Why do you think this is called a high-pressure system?

High in the atmosphere, air pressure decreases. With fewer air molecules above, there is less pressure from the weight of the air above. Places where the air pressure is high, are called high pressure systems. A low pressure system has lower pressure at its center than the areas around it.

Explanation:

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what is the actual question though?

answer a fan shaped mass of sediment deposited at the base of a mountain front do to an abrupt change in stream gradient;