Description from the US Census Bureau (see link for source): Census blocks are statistical areas
bounded on all sides by visible features, such as streets, roads,
streams, and railroad tracks, and by non-visible boundaries such as city, town, township, and county
limits, and short line-of-sight extensions of streets and roads. Generally, census blocks are small in area;
for example, a block in a city. Census blocks in suburban and rural areas may be large, irregular and
bounded by a variety of features, such as roads, streams, and/or transmission line rights-of-way. In
remote areas census blocks may encompass hundreds of square miles. Census blocks cover all territory
in the United States, Puerto Rico, and the Island areas. Blocks do not cross the boundaries of any entity
for which the Census Bureau tabulates data.