What is the purpose of Deuteronomy 11:30?

Question

Deuteronomy 11:29 (NIV) commands:

When the LORD your God has brought you into the land you are entering to possess, you are to proclaim on Mount Gerizim the blessings, and on Mount Ebal the curses.

Now these mountains, Gerizim and Ebal are very prominent, being on the south and north respectively of the city of Shechem (Nablus today). There aren't any other mountains with these same names. The descendants of Israel certainly knew about these mountains from the traditions of the Jacob in Shechem related in Genesis, and from the spies who scouted the land in Numbers. Why then, does the very next verse, Deuteronomy 11:30 (NIV)

As you know, these mountains are across the Jordan, westward, toward the setting sun, near the great trees of Moreh, in the territory of those Canaanites living in the Arabah in the vicinity of Gilgal.

need to go into such detail about the location of two landmarks that are impossible to miss? What does this verse add to the narrative?

Answer 1

Professor Elia Samuele Artom suggests that this location was not known to them at the time.

(Note that the wording "הלא המה" is used throughout the bible to reference something known to the reader. See, for example, I Kings 14:29, 15:23.)

In the comments, Bach has suggested that the Bible may have wanted to record the exact location for future generations, as seen a number of times throughout the Bible, where locations known to them are recorded in detail.

The Talmud (Sotah 33b) suggests that this verse comes to specify a path that the Jewish people should take on their way in to Canaan:

Rabbi Eliezer ben Ya’akov says: The verse does not come to establish the location of Mount Gerizim and Mount Ebal. Rather, it comes to show the Jewish people the way the second time, when they were entering the land of Canaan, like the way He showed them the first time, when they left Egypt and a pillar of cloud went before them and made the terrain easier to transverse. The purpose of the verse is to instruct the Jewish people how to enter the land of Canaan with relative ease, despite the absence of the pillar of cloud. The word “way” instructs them to go along a pre-established way, and not in fields and vineyards. The phrase “that dwell” instructs them to go in settled areas and not in the wilderness. “In the Arabah,” which means plain, teaches them to go in the plains and not over mountains and hills.

Hezekiah B. Manoah suggests that this is to tell them exactly which portion of the mountains to deliver the blessings and curses on, as they are very large.

Answer 2

The significance seems to be אֵצֶל אֵלֹונֵי מֹרֶֽה, "by the terebinths, (or oaks) of Moreh:"

Both are on the other side of the Jordan, beyond the west road that is in the land of the Canaanites who dwell in the Arabah - near Gilgal, by the terebinths of Moreh? (JPS)

The mountains are the general landmarks, the terebinths of Moreh, the specific location. That is, simply saying Mount Gerizim and Mount Ebal would allow the Israelites to set up anywhere in the valley, but Moses wants them specifically by the terebinths of Moreh.

This seems to set up a type of reenactment of Abram's arrival in Canaan:

¹ Now the LORD said to Abram, “Go from your country and your kindred and your father's house to the land that I will show you. ² And I will make of you a great nation, and I will bless you and make your name great, so that you will be a blessing. ³ I will bless those who bless you, and him who dishonors you I will curse, and in you all the families of the earth shall be blessed.” ⁴ So Abram went, as the LORD had told him, and Lot went with him. Abram was seventy-five years old when he departed from Haran. ⁵ And Abram took Sarai his wife, and Lot his brother's son, and all their possessions that they had gathered, and the people that they had acquired in Haran, and they set out to go to the land of Canaan. When they came to the land of Canaan, ⁶ Abram passed through the land to the place at Shechem, to the oak of Moreh. At that time the Canaanites were in the land. ⁷ Then the LORD appeared to Abram and said, “To your offspring I will give this land.” So he built there an altar to the LORD, who had appeared to him.
(Genesis 12 ESV)

Abram left Haran as the LORD instructed. His first stop in Canaan was עַד אֵלֹון מֹורֶה, at the terebinth (singular) of Moreh. There the LORD promised the land to Abram's offspring.

Moses instructs the Israelites to return not to the general vicinity between the two mountains, but to the same location where the LORD first promised to give Abram's offspring the land.

This reenactment is in the reverse order of the original events:

A:  I will bless those who bless you and curse those who curse you
 B:  Abram arrives at the oak of Moreh and builds an altar
 B': The Israelites arrive at the oaks of Moreh with the Ark of the Covenant
A': The Israelites pronounce blessings from Mt Gerizim and curses from Mt Ebal