@ Jalyha - not always true. In the section of the Illinois River, where I live (in north central Illinois) are featured with large sandstone bluffs formed from ancient sea sand turned to stone, then about 10,000 years ago a huge ice dammed lake formed as the glaciers melted back broke open and millions of gallons of water poured out forming the channels and cut sandstone bluffs of the Illinois River valley over the course of 3 days - not millions of years like most erosion. Many of the islands here are kind of mushroom in shape, where the portions under the rivers surface actually cut under the islands above the water, as in the illustration below. Granted it was fairly unusual geologic formation due to catastrophic erosion event, but this happened at many places around the world at the end of the ice age.

il-rvr-isle.jpg