Background: When rock units are placed under horizontal compression, they shorten and become thicker. Because rock units, other than muds, do not significantly change in volume, this is accomplished in two primary ways: through faulting and folding. In the shallow crust, where brittle deformation can occur, thrust faults form, which causes deeper rock to move on top of shallower rock. Because deeper rock is often older, as noted by the principle of superposition, this can result in older rocks moving on top of younger ones. Movement along faults can result in folding, either because the faults are not planar or because rock layers are dragged along, forming drag folds as slip occurs along the fault. Deeper in the Earth, rocks behave plastically and fold instead of faulting. These folds can either be those where the material in the center of the fold buckles upwards, creating "antiforms", or where it buckles downwards, creating "synforms". If the tops of the rock units within the folds remain pointing upwards, they are called anticlines and synclines, respectively. If some of the units in the fold are facing downward, the structure is called an overturned anticline or syncline, and if all of the rock units are overturned or the correct up-direction is unknown, they are simply called by the most general terms, antiforms and synforms.

Situation: Martha is currently walking around the forest when she notices two rocks that catch her attention. She quickly picks them up, stows them in her backpack and runs to the local geologist. She shows them to Linda, the geologist, and asks what she can tell her about the rocks. Linda informs her that the yellow rock has been subject to horizontal compression, but the blue rock has not been horizontally compressed. She promises Martha she will look into the rocks in more detail to answer more questions for her.

Question: Which color rock is more likely to be shorter?
Answer: yellow

Situation: Martha is currently walking around the forest when she notices two rocks that catch her attention. She quickly picks them up, stows them in her backpack and runs to the local geologist. She shows them to Linda, the geologist, and asks what she can tell her about the rocks. Linda informs her that the yellow rock has been subject to horizontal compression, but the blue rock has not been horizontally compressed. She promises Martha she will look into the rocks in more detail to answer more questions for her.

Question: Which color rock is more likely to be longer?
Answer: blue

Situation: Martha is currently walking around the forest when she notices two rocks that catch her attention. She quickly picks them up, stows them in her backpack and runs to the local geologist. She shows them to Linda, the geologist, and asks what she can tell her about the rocks. Linda informs her that the yellow rock has been subject to horizontal compression, but the blue rock has not been horizontally compressed. She promises Martha she will look into the rocks in more detail to answer more questions for her.

Question: Which color rock is more likely to be thicker?
Answer: yellow

Situation: Martha is currently walking around the forest when she notices two rocks that catch her attention. She quickly picks them up, stows them in her backpack and runs to the local geologist. She shows them to Linda, the geologist, and asks what she can tell her about the rocks. Linda informs her that the yellow rock has been subject to horizontal compression, but the blue rock has not been horizontally compressed. She promises Martha she will look into the rocks in more detail to answer more questions for her.

Question: Which color rock is more likely to be thinner?
Answer: blue