Observation of blue clay slopes outcropping along the Italian peninsula (Apennine foredeep and other inland areas) promptedinvestigation into several factors whichgovern their actual slope angle. Though characterized by good gradient, slope angle may differ from slope to slope, as well as within certain limited areas.

The study attempts to connect structural order (dip strata) and shear strength of the soil to explain slope angle. In many cases it is possible to distinguish (i) slope angle controlled by substratum and, thus, characterized by shear strength higher than the residual condition,and (ii) slope angle controlled by colluvium cover,characterized by shear strength reduced to residual.

Following an infinite slope stability model approach, the above conditions are connected to the half residual friction angle value, ϕR′/2, which splits the slopes intotwo main types: cover-controlled, characterized by β≤ϕR′/2 and completely or in part substratum-controlled, characterized by β>ϕR′/2.

Slope angle distribution, deduced from slope map,can help us to understand the relative role of footslope removal processes and to distinguish slope type, substratum-controlled or colluvium-controlled.

This study cannot embrace all the factors and processes leading to a blue clay slope angle, and local geomorphological processes may complicate the outline proposed, though it does provide a useful first approach.

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