A quartz cast after the crack filling of a septerian nodule or possibly after similar crack-filling spherulite" (p.c. J.S. White June 1985). Solutions acting on septaria nay remove the filings of the cracks, in which event the septaria fall apart, or may remove the material between the cracks, thus leaving a skeletal structure made up of the the veins (Twenhofelm, W.H. Principles of sedimentation. 1950, 604). In plain English, these are cool! These unusual quartz mineral specimens were mined in Santa Cantarina, Brazil.

SEPTARIA

Concretion

A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity (i.e. the spaces between the sediment grains). Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and cresco meaning 'to grow'. Concretions form within layers of sedimentary strata that have already been deposited. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.

Burrow

A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the animals. Burrows are also commonly preserved in the fossil record as a type of trace fossil.

Raindrop impresions

Raindrops striking unconsolidated sediment can form miniature impact craters. Once the sediment dries, these impressions can be preserved if they become covered with sediment soon after they were formed. Below are numerous raindrop impressions on modern sediment, with a shoe for scale.

Ripple Mark

One of a series of small marine, lake, or riverine features, consisting of repeating wavelike forms with symmetric slopes, sharp peaks, and rounded troughs.

Ripple marks are formed in sandy bottoms by oscillation waves, in which only the wave form advances rapidly, the actual water-particle motion consisting of almost closed vertical orbits. The presence of the bottom restricts the lowermost orbits into nearly flat ellipses, and the bottom water moves back and forth rhythmically. If the maximum horizontal velocity of this motion is capable of moving the grains composing the bed, ripple marks develop.

Flute Casts

Flute casts are irregular ridges that display a marked asymmetry. The asymmetry of these structures allows for determination of paleocurrent direction—flows originate from the direction of the steeper side of the flute. Flute casts typically have a "pointy" end, oriented in the upstream direction, and a more gentle, flared end that merges downstream into the bed surface. Flute casts along the base of a given bed are usually of approximately the same size, but between beds or formations, may vary greatly in size.

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Concretion

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