The Sandy Beaches of Twin Creeks
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This is an Earthcache in the middle of a very nice and quiet park with a creek running through. Depending on which way you take to the cache, the terrain rating will differ. Care should be taken while walking or riding a bike as the creek runs right along the path. During the rainy season, the creek isn't passable and the path is muddy. Use caution!
When it rains hard enough, this geological formation can be covered by the creek water.
Poison Oak is known to be in the area. There is none by the cache site, however. Use caution regardless.
For this Earthcache, we will be looking at the, somewhat out-of-place, sandy beaches of Twin Creeks. This cache will hopefully educate how sand can end up in a place like this, where it came from, how it formed, and what type of sand is present.
What is Sand?
Sand is a natural unconsolidated granular material. Sand is composed of sand grains which range in size from 1/16 to 2 mm (62.5…2000 micrometers). Sand grains are either mineral particles, rock fragments or biogenic in origin. Finer granular material than sand is referred to as silt. Coarser material is gravel. Majority of sand is dominantly composed of silicate minerals or silicate rock fragments. By far the most common mineral in sand is Quartz. Sand is a natural unconsolidated granular material. Sand is composed of sand grains which range in size from 1/16 to 2 mm (62.5…2000 micrometers). Sand grains are either mineral particles, rock fragments or biogenic in origin. Finer granular material than sand is referred to as silt. Coarser material is gravel. Majority of sand is dominantly composed of silicate minerals or silicate rock fragments. By far the most common mineral in sand is quartz. Hence, the term “sand” without qualification is imagined to be composed of quartz mostly. However, sand is a natural mixture which means that it is never pure. By no means can one say that quartz and sand are the same thing.
The Formation of Sand
Sand forms mostly by the chemical and/or physical breakdown of rocks. This process is collectively known as weathering. Physical and chemical weathering are usually treated separately, but in reality they usually go hand in hand and it is often difficult to separate one from another because they tend to support each other. Chemical weathering is much more important sand-producing factor overall. It operates most efficiently in humid and hot climate. Physical weathering dominates in cold and/or dry areas. Weathering of bedrock which produces sand usually takes place in soil. Soil covers bedrock as a thin layer, providing moisture for the disintegration process of rocks. Granite is a common rock type and serves as a great example of sand forming processes. Granite is composed of feldspar, which is pink and white, which decomposes chemically into clay minerals. Another important constituent of granite is quartz (gray). Quartz is very resistant to chemical weathering. It does not alter to any other mineral — quartz is quartz and will remain that way. It eventually goes into solutions but VERY slowly. Hence, disintegrated granite yields lots of quartz grains which will be transported mostly by running water as sand grains.
The Composition of Sand
Sand is a residual material of preexisting rocks. It is therefore composed of minerals that were already there in the rocks before the disintegration commenced. However, there is one important aspect; sand occurs in a harsh environment where only the toughest material survives. Quartz is one of these minerals but not the only one. It is so dominant in most sand samples because it is so abundant. 12% of the crust is composed of it. Only feldspars are more abundant than quartz. Heavy minerals may sometimes occur in sand in much higher concentrations. This is usually a result of hydrodynamic sorting. Either sea waves or river flow sort out heavier grains and carry lighter ones away. Such occurrences are known as placers and they are often used as a valuable mineral resource. An example of such material is gold. Most sand samples consist of sand grains which are composed of a single mineral — quartz grains, feldspar grains, etc. But sand may also contain grains that are aggregates of crystals i.e. fragments of rocks (known also as lithic fragments). Lithic sand is usually immature and it also tends to form when rocks are very fine-grained. Granite usually disintegrates into distinct mineral grains, but phyllite and basalt for example are often so fine-grained that they tend to occur in sand as lithic fragments. Lithic fragments are also common in regions where erosion is rapid (mountainous terrain). Sometimes sand contains new minerals or mineral aggregates that were non-existent in the source rocks. Third major and versatile component of sand (two others were mineral grains and lithic fragments) are grains of biogenic origin. Biogenic sand is composed of fragments of exoskeletons of marine organisms. Common contributors are corals, foraminifera, sea urchins, sponges, mollusks, algae, etc. This is known as coral sand even if it didn't come from coral. This is most likely not the cause of the sand here. Sand does not need to be a pure collection of either mineral, lithic, or biogenic grains. In many cases two of them and sometimes even three are mixed.
The Texture and Transport of Sand
Geologists describe sand by measuring the roundness of grains and the distribution of grain sizes. By doing that they hope to shed some light on the origin of the grains being measured. Roundedness usually gives information about the length of transport route and distribution of grain sizes helps to determine from which environment these grains come from. River sand is usually poorly sorted and compositionally immature. Beach sand is more rounded and eolian dune sand is generally well sorted. The average size of grains is determined by the energy of the transport medium. Higher current velocity (either stream flow or sea waves) can carry heavier load. Coarse-grained sediments therefore reveal that they were influenced by energetic medium because finer material is carried away. Sand is mostly transported by rivers, but average sand grains are too large and heavy for average river to carry them in suspension. Hence, sand grains tend to move in jumps. They are lifted up by more energetic current and settle out when current velocity decreases and then wait for the next jump. This mode of movement is known as saltation. Average silt grain moves differently. It is light-weight enough to be carried in suspension for a long time and this is actually one of the most important reasons why we treat silt separately from sand. Most sand grains carried by the rivers are eventually deposited at the rivermouths where the current velocity suddenly drops. Then sea waves (longshore currents) take over and carry sand along the coastline. Sand grains carried by the rivers are also deposited on alluvial flood plains and point bars (inside bend of streams where current flow is the slowest). Sand is also transported by wind, ocean currents, glaciers, turbidity currents, etc. Moving sand forms landforms like ripples and dunes. If it's the right time of year, you can see ripples in the sand here.
Logging Requirements:
Please answer the following questions, to your best ability, using the information above. When you feel you have them answered correctly, send me your answers via the Geocaching website's messaging feature, or through email. Any "Found It" logs without the requirements being sent in, will be deleted.
Please include the GC code of this cache and the number of people in your party in your message.
1) What type of process most likely formed the sand you see? Why?
2) What type of material do you think makes up these sand bars?
3) How big or small are the granules of sand? If possible, tell me what the sand feels like. (If you cannot reach the sand, that's fine.)
4) How do you think the sand got from where it was formed to where you see it now?
5) This is NOT a requirement: Pictures are no longer allowed to be a logging requirement, however, they are most definitely encouraged! Snap a few while you're out if you'd like.
Email or send me the answers via the Geocaching website. Any "Found It" logs without answers will be deleted. There is no container at this geocache as it is an Earthcache.
Sources: www.sandatlas.org, Wikipedia, Personal knowledge of earth science and geology
Congrats to LewisClan77 and LiTtLeBrObRa for the Co-FTF!
CAA4JN
Additional Hints
(Decrypt)
[Read the information above for the best chances at correctly answering the logging requirements. And please, don't fall in the creek.]