If the point of migdal bavel was to reach the heavens, why not build it on a mountain instead of a valley?
וַיְהִי כָל הָאָרֶץ שָׂפָה אֶחָת וּדְבָרִים אֲחָדִים:(ב) וַיְהִי בְּנָסְעָם מִקֶּדֶם וַיִּמְצְאוּ בִקְעָה בְּאֶרֶץ שִׁנְעָר וַיֵּשְׁבוּ שָׁם:(ג) וַיֹּאמְרוּ אִישׁ אֶל רֵעֵהוּ הָבָה נִלְבְּנָה לְבֵנִים וְנִשְׂרְפָה לִשְׂרֵפָה וַתְּהִי לָהֶם הַלְּבֵנָה לְאָבֶן וְהַחֵמָר הָיָה לָהֶם לַחֹמֶר:(ד) וַיֹּאמְרוּ הָבָה נִבְנֶה לָּנוּ עִיר וּמִגְדָּל וְרֹאשׁוֹ בַשָּׁמַיִם וְנַעֲשֶׂה לָּנוּ שֵׁם פֶּן נָפוּץ עַל פְּנֵי כָל הָאָרֶץ
1) And the whole earth was of one language and of one speech. 2) And it came to pass, as they journeyed east, that they found a plain in the land of Shinar; and they dwelt there. 3) And they said one to another: 'Come, let us make brick, and burn them thoroughly.' And they had brick for stone, and slime had they for mortar. 3) And they said: 'Come, let us build us a city, and a tower, with its top in heaven, and let us make us a name; lest we be scattered abroad upon the face of the whole earth.'
Bereshis 11:1-4