C) granite
D) diorite
The Mississippi River today is a classic meandering river. 20,000 years ago the
Mississippi River was a major outwash channel for the Laurentide ice sheet, and was
characterized by dramatic variations in flow with huge discharge during summer ice
melt and a dramatic falloff in discharge during the winter. The river also carried coarser
sediment than today, with abundant sand and even gravel in its upper reaches. What
does this suggest about the ancient Mississippi?
A) It probably looked similar to the river today, but was characterized by more dramatic
summer floods that occurred every year.
B) It probably resembled modern rivers in Alaska with a broad braided stream valley
that dried up during the winter months, producing winter dust storms.
C) It was probably a lush heavily vegetated valley due to the nutrients brought down by
the glacial meltwater, producing a swampy landscape with numerous channels.
D) It is impossible to predict what the river might have looked like.
How did the lunar maria most likely originate?
A) Huge impact basins were nearly filled with basaltic lava flows.
B) Huge impact craters filled with frozen carbon dioxide and dark-colored silt and dust.