Mima Mounds are low, dome-like, roughly symmetrical natural mounds on terrestrial terrain - mainly composed of loose, unstratified, often gravelly sediment. They have a diameter of 3 to 50 m, and a height of 0.2 to 2 m.

They are found mainly in the northwestern United States - though similar structures have also been found on every continent except Antarctica. They sometimes occur in groups of thousands.

After 150 years or so of research, there are now several theories attempting to explain their origins - none of which has been generally accepted.

As their construction tends to vary, it's currently thought that different mounds may have differing origins.

The main theories are that they were formed by ;

• Gophers (moles)
• Wind
• Seismic activity
• Clay expansion/contraction
• Termites, ants, rodents and plant roots
• Glacial flooding

See : BBC Earth

Also see : Drumlinsplugin-autotooltip__plain plugin-autotooltip_bigDrumlins

"Drumlins are elongated hills of glacial deposits. They can be 1 km long and 500 m wide, often occurring in groups. A group of drumlins is called a drumlin swarm or a basket of eggs, eg Vale of Eden.[UK]."

Source : BBC Glacial Depostion"

They are formed from loose debris (clay, sill, boulders etc) that would have been dragged along by a previously existing glacier or ice sheet. They are clearly directional, with their axes showing the direction of flow of the glacier.