El Salvador was the easiest way to get through north America instead of going all the way around towards the arctic oceans.
Answer:
Ohio has had many different geologic periods ranging from the Precambrian Period to the Permian Period. In the 245 million years following the Permian Period rocks were formed in what is now Ohio but were erased by natural forces, this is called the geologic gap. Around a million years ago the Ice Age caused glaciers to move across Ohio which covered around 2/3 of the state, which scraped much of Ohio's surface. After all the glaciers melted around 10,000 years ago the Earth entered the recent period of geologic history. Now new igneous rocks are being formed.
The Australian people that leave more inland where the climate is very harsh, and it is mostly arid and semi arid, have troubles with getting water, because it lacks on most of the country. They use the underground waters, but they have to be filtered and desalinized because big portion of this water is either not suitable for direct consumption, or contains salt. <span />
The Sahara desert is vast, large, dry, and empty of resources. This means people/armies wouldn't walk through there because it's basically suicide. This acts as a "natural barrier" because nobody would cross the desert in the first place.
Hope this helps!
