I live near the foothills in Colorado, and have a very interesting backyard. We have lots of sandstone concretions and quartz. I’m used to seeing bowl shaped deformities in all sorts of rock types caused by natural erosion, but many of these are very deep and narrow, and some have progressed all the way to form tubes. Picture 2 is about 6 inch deep with a 1X2 inch opening. pictures 3-5 are about 18 inches long with a hollow and fairly uniform center, like a straw. I’m guessing this was some sort of very soft cylindrical concretion that eroded out leaving the harder outside sandstone? Either way very cool to see these all over the place!