Seam

The transition area between patches is often a sharp delineation. When a new Patch appears, this is literal, a two-dimensional line between areas, and while it softens somewhat over time, the Seams are never more than a few dozen yards wide. A person standing on a patch of desert salt flat abutting directly against one of frozen mountains, one can literally take a single step to go from sweltering dry heat to the freezing cold and blowing snow.

Manifestation

Initially seams are not visible, being no more than knife's edge between two terrains. After a year or two, small splotches of terrain from one patch start appearing in the other along the seam, no more than a few inches at first, then larger areas within a few months of the first appearances. After ten years, the seam may be up to twenty feet, with a more gradual, but still rapid transition between the patches, although the terrains never "mix," remaining discrete areas within the seam, like stones in a flagstone walk.
Weather across a seam can be downright weird! Given this is Patchwork, that’s saying something. Weather and temperature in one patch has no direct affect on an adjacent one. Thus, someone standing in a patch that’s currently experiencing a bright sunny Spring day with pleasant temperatures, can stand near the edge and see across a 20’ seam to the next patch’s freezing cold hail storm with dark heavy clouds. If they have any width, the seams experience weather from each of the adjacent Patches, but in random spots with the full effects from one of the patches. The closest patch has the most effect on the seam, but it’s not an absolute.
The effects of weather also do not cross seams. Wind, for instance, does not cross, nor does fog, billowing clouds of dust, or other atmospheric activity. Similarly, rain, hail, snow, and other precipitation also does not cross. (Close observation shows that it falls to the ground at the very edge of the seam.)
Although the seams themselves are not visible, it's often not difficult to spot the transitional effects. This may appear as a curtain of rain, like one might see across a valley, although in a much sharper line than normal, while clouds appear sliced through like a cake. For unknown reasons, this boundary is often difficult to see until one is within a few dozen yards of the seam.

Macro Effects?

What can cross a seam "naturally" isn't always clear. While rain stops at a seam, a river continues on unabated, often resulting in a waterfall or a body of water like The Mudhole. Hail falling from the sky also stops, creating a rather unpleasant experience for anyone walking through the resulting "hail curtain," but rocks rolling down a slope pass right on through without impediment.
What's the difference there?
Most who study this sort of thing chalk it up to micro vs. macro effects. Small particles without will or self-motivation (e.g., rain or hail) are stopped by the seam, while larger objects (boulders, street cars) aren't affected, or at least not affected at a level that brings them to a halt. Insects and other normal vermin also pass across freely, possibly due to having just enough will to cross the transom.

Type

Natural