We bring together the topics of geographical clusters and technological
trajectories, and shift the focus of the analysis of regional innovation
to main technological trends rather than firms. We define a number of
inventive clusters in the US space and show that long chains of
citations mostly take place between these clusters. This is reminiscent
of the idea of global pipelines of knowledge transfer that is found in
the geographical literature. The deep citations are used to identify
technological trajectories, which are the main directions along which
incremental technological progress accumulates into larger changes.
While the origin and destination of these trajectories are concentrated
in space, the intermediate nodes travel long distances and cover many
locations across the globe. We conclude by calling for more theoretical
and empirical attention to the "deep rivers" that connect the "high
mountains" of local knowledge production.
JEL Classification: O33, O31, R11
Keywords: patent citations; regional concentration of inventive activities; technological trajectories