diff --git a/pageserver/src/layered_repository/layer_map.rs b/pageserver/src/layered_repository/layer_map.rs
index 2957a55d03..ea91c6be01 100644
--- a/pageserver/src/layered_repository/layer_map.rs
+++ b/pageserver/src/layered_repository/layer_map.rs
@@ -271,15 +271,16 @@ impl LayerMap {
     /// Is there a newer image layer for given segment?
     ///
     /// This is used for garbage collection, to determine if an old layer can
-    /// be deleted. We ignore in-memory layers because they are not durable
-    /// on disk, and delta layers because they depend on an older layer.
+    /// be deleted.
     pub fn newer_image_layer_exists(&self, seg: SegmentTag, lsn: Lsn) -> bool {
         if let Some(segentry) = self.segs.get(&seg) {
+            // We only check on-disk layers, because
+            // in-memory layers are not durable
             for (newer_lsn, layer) in segentry
                 .historic
                 .range((Included(lsn), Included(Lsn(u64::MAX))))
             {
-                // Ignore delta layers.
+                // Ignore layers that depend on an older layer.
                 if layer.is_incremental() {
                     continue;
                 }
diff --git a/pageserver/src/layered_repository/storage_layer.rs b/pageserver/src/layered_repository/storage_layer.rs
index cfae4c81c1..2cb4850c10 100644
--- a/pageserver/src/layered_repository/storage_layer.rs
+++ b/pageserver/src/layered_repository/storage_layer.rs
@@ -105,12 +105,24 @@ pub enum PageReconstructResult {
 /// in-memory and on-disk layers.
 ///
 pub trait Layer: Send + Sync {
-    // These functions identify the relish segment and the LSN range
-    // that this Layer holds.
+    /// Identify the timeline this relish belongs to
     fn get_timeline_id(&self) -> ZTimelineId;
+
+    /// Identify the relish segment
     fn get_seg_tag(&self) -> SegmentTag;
+
+    /// Inclusive start bound of the LSN range that this layer hold
     fn get_start_lsn(&self) -> Lsn;
+
+    /// 'end_lsn' meaning depends on the layer kind:
+    /// - in-memory layer is either unbounded (end_lsn = MAX_LSN) or dropped (end_lsn = drop_lsn)
+    /// - image layer represents snapshot at one LSN, so end_lsn = lsn
+    /// - delta layer has end_lsn
+    ///
+    /// TODO Is end_lsn always exclusive for all layer kinds?
     fn get_end_lsn(&self) -> Lsn;
+
+    /// Is the segment represented by this layer dropped by PostgreSQL?
     fn is_dropped(&self) -> bool;
 
     /// Filename used to store this layer on disk. (Even in-memory layers