I did not find installing rgeos on Mac the most straightforward thing. I thought I’d share here what worked for me.
Step 1: Install GDAL
Step 2: Download rgeos (do not decompress)
Step 3: Open terminal and run the following command
sudo R CMD INSTALL ~/Downloads/ rgeos_VERSION.tar.gz –configure-args=’–with-geos-config=/Library/Frameworks/GEOS.framework/unix/bin/geos-config’
replacing VERSION in rgeos_VERSION.tar.gz with the rgeos version you have just downloaded.
Step 4: Open R and require(rgeos) to see if everything ran smoothly.