How do I create a surface with limited data using Geo model tool?

PamKoi

Hi there, I have some interval data from short drill holes (<20m) in a weathered zone. The drilling was done to determine the contact between the weathered zone and hard rock. What would be the best tool to use or way to go about it? I attempted to create a geological model using the data but did not have more options show up for the different surface types (deposit, erosional or intrusion etc) to begin with. There's only two litho types here, very straightforward wtih no grouping or interps required for interval selection tool here (I think). I don't know what I did wrong as the output volume only showed the 'unknown' surface. How best can I approach this to generate the contact surface?