The time-dependent viscoelastic deformation and flow of various types of silicate glasses are examined by the use of a pyramidal Berkovich indenter. It is demonstrated that a pyramidal indenter is an efficient microprobe for viscoelastic studies of glass-forming materials at temperatures near the glass transition point. Some important rheological functions of silicate glasses are determined as functions of time on the basis of a linear viscoelastic constitutive equation for pyramidal indentation. The test results are theoretically related to the dimension of flow units of these glasses, suggesting that a model of thermally fragmented silicate clusters is appropriate for consistently understanding the present rheological test results. However, the microstructural details (crystallite-like or network-like microstructures) of fragmented clusters are not inferred from the rheological information.