Fill in the Blanks Mexico's Physical GeographyOnline version Lesson 1 Fill in the blanks with the correct word. by Bonnie Stolz 1 river is known in the United States as the Rio Grande . This river forms part of Mexico's border with the . The separates the of from the . Mexico City lies at the end of the plateau in the Valley of . In the Peninsula , erosion of rock has created many and . is one of most important resources . The Gulf of Mexico is Mexico's border . peninsula stretches from the Mexico into the Ocean . is Mexico's most important product .