Inland regions were mostly unexplored by the Spanish, leaving them generally outside the control of the colonial authorities. Mountain ranges of the Peninsular Ranges, eastern Transverse Ranges, and the Sierra Nevada, along with the arid Colorado Desert, Mojave Desert, and Great Basin Desert in their eastern rain shadows, served as natural barriers to Spanish settlement. The eastern border of upper Las Californias was never officially defined under either Spanish or subsequent Mexican rule.[4] The 1781 Instrucciones and government correspondence described Alta California ("Upper California") as the areas to the west of the Sierra Nevada and the lower part of the Colorado River in the Lower Colorado River Valley (the river forms the present day border between the states of California and Arizona).[5]