Inclusion

When a sediment is laid, it often happens that existing pieces of rock debris are included in the laying of the sediment. This example of a clast included in the sediment before the sediment is turned to stone, or Lithified. ( we would call it a pebble, Geologists term it a clast) There is a Law that says the age of the inclusion is older than the age of the sediment, and this is useful in determining how old certain rocks are. Using the age of an inclusion to determine the age of a sedimentary rock is known as relative aging, and it appears rather obvious. A rock that has an inclusion within has to be younger than that inclusion.

3. Joints

Sediments that have formed with an ample supply of substrate, and have formed over a number of Millennia, beneath water; When the water recedes, and the upper strata is exposed to erosion, the removal of the upper strata releases the some of the pressure being borne by the under laying strata. Over time and in the exposed environment (without the protective cover of water) the sediment cracks. Usually these cracks, or joints as the geologists term them, are at right angles one to another. They are one consequence of Lithification by pressure, and the pressure being released. This phenomena can best be seen at a quarry where the cliff face is 'boxed' by the joints.

4. Leaching

The dark brown and black colourings on rocks is a result of the leaching process. In this setting, where there is an abundance of moisture, the water will fill the rock pores, and in doing so will dissolve most of the chemical compounds that make up the rock. As new water enters the pores, and being more able to combine with the constituent compounds, the newer water displaces the existing water, which permeates to the outer face of the rock. Upon being directly exposed to the sun and wind, the water itself evaporates, leaving behind the dissolved chemical compounds. This results in the outer face of the rock becoming coloured, according to the prevalent chemical of the rock. For instance, rock with a high Iron make up would result in the darken or black face that this sample exhibits.

5. Lichen

Some say that these organisms were the first plants on Earth, but before commenting we must understand their role first. Lichen look like a fungus but act like an algae, and yet are neither. Closer to Bacteria, it has been discovered that recorded lichen evidence goes back as far as 400 million years ago. (as an aside, it has been demonstrated that they can withstand the rigors or exposure to space for up to 28 days!) But enough introduction. Lichen are the first to populate a rock. They are able to survive extreme periods of drought, and only require slight amounts of water to survive. They attach to the rock face via tendrils attached to the micro fissures of the rock surface. Whilst allowing nutrients to pass into the lichen, this process also applies a slight acidic compound to the rock surface. Over time, this weakens the face of the rock, and allows for the weathering and erosion activities to break the rock down. As a primary propagation agent, you would expect to find a variety of instances of lichen within any groupings of rocks. This site is no different.

6. Man Made Breccia

Breccia is formed when angled particles, over 2mm in diameter; are bound together in a matrix of mineral cement. The particles, or clasts, may be from the same sedimentary rock, or they may be from differing rocks. Likewise, the spaces between the individual clasts could be filled with silt (iron oxide), carbonate (e.g., calcite), or silica, all forming a binding cement that holds the clasts together. Breccia are found world wide, and samples have been recorded upon the moon as well as Mars.

7. Moraine

The placing of chaotic supra glacial sediments on to the landscape with limited reworking, has resulted in this rock. Moraine can be formed by the accumulation of sand and gravel deposits conveyed by glacial streams, created along the margins of the ice. These forms are characterised by well rounded particles and a compact bedding once the ice carrier has retreated (melted) and the water evaporated. They are usually continuous, but are often broken up post glaciation. Moraine can best be described as the Lithification of Till.

8. Organic Erosion

Organic weathering of rocks can occur in two forms. In the one case, dust and particles collect in cracks and hollows of the unmoving rock. When the seed of a colonising plant lands up in this place, germination takes place and the seed grows. As the small plant grows, it's roots find tiny fissures within the rock, and in growing the root expansion will ultimately force the fissure to crack the rock. In the other case, the simple ground cover of mosses and such are nourished by the salts and nitrates of the rock composition, and the growth absorbs these compounds from the body of the rock, thus draining the rock of the very chemical bonding that holds it together. As the rock weakens, the ground cover grows, and increases the demand from the rock, until eventually the rock is destroyed and the plant life will wither.

9. Unconformity

This is a change in the profile of the upper strata that does not conform to the lower strata, and subsequent strata continue to be laid over this variance. Notably when a sediment is uplifted, and the upper strata is weathered or eroded sufficient to adjust the surface, and further sediments are laid down thereafter; this would be an unconformity. This example would have undergone an event that resulted in sufficient force (possibly due to heat) to allow the silicate compounds within the sediment to crystalize. And subsequent layers of sediment are placed above this bed of crystals.

10. Weathering

When sediments are exposed to the air, they are constantly eroded with their shape changing as a consequence. In areas of extreme day and night temperature changes, and particularly in dry conditions, the expansion and contraction process results in the outer face of the rock 'peeling off' (similar to an onion skin). This result can also be achieved in conditions of high humidity and fluctuating temperature ranges. This gradual and almost imperceptible erosion will result in the rock being rounded entirely, flake of outer layer by flake. Although more common in Desert settings, this process will be carried out provided there is a range of hot and cold temperatures closely following each other.

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Have fun putting into practice everything you know of this Rocky Road!