Within the collaborative research centre SFB350 at the University of Bonn a component software called GeoToolKit has been developed, which is intended to facilitate the design and implementation of 3D/4D geological applications. It provides a range of geo-oriented software building blocks for application developers involving database management system based spatial and temporal data maintenance, support for efficient spatial and temporal retrieval, communication, visualization and graphical interfaces which the user could assemble in a ready-to-use application. As such, GeoToolKit is not a GIS-in-a-box package - rather it is a library of C++ classes that allows the incorporation of spatial functionality within an application under development. It is primarily oriented towards software engineers with the C++ experience involved in the development of specialpurpose geological applications, which can hardly be modelled within standard GISs. We present applications using GeoToolKit, which have been developed to support the geological reconstruction of the Lower Rhine Basin.