"Plain mountains" is not a commonly used term in geography or geology. It could potentially refer to mountains that have relatively flat or level surfaces on top, but without more context it is unclear what specifically is being referred to.
Appalachian mountains
The Vindhya Mountains divide the Narmada River basin from the Indo-Gangetic plain.
The colonists referred to the foothills between the Appalachian Mountains and the Atlantic Coastal Plain as the Piedmont region.
The West Siberian Plain lies east of the Ural Mountains. It is one of the largest flat areas in the world, covering over 2.6 million square kilometers.
The Ural Mountains separate the Northern European Plain from the West Siberian Plain and serve as the traditional boundary between Europe and Asia. The range extends from the Arctic Ocean to the Ural River in Kazakhstan.
applachian mountains
No, Oman has a narrow coastal plain, but mountains inland and desert inland of those mountains.
Coastal Plain
Appalachian mountains
the Hindu Kush Mountains
A plain.
a plain
The Rocky Mountains.
They both are in between of mountains and hills
The Vindhya Mountains divide the Narmada River basin from the Indo-Gangetic plain.
The state of North Carolina is separated into 3 regions. These regions are the Coastal Plain, the Piedmont, and the Mountains.
The Vindhya Mountains divide the Narmada River basin from the Indo-Gangetic plain.