How wide was the narrow sea between the western continent and adjacent volcanic islands 20 million years ago?

hello!

let's go through all of these words' definitions.

u-shaped valley - where the valley curves down like a u shape

corrie - similar to the hole created at the top of a volcano when it erupts, it's almost like a dent in a mountain

pyramidal peak - a mountain where the peak is very sharp, such as the matterhorn.

arete - two valleys separated by a thin mountain range

i'm going to assume you know how to read a topographic map, but in case you don't, feel free to comment.

now, let's figure out which one's which:

a - seems to be a mountain range separating two valleys, with a mountain running from top center to bottom center and two valleys on either side. therefore, i'd consider this an arete.

b - it looks like a dent into the mountain, like what a corrie is.

c - looks to be a valley, with two steeper sides curving up around the bottom of the valley which has a river in it. i would consider this an u-shaped valley.

hope this !

81 is a composite number.