NOTE: In order to view the sedimentary layers that this EarthCache covers, you will have to approach GZ from the beach (which can be easily approached from the Scripps pier area). At low or moderate tide, it is a straightforward walk along the beach. Do not attempt to search for this cache at high tide or in stormy weather.
SANDSTONE COLORATION
There are three basic components of sandstones: (1) detrital grains, mainly transported, sand-size minerals such as quartz and feldspar, (2) a detrital matrix of clay or mud, which is absent in “clean” sandstones, and (3) a cement that is chemically precipitated in crystalline form from solution and that serves to fill up original pore spaces.
The colour of a sandstone depends on its detrital grains and bonding material. An abundance of potassium feldspar often gives a pink colour; this is true of many feldspathic arenites, which are feldspar-rich sandstones. Fine-grained, dark-coloured rock fragments, such as pieces of slate, chert, or andesite, however, give a salt-and-pepper appearance to a sandstone. Iron oxide cement imparts tones of yellow, orange, brown, or red, whereas calcite cement imparts a gray colour. A sandstone consisting almost wholly of quartz grains cemented by quartz may be glassy and white. A chloritic clay matrix results in a greenish black colour and extreme hardness; such rocks are wackes. Source: Britannica
SEDIMENTARY STRUCTURES
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particle sizes are deposited on top of each other. These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick. Source: Wikipedia
Imbrication is a stack of large and usually flat clasts—cobbles, gravels, mud chips, etc—that are aligned in the direction of fluid flow. The clasts may be stacked in rows, with their edges dipping down and flat surfaces aligned to face the flow (see figure). Or their flat surfaces may be parallel to the layer and long axes aligned with the flow. Imbrications are useful for analyzing paleocurrents, or currents found in the geologic past, especially in alluvial deposits. Source: LibreTexts Geosciences
Cobbles in this conglomerate are positioned in a way that they are stacked on each other, which occurred as flow went from left to right. Source: LibreTexts
Hummocky cross-stratification is a type of sedimentary structure found in sandstones. It is a form of cross-bedding usually formed by the action of large storms, such as hurricanes. It takes the form of a series of "smile"-like shapes, crosscutting each other. Source: Wikpedia
Hummocky cross-stratification in Nova Scotia. Source: Wikimedia commons
Now let's learn about a specific kind of sedimentary structure that often goes hand-in-hand with areas that have been heavily changed by water.
Soft-sediment deformation structures develop at deposition or shortly after, during the first stages of the sediment's consolidation. This is because the sediments need to be "liquid-like" or unsolidified for the deformation to occur. These formations have also been put into a category called water-escape structures by Lowe (1975). The most common places for soft-sediment deformations to materialize are in deep water basins with turbidity currents, rivers, deltas, and shallow-marine areas with storm impacted conditions. This is because these environments have high deposition rates, which allows the sediments to pack loosely. Source: Wikipedia
Load casts are bulges, lumps, and lobes that can form on the bedding planes that separate the layers of sedimentary rocks. The lumps "hang down" from the upper layer into the lower layer, and typically form with fairly equal spacing. Source: Wikipedia
Load casts in France. Source: Wikimedia Commons
Ball-and-pillow structures are masses of clastic sediment that take the form of isolated pillows or protruding ball structures. Source: Wikipedia
Ball-and-pillow strucutres in Australia. Source: Wikimedia Commons
A dish structure is a type of sedimentary structure formed by liquefaction and fluidization of water-charged soft sediment either during or immediately following deposition. The bounding surface of the dish can take on variable shapes, from substantially flat to bowl-like and to strongly concave up. Commonly the dishes are separated by vertical streaks of massive sand called 'pillars'. Source: Wikipedia
Dish structure in Peru. Source: Wikimedia Commons
Sole marks are sedimentary structures found on the bases of certain strata, that indicate small-scale (usually on the order of centimetres) grooves or irregularities. This usually occurs at the interface of two differing lithologies and/or grain sizes. They are commonly preserved as casts of these indents on the bottom of the overlying bed (like flute casts). This is similar to casts and molds in fossil preservation. Occurring as they do only at the bottom of beds, and their distinctive shapes, they can make useful way up structures and paleocurrent indicators. Source: Wikpedia
Sandstone sole marks in Nova Scotia. Source: Wikimedia Commons
QUESTIONS
Describe how the sediment layers' colors and textures change from top to bottom.
Can you find any imbrication on the cliff? (Hint: is there a part of the cliff that contains a lot of cobblestones and gravel?)
Are there any parts of the sediment that seem like they contain evidence of hummocky cross-stratification? Would you say that the sediment layering is evenly stacked, or more unstably stacked with layers that overlap each other?
Can you find any soft-sediment deformation structures on the cliff? Use the pictures of load casts, ball-and-pillow structures, dish structures, and sole marks to figure it out. Your answer should include what they look like (feel free to just use adjectives like round, bulging, etc.), and if they are around the bottom, top, or middle of the cliff.
Since sole marks are only found at the bottom of beds, why do you think there are sole mark-like structures high up on the cliff?