These merges are called seams or breaks. The coming together of two areas that were developed at different times is one of the major reasons for seams. Seams are also produced when two expanding communities with two distinct patterns grow into each other. Bodies of water and rivers provided a natural break in a street pattern. Many cities have a regular, planned street pattern or irregular, unplanned street pattern with various combinations.
Some grided cities have deviations from the regular pattern.
These deviations might be the result of preserving an important building or a topographic feature such as a hill or a lake. Other planned layouts have irregular patterns within them which may indicate that an older settlement has grown into a larger city. Each city has its own distinct layout. Older cities tend to have small city blocks outlined by narrow streets that were developed by individuals. Whereas newer cities were developed by city planners with larger city blocks and wider streets.