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Wonders of Geology

Mammoth Caves in Kentucky and Carlsbad Caverns in New Mexico

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Two of the world’s most spectacular caves are Mammoth Caves in central Kentucky and Carlsbad Caverns in southeastern New Mexico . Find out how they formed.

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Underground worlds of mystery and grandeur, caves are labyrinths of passageways and streams, homes to unusual creatures and stunning formations . Two of the world’s most spectacular caves are Mammoth Caves in central Kentucky and Carlsbad Caverns in southeastern New Mexico . Mammoth is known for its long passages while Carlsbad is known for its large rooms and bat colonies . These two caves provide good examples of most of the caves found around the world—limestone caves.

In its pure state, water is acid-free . However, as rainwater falls through the atmosphere and as it passes through decaying organic matter in the soil, it picks up carbon dioxide (CO 2 ), making the water acidic . As this water with carbonic acid makes its way underground through cracks and fractures in the earth’s surface, it reacts with the calcite that makes up most of the limestone, putting it into solution . It is then removed with the help of underground streams . As these areas of limestone are dissolved and removed, large openings under the earth’s surface are left, thus creating caves . In addition to carbonic acid, evidence shows that hydrogen sulfide from oil and gas deposits, and a whole range of microbes, combines with oxygen in the underground water table, producing sulfuric acid . This strong acid causes very aggressive dissolution of passageways at the level of the water table along cracks, fractures, and faults in the limestone.

Caves vary in size, shape and content; but they all include spectacular formations . The two most well-known speleothems are stalactites and stalagmites, but many confuse the two . One way to remember the one on the cave’s ceiling is that they hang “tite” to the ceiling—stalactites . Stalagmites, on the other hand, “mite” rise from the floor of these underground worlds . These formations result when this mixture of water and calcite enters a cave . The solution goes through a reaction where CO 2 bubbles away, and the calcite is deposited in formations called speleothems . Stalagmites result from the calcite collecting on the floor directly below a stalactite, which forms by calcite collecting on the ceiling . If the two join together, they become known as a column . Other speleothems include flowstones, draperies, soda straws, and helectites.

To allow enough time for peer assessments to be performed after learners have submitted their own responses, set the response due date and time at least one week before the peer assessment due date and time.

If the response due time and peer assessment due time are too close together, and a learner submits a response just before responses are due, other learners may not have time to perform peer assessments before peer assessments are due.

Use course discussion posts to provide guidance for peer grading of ORA assignments.

Consider extending due dates to allow the discussion moderation team to monitor course discussions for questions about, or reactions to, peer grading, and to address issues when necessary.

If learners raise concerns about ORA assignments in course discussions, course team members can perform actions such as New Balance Hook and Loop Recovery Slide Mens Slides Shoes SDL230 MQ56eUhhW
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, or removing a learner response from peer grading. If there are more widespread issues with peer grading, the course team can reduce the weight of the peer assessment within the final course grade or allow learners to drop the lowest graded assignment from their grades.

In ORA assignments, you can ask your learners to upload images, .pdf files, or other types of files as a part of their responses. Other learners evaluate the responses and their accompanying files during the peer assessment. Offering the option to upload files in addition to a text response can give learners the opportunity to use tools and develop skills that are relevant to your course.

Before you decide to ask learners to upload files along with their text responses, however, be aware of the following limitations and best practices.

For more information, see Specify the Response Type .

10.30.1.5.1. Prohibited File Extensions

Learners cannot upload file types that have the following file name extensions as part of an open response assessment on edx.org or edX Edge. When you define a set of custom file types for learners to upload with their responses, you cannot specify these file types. The extensions on this list are selected and maintained by the development operations team at edX, and are subject to change.

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