This phenomenon, the process of which is called cap formation, was discovered in 1971 on lymphocytes[1] and is a property of amoebae and all locomotory animal cells except sperm.
[2] In this case, crawling fibroblasts were held in a medium containing small (~1 micrometre in size) carbon particles.
They did so in a roughly straight line, with the particle remaining initially stationary with respect to the substratum.
In view of what we know of capping, this phenomenon is now interpreted as follows: The particle is presumably stuck to many surface molecules, crosslinking them and forming a patch.
An alternative view is that the patches are moved to the rear of the cell by direct attachment to the actin cytoskeleton.
In this scheme, the nature of the tines of the rake are not specified but could, for example, be surface integrins that often act as the feet of the cell to attach it to the substrate.