Historically, there have been cyclical ice ages in which glacial sheets periodically covered the higher latitudes of the continents.

The Cordilleran ice sheet was a major ice sheet that periodically covered, during glacial periods over the last 2.6 million years, a large area of North America.

Fox Sports Oklahoma also broadcasts "OSSAA Championship Spotlight" periodically covering the full range of OSSAA athletic competition.

For perspective, it periodically covered over one million km, and was often larger than 150,000 km over its 4000-year history.

Bryant periodically covers Terminal songs in his current band, Alive in Wild Paint.

Popular New York City education blog Gotham Schools covered Teach For Us in 2008 and links to bloggers periodically in its regular coverage.

Jon also periodically covers national security, e-voting, and tech policy.

Imagine a sun so variable that it periodically covers practically the entire possible range.

In addition, he periodically covers human rights stories and the international impasse over North Korea's nuclear weapons program.

Retreating glaciers often left recessional moraines that impounded lakes such as Lake Yosemite (a shallow lake that periodically covered much of the floor of Yosemite Valley).